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1.  Simultaneous use of solution NMR and X-ray data in REFMAC5 for joint refinement/detection of structural differences 
Paramagnetic NMR data (pseudocontact shifts and self-orientation residual dipolar couplings) and diamagnetic residual dipolar couplings can now be used in the program REFMAC5 from CCP4 as structural restraints together with X-ray crystallographic data. These NMR restraints can reveal differences between solid state and solution conformations of molecules or, in their absence, can be used together with X-ray crystallographic data for structural refinement.
The program REFMAC5 from CCP4 was modified to allow the simultaneous use of X-ray crystallographic data and paramagnetic NMR data (pseudocontact shifts and self-orientation residual dipolar couplings) and/or diamagnetic residual dipolar couplings. Incorporation of these long-range NMR restraints in REFMAC5 can reveal differences between solid-state and solution conformations of molecules or, in their absence, can be used together with X-ray crystallographic data for structural refinement. Since NMR and X-ray data are complementary, when a single structure is consistent with both sets of data and still maintains reasonably ‘ideal’ geometries, the reliability of the derived atomic model is expected to increase. The program was tested on five different proteins: the catalytic domain of matrix metalloproteinase 1, GB3, ubiquitin, free calmodulin and calmodulin complexed with a peptide. In some cases the joint refinement produced a single model consistent with both sets of observations, while in other cases it indicated, outside the experimental uncertainty, the presence of different protein conformations in solution and in the solid state.
PMCID: PMC4306559  PMID: 24699641
structure refinement; PCS; RDC; X-ray; REFMAC
2.  Motions on the Millisecond Timescale and Multiple Conformations of HIV-1 Capsid Protein: Implications for Structural Polymorphism of CA Assemblies 
The capsid protein (CA) of human immunodeficiency virus 1 (HIV-1) assembles into a cone-like structure that encloses the viral RNA genome. Interestingly, significant heterogeneity in shape and organization of capsids can be observed in mature HIV-1 virions. In vitro, CA also exhibits structural polymorphism and can assemble into various morphologies, such as cones, tubes and spheres. Many intermolecular contacts that are critical for CA assembly are formed by its C-terminal domain (CTD), a dimerization domain, which was found to adopt different orientations in several X-ray and NMR structures of the the CTD dimer and full-length CA proteins. Tyr145 (Y145), residue two in our CTD construct used for NMR structure determination, but not present in the crystallographic constructs, was found to be crucial for infectivity and engaged in numerous interactions at the CTD dimer interface. Here we investigate the origin of CA structural plasticity using solid-state and solution NMR spectroscopy. In the solid-state, the hinge region connecting the NTD and CTD is flexible on the millisecond timescale, as evidenced by the backbone motions of Y145 in CA conical assemblies and in two CTD constructs (137–231 and 142–231), allowing the protein to access multiple conformations essential for pleimorphic capsid assemblies. In solution, the CTD dimer exists as two major conformers, whose relative populations differ for the different CTD constructs. In the longer CTD (144–231) construct that contains the hinge region between the NTD and CTD, the populations of the two conformers are likely determined by the protonation state of the E175 side chain that is located at the dimer interface and within hydrogen bonding distance of the W184 side chain on the other monomer. At pH 6.5, the major conformer exhibits the same dimer interface as full-length CA. In the short CTD (150–231) construct, no pH-dependent conformational shift is observed. These findings suggest that the presence of structural plasticity at the CTD dimer interface permits pleiotropic HIV-1 capsid assembly, resulting in varied capsid morphologies.
PMCID: PMC3325613  PMID: 22428579
3.  Membrane Protein Structure Determination in Membrana 
Accounts of chemical research  2013;46(9):2182-2190.
The two principal components of biological membranes, the lipid bilayer and the proteins integrated within it, have coevolved for specific functions that mediate the interactions of cells with their environment. Molecular structures can provide very significant insights about protein function. In the case of membrane proteins, the physical and chemical properties of lipids and proteins are highly interdependent; therefore structure determination should include the membrane environment. Considering the membrane alongside the protein eliminates the possibility that crystal contacts or detergent molecules could distort protein structure, dynamics, and function and enables ligand binding studies to be performed in a natural setting.
Solid-state NMR spectroscopy is compatible with three-dimensional structure determination of membrane proteins in phospholipid bilayer membranes under physiological conditions and has played an important role in elucidating the physical and chemical properties of biological membranes, providing key information about the structure and dynamics of the phospholipid components. Recently, developments in the recombinant expression of membrane proteins, sample preparation, pulse sequences for high-resolution spectroscopy, radio frequency probes, high-field magnets, and computational methods have enabled a number of membrane protein structures to be determined in lipid bilayer membranes.
In this Account, we illustrate solid-state NMR methods with examples from two bacterial outer membrane proteins (OmpX and Ail) that form integral membrane β-barrels. The ability to measure orientation-dependent frequencies in the solid-state NMR spectra of membrane-embedded proteins provides the foundation for a powerful approach to structure determination based primarily on orientation restraints. Orientation restraints are particularly useful for NMR structural studies of membrane proteins because they provide information about both three-dimensional structure and the orientation of the protein within the membrane. When combined with dihedral angle restraints derived from analysis of isotropic chemical shifts, molecular fragment replacement, and de novo structure prediction, orientation restraints can yield high-quality three-dimensional structures with few or no distance restraints. Using complementary solid-state NMR methods based on oriented sample (OS) and magic angle spinning (MAS) approaches, one can resolve and assign multiple peaks through the use of 15N/13C labeled samples and measure precise restraints to determine structures.
PMCID: PMC3970975  PMID: 24041243
4.  A Geometric Arrangement Algorithm for Structure Determination of Symmetric Protein Homo-Oligomers from NOEs and RDCs 
Journal of Computational Biology  2011;18(11):1507-1523.
Nuclear magnetic resonance (NMR) spectroscopy is a primary tool to perform structural studies of proteins in physiologically-relevant solution conditions. Restraints on distances between pairs of nuclei in the protein, derived from the nuclear Overhauser effect (NOE), provide information about the structure of the protein in its folded state. NMR studies of symmetric protein homo-oligomers present a unique challenge. Using X-filtered NOESY experiments, it is possible to determine whether an NOE restrains a pair of protons across different subunits or within a single subunit, but current experimental techniques are unable to determine in which subunits the restrained protons lie. Consequently, it is difficult to assign NOEs to particular pairs of subunits with certainty, thus hindering the structural analysis of the oligomeric state. Computational approaches are needed to address this subunit ambiguity, but traditional solutions often rely on stochastic search coupled with simulated annealing and simulations of simplified molecular dynamics, which have many tunable parameters that must be chosen carefully and can also fail to report structures consistent with the experimental restraints. In addition, these traditional approaches rarely provide guarantees on running time or solution quality. We reduce the structure determination of homo-oligomers with cyclic symmetry to computing geometric arrangements of unions of annuli in a plane. Our algorithm, disco, runs in expected O(n2) time, where n is the number of distance restraints, potentially assigned ambiguously. disco is guaranteed to report the exact set of oligomer structures consistent with the distance restraints and also with orientational restraints from residual dipolar couplings (RDCs). We demonstrate our method using two symmetric protein complexes: the trimeric E. coli diacylglycerol kinase (DAGK) and a dimeric mutant of the immunoglobulin-binding domain B1 of streptococcal protein G (GB1). In both cases, disco computes oligomer structures with high precision and also finds distance restraints that are either mutually inconsistent or inconsistent with the RDCs. The entire protocol DISCO has been completely automated in a software package that is freely available and open-source at
PMCID: PMC3216109  PMID: 22035328
algorithms; computational molecular biology; protein structure
5.  A refinement protocol to determine structure, topology, and depth of insertion of membrane proteins using hybrid solution and solid-state NMR restraints 
Journal of biomolecular NMR  2009;44(4):195-205.
To fully describe the fold space and ultimately the biological function of membrane proteins, it is necessary to determine the specific interactions of the protein with the membrane. This property of membrane proteins that we refer to as structural topology cannot be resolved using X-ray crystallography or solution NMR alone. In this article, we incorporate into XPLOR-NIH a hybrid objective function for membrane protein structure determination that utilizes solution and solid-state NMR restraints, simultaneously defining structure, topology, and depth of insertion. Distance and angular restraints obtained from solution NMR of membrane proteins solubilized in detergent micelles are combined with backbone orientational restraints (chemical shift anisotropy and dipolar couplings) derived from solid-state NMR in aligned lipid bilayers. In addition, a supplementary knowledge-based potential, Ez (insertion depth potential), is used to ensure the correct positioning of secondary structural elements with respect to a virtual membrane. The hybrid objective function is minimized using a simulated annealing protocol implemented into XPLOR-NIH software for general use.
PMCID: PMC2824793  PMID: 19597943
Hybrid method; Membrane protein; Molecular modeling; Structural topology; PISEMA; Solid-state NMR
6.  Systematic Comparison of Crystal and NMR Protein Structures Deposited in the Protein Data Bank 
Nearly all the macromolecular three-dimensional structures deposited in Protein Data Bank were determined by either crystallographic (X-ray) or Nuclear Magnetic Resonance (NMR) spectroscopic methods. This paper reports a systematic comparison of the crystallographic and NMR results deposited in the files of the Protein Data Bank, in order to find out to which extent these information can be aggregated in bioinformatics. A non-redundant data set containing 109 NMR – X-ray structure pairs of nearly identical proteins was derived from the Protein Data Bank. A series of comparisons were performed by focusing the attention towards both global features and local details. It was observed that: (1) the RMDS values between NMR and crystal structures range from about 1.5 Å to about 2.5 Å; (2) the correlation between conformational deviations and residue type reveals that hydrophobic amino acids are more similar in crystal and NMR structures than hydrophilic amino acids; (3) the correlation between solvent accessibility of the residues and their conformational variability in solid state and in solution is relatively modest (correlation coefficient = 0.462); (4) beta strands on average match better between NMR and crystal structures than helices and loops; (5) conformational differences between loops are independent of crystal packing interactions in the solid state; (6) very seldom, side chains buried in the protein interior are observed to adopt different orientations in the solid state and in solution.
PMCID: PMC3032220  PMID: 21293729
Large scale structure comparison; NMR spectroscopy; Protein Data Bank; structure similarity; X-ray crystallography.
7.  Improving NMR Protein Structure Quality by Rosetta Refinement: A Molecular Replacement Study 
Proteins  2009;75(1):147-167.
The structure of human protein HSPC034 has been determined by both solution NMR spectroscopy and X-ray crystallography. Refinement of the NMR structure ensemble, using a Rosetta protocol in the absence of NMR restraints, resulted in significant improvements not only in structure quality, but also in molecular replacement (MR) performance with the raw X-ray diffraction data using MOLREP and Phaser. This method has recently been shown to be generally applicable with improved MR performance demonstrated for eight NMR structures refined using Rosetta.1 Additionally, NMR structures of HSPC034 calculated by standard methods that include NMR restraints, have improvements in the RMSD to the crystal structure and MR performance in the order DYANA, CYANA, XPLOR-NIH, and CNS with explicit water refinement (CNSw). Further Rosetta refinement of the CNSw structures, perhaps due to more thorough conformational sampling and/or a superior force field, was capable of finding alternative low energy protein conformations that were equally consistent with the NMR data according to the RPF scores. Upon further examination, the additional MR-performance shortfall for NMR refined structures as compared to the X-ray structure MR performance were attributed, in part, to crystal-packing effects, real structural differences, and inferior hydrogen bonding in the NMR structures. A good correlation between a decrease in the number of buried unsatisfied hydrogen-bond donors and improved MR performance demonstrates the importance of hydrogen-bond terms in the force field for improving NMR structures. The superior hydrogen-bond network in Rosetta-refined structures, demonstrates that correct identification of hydrogen bonds should be a critical goal of NMR structure refinement. Inclusion of non-bivalent hydrogen bonds identified from Rosetta structures as additional restraints in the structure calculation results in NMR structures with improved MR performance
PMCID: PMC2878636  PMID: 18816799
NMR; X-ray; HSPC034; PP25; C1orf41; Northeast Structural Genomics Consortium; structural genomics; comparison of NMR and X-ray structures; Rosetta; NMR force field refinement; molecular replacement; hydrogen bonding; X-ray crystallography; refinement methods
8.  NMR structure of the protein NP_247299.1: comparison with the crystal structure 
Comparison of the NMR and crystal structures of a protein determined using largely automated methods has enabled the interpretation of local differences in the highly similar structures. These differences are found in segments of higher B values in the crystal and correlate with dynamic processes on the NMR chemical shift timescale observed in solution.
The NMR structure of the protein NP_247299.1 in solution at 313 K has been determined and is compared with the X-ray crystal structure, which was also solved in the Joint Center for Structural Genomics (JCSG) at 100 K and at 1.7 Å resolution. Both structures were obtained using the current largely automated crystallographic and solution NMR methods used by the JCSG. This paper assesses the accuracy and precision of the results from these recently established automated approaches, aiming for quantitative statements about the location of structure variations that may arise from either one of the methods used or from the different environments in solution and in the crystal. To evaluate the possible impact of the different software used for the crystallographic and the NMR structure determinations and analysis, the concept is introduced of reference structures, which are computed using the NMR software with input of upper-limit distance constraints derived from the molecular models representing the results of the two structure determinations. The use of this new approach is explored to quantify global differences that arise from the different methods of structure determination and analysis versus those that represent interesting local variations or dynamics. The near-identity of the protein core in the NMR and crystal structures thus provided a basis for the identification of complementary information from the two different methods. It was thus observed that locally increased crystallographic B values correlate with dynamic structural polymorphisms in solution, including that the solution state of the protein involves a slow dynamic equilibrium on a time scale of milliseconds or slower between two ensembles of rapidly interchanging conformers that contain, respectively, the cis or trans form of the C-terminal proline and represent about 25 and 75% of the total protein.
PMCID: PMC2954228  PMID: 20944234
structure comparison in crystals and in solution; structure-determination software; reference structures; nitrogenase iron–molybdenum cofactor
9.  NMR and X-ray analysis of structural additivity in metal binding site-swapped hybrids of rubredoxin 
Chimeric hybrids derived from the rubredoxins of Pyrococcus furiosus (Pf) and Clostridium pasteurianum (Cp) provide a robust system for the characterization of protein conformational stability and dynamics in a differential mode. Interchange of the seven nonconserved residues of the metal binding site between the Pf and Cp rubredoxins yields a complementary pair of hybrids, for which the sum of the thermodynamic stabilities is equal to the sum for the parental proteins. Furthermore, the increase in amide hydrogen exchange rates for the hyperthermophile-derived metal binding site hybrid is faithfully mirrored by a corresponding decrease for the complementary hybrid that is derived from the less thermostable rubredoxin, indicating a degree of additivity in the conformational fluctuations that underlie these exchange reactions.
Initial NMR studies indicated that the structures of the two complementary hybrids closely resemble "cut-and-paste" models derived from the parental Pf and Cp rubredoxins. This protein system offers a robust opportunity to characterize differences in solution structure, permitting the quantitative NMR chemical shift and NOE peak intensity data to be analyzed without recourse to the conventional conversion of experimental NOE peak intensities into distance restraints. The intensities for 1573 of the 1652 well-resolved NOE crosspeaks from the hybrid rubredoxins were statistically indistinguishable from the intensities of the corresponding parental crosspeaks, to within the baseplane noise level of these high sensitivity data sets. The differences in intensity for the remaining 79 NOE crosspeaks were directly ascribable to localized dynamical processes. Subsequent X-ray analysis of the metal binding site-swapped hybrids, to resolution limits of 0.79 Å and 1.04 Å, demonstrated that the backbone and sidechain heavy atoms in the NMR-derived structures lie within the range of structural variability exhibited among the individual molecules in the crystallographic asymmetric unit (~0.3 Å), indicating consistency with the "cut-and-paste" structuring of the hybrid rubredoxins in both crystal and solution.
Each of the significant energetic interactions in the metal binding site-swapped hybrids appears to exhibit a 1-to-1 correspondence with the interactions present in the corresponding parental rubredoxin structure, thus providing a structural basis for the observed additivity in conformational stability and dynamics. The congruence of these X-ray and NMR experimental data offers additional support for the interpretation that the conventional treatment of NOE distance restraints contributes substantially to the systematic differences that are commonly reported between NMR- and X-ray-derived protein structures.
PMCID: PMC2249605  PMID: 18053245
10.  Statistical analysis of interface similarity in crystals of homologous proteins 
Journal of molecular biology  2008;381(2):487-507.
Many proteins function as homooligomers and are regulated via their oligomeric state. For some proteins, the stoichiometry of homooligomeric states under various conditions has been studied using gel filtration or analytical ultracentrifugation experiments. The interfaces involved in these assemblies may be identified using crosslinking and mass spectrometry, solution-state NMR, and other experiments. But for most proteins, the actual interfaces that are involved in oligomerization are inferred from X-ray crystallographic structures using assumptions about interface surface areas and physical properties. Examination of interfaces across different PDB entries in a protein family reveals several important features. First, similarity of space group, asymmetric unit size, and cell dimensions and angles (within 1%) does not guarantee that two crystals are actually the same crystal form, that is containing similar relative orientations and interactions within the crystal. Conversely, two crystals in different space groups may be quite similar in terms of all of the interfaces within each crystal. Second, NMR structures and an existing benchmark of PDB crystallographic entries consisting of 126 dimers and larger structures and 132 monomers was used to determine whether the existence or lack of existence of common interfaces across multiple crystal forms can be used to predict whether a protein is an oligomer or not. Monomeric proteins tend to have common interfaces across only a minority of crystal forms, while higher order structures exhibit common interfaces across a majority of available crystal forms. The data can be used to estimate the probability that an interface is biological if two or more crystal forms are available. Finally, the PISA database available from the EBI is more consistent in identifying interfaces observed in many crystal forms than is the PDB or EBI’s Protein Quaternary Server (PQS). The PDB in particular is missing highly likely biological interfaces in its biological unit files for about 10% of PDB entries.
PMCID: PMC2573399  PMID: 18599072
11.  Alternate states of proteins revealed by detailed energy landscape mapping 
Journal of molecular biology  2010;405(2):607-618.
What conformations do protein molecules populate in solution? Crystallography provides a high-resolution description of protein structure in the crystal environment, while NMR describes structure in solution but using less data. NMR structures display more variability, but is this because crystal contacts are absent or because of fewer data constraints? Here we report unexpected insight into this issue obtained through analysis of detailed protein energy landscapes generated by large-scale, native-enhanced sampling of conformational space with Rosetta@HOME for 111 protein domains. In the absence of tightly associating binding partners or ligands, the lowest-energy Rosetta models were nearly all <2.5Å CαRMSD from the experimental structure; this result demonstrates that structure prediction accuracy for globular proteins is limited mainly by the ability to sample close to the native structure. While the lowest-energy models are similar to deposited structures, they are not identical; the largest deviations are most often in regions involved in ligand, quaternary, or crystal contacts. For ligand binding proteins, the low energy models may resemble the apo structures, and for oligomeric proteins, the monomeric assembly intermediates. The deviations between the low energy models and crystal structures largely disappear when landscapes are computed in the context of the crystal lattice or multimer. The computed low-energy ensembles, with tight crystal-structure-like packing in the core, but more NMR-structure-like variability in loops, may in some cases resemble the native state ensembles of proteins better than individual crystal or NMR structures, and can suggest experimentally testable hypotheses relating alternative states and structural heterogeneity to function.
PMCID: PMC3046547  PMID: 21073878
Rosetta; alternative conformations; protein mobility; structure prediction; validation
12.  Structural constraints for the Crh protein from solid-state NMR experiments 
Journal of Biomolecular Nmr  2008;40(4):239-250.
We demonstrate that short, medium and long-range constraints can be extracted from proton mediated, rare-spin detected correlation solid-state NMR experiments for the microcrystalline 10.4 × 2 kDa dimeric model protein Crh. Magnetization build-up curves from cross signals in NHHC and CHHC spectra deliver detailed information on side chain conformers and secondary structure for interactions between spin pairs. A large number of medium and long-range correlations can be observed in the spectra, and an analysis of the resolved signals reveals that the constraints cover the entire sequence, also including inter-monomer contacts between the two molecules forming the domain-swapped Crh dimer. Dynamic behavior is shown to have an impact on cross signals intensities, as indicated for mobile residues or regions by contacts predicted from the crystal structure, but absent in the spectra. Our work validates strategies involving proton distance measurements for large and complex proteins as the Crh dimer, and confirms the magnetization transfer properties previously described for small molecules in solid protein samples.
Electronic supplementary material
The online version of this article (doi:10.1007/s10858-008-9229-3) contains supplementary material, which is available to authorized users.
PMCID: PMC2579321  PMID: 18320329
Catabolite repression histidine-containing phosphocarrier protein (Crh); Distance constraints; MAS; 3D Protein structure; Solid-state NMR spectroscopy
13.  Mitochondrial uncoupling protein 2 structure determined by NMR molecular fragment searching 
Nature  2011;476(7358):109-113.
Mitochondrial solute carriers are a family of membrane proteins that catalyze the transport of small molecules across the mitochondrial inner membrane1. We describe here a solution nuclear magnetic resonance (NMR) method for structural characterization of mitochondrial uncoupling protein 2 (UCP2), a member of the carrier family that translocates protons across the mitochonrial inner membrane2. The method, which overcomes some of the challenges associated with membrane-protein structure determination3, combines orientation restraints derived from NMR residual dipolar couplings (RDCs) and semi-quantitative distance restraints from paramagnetic relaxation enhancement (PRE) measurements. The local and secondary structures of the protein were determined by piecing together molecular fragments from the Protein Data Bank (PDB) that best fit experimental RDCs from samples weakly aligned in a DNA nanotube liquid crystal. The RDCs also determine the relative orientation of the secondary structural segments, and the PRE restraints provide their spatial arrangement in the tertiary fold. We determined the structure of UCP2 in the presence of GDP. We also mapped the interaction between UCP2 and GDP by measuring paramagnetic broadening of the protein NMR resonances from nitroxide-labeled GDP. UCP2 closely resembles the bovine ADP/ATP carrier (the only carrier protein of known structure4), but the relative orientations of the helical segments are different, resulting in a wider opening on the matrix side of the inner membrane. Moreover, the nitroxide-labeled GDP binds inside the channel and appears to be closer to transmembrane helix 1–4. We believe this biophysical approach, based on solution NMR spectroscopy, can be applied broadly to other membrane proteins and in particular to other mitochondrial carriers, not only for structure determination but also for characterizing various conformational states of these proteins linked to substrate transport.
PMCID: PMC3150631  PMID: 21785437
14.  αB-Crystallin: A Hybrid Solid-Solution State NMR Investigation Reveals Structural Aspects of the Heterogeneous Oligomer 
Journal of molecular biology  2008;385(5):1481-1497.
Atomic level structural information on αB-Crystallin (αB), a prominent member of the small Heat Shock Protein (sHSP) family has been a challenge to obtain due its polydisperse, oligomeric nature. We show that magic-angle spinning solid-state NMR can be used to obtain high-resolution information on ∼ 580 kDa human αB assembled from 175-residue, 20 kDa subunits. An ∼100-residue α-crystallin domain is common to all sHSPs and solution-state NMR was performed on two different α-crystallin domain constructs isolated from αB. In vitro, the chaperone-like activities of full-length αB and the isolated α-crystallin domain are identical. Chemical shifts of the backbone and the Cβ resonances have been obtained for residues 64-162 (α-crystallin domain plus part of the C-terminus) in αB and the isolated α-crystallin domain by solid- and solution-state NMR, respectively. Both sets of data strongly predict six β-strands in the α-crystallin domain. A majority of residues in the α-crystallin domain have similar chemical shifts in both solid- and solution-state indicating a similar structure for the domain in its isolated and oligomeric forms. Sites of inter-subunit interaction are identified from chemical shift differences that cluster to specific regions of the α-crystallin domain. Multiple signals are observed for the resonances of M68 in the oligomer, identifying the region containing this residue as existing in heterogeneous environments within αB. Evidence for a novel dimerization motif in the human α-crystallin domain is obtained by a comparison of (i) solid- and solution-state chemical shift data and (ii) 1H-15N HSQC spectra as a function of pH. The isolated α-crystallin domain undergoes a dimer-monomer transition over the pH range of 7.5 to 6.8. This steep pH-dependent switch may be important for αB to function optimally, e.g., to preserve the filament integrity of cardiac muscle proteins such as actin and desmin during cardiac ischemia which is accompanied by acidosis.
PMCID: PMC2692910  PMID: 19041879
αB-Crystallin; chaperone; sHSP; solid-state NMR; solution-state NMR
15.  NMR Crystallography of Enzyme Active Sites: Probing Chemically-Detailed, Three-Dimensional Structure in Tryptophan Synthase 
Accounts of chemical research  2013;46(9):2008-2017.
NMR crystallography – the synergistic combination of X-ray diffraction, solid-state NMR spectroscopy, and computational chemistry – offers unprecedented insight into three-dimensional, chemically-detailed structure. From its initial role in refining diffraction data of organic and inorganic solids, NMR crystallography is now being developed for application to active sites in biomolecules, where it reveals chemically-rich detail concerning the interactions between enzyme site residues and the reacting substrate that is not achievable when X-ray, NMR, or computational methodologies are applied in isolation. For example, typical X-ray crystal structures (1.5 to 2.5 Å resolution) of enzyme-bound intermediates identify possible hydrogen-bonding interactions between site residues and substrate, but do not directly identify the protonation state of either. Solid-state NMR can provide chemical shifts for selected atoms of enzyme-substrate complexes, but without a larger structural framework in which to interpret them, only empirical correlations with local chemical structure are possible. Ab initio calculations and molecular mechanics can build models for enzymatic processes, but rely on chemical details that must be specified. Together, however, X-ray diffraction, solid-state NMR spectroscopy, and computational chemistry can provide consistent and testable models for structure and function of enzyme active sites: X-ray crystallography provides a coarse framework upon which models of the active site can be developed using computational chemistry; these models can be distinguished by comparison of their calculated NMR chemical shifts with the results of solid-state NMR spectroscopy experiments. Conceptually, each technique is a puzzle piece offering a generous view of the big picture. Only when correctly pieced together, however, can they reveal the big picture at highest resolution. In this Account, we detail our first steps in the development of NMR crystallography for application to enzyme catalysis. We begin with a brief introduction to NMR crystallography and then define the process that we have employed to probe the active site in the β-subunit of tryptophan synthase with unprecedented atomic-level resolution. This approach has resulted in a novel structural hypothesis for the protonation state of the quinonoid intermediate in tryptophan synthase and its surprising role in directing the next step in the catalysis of L-Trp formation.
PMCID: PMC3778090  PMID: 23537227
16.  Developing advanced x-ray scattering methods combined with crystallography and computation 
Methods (San Diego, Calif.)  2013;59(3):363-371.
The extensive use of small angle x-ray scattering (SAXS) over the last few years is rapidly providing new insights into protein interactions, complex formation and conformational states in solution. This SAXS methodology allows for detailed biophysical quantification of samples of interest. Initial analyses provide a judgment of sample quality, revealing the potential presence of aggregation, the overall extent of folding or disorder, the radius of gyration, maximum particle dimensions and oligomerization state. Structural characterizations include ab initio approaches from SAXS data alone, and when combined with previously determined crystal/NMR, atomistic modeling can further enhance structural solutions and assess validity. This combination can provide definitions of architectures, spatial organizations of protein domains within a complex, including those not determined by crystallography or NMR, as well as defining key conformational states of a protein interaction. SAXS is not generally constrained by macromolecule size, and the rapid collection of data in a 96-well plate format provides methods to screen sample conditions. This includes screening for co-factors, substrates, differing protein or nucleotide partners or small molecule inhibitors, to more fully characterize the variations within assembly states and key conformational changes. Such analyses may be useful for screening constructs and conditions to determine those most likely to promote crystal growth of a complex under study. Moreover, these high throughput structural determinations can be leveraged to define how polymorphisms affect assembly formations and activities. This is in addition to potentially providing architectural characterizations of complexes and interactions for systems biology-based research, and distinctions in assemblies and interactions in comparative genomics. Thus, SAXS combined with crystallography/NMR and computation provides a unique set of tools that should be considered as being part of one’s repertoire of biophysical analyses, when conducting characterizations of protein and other macromolecular interactions.
PMCID: PMC3684416  PMID: 23376408
SAXS; crystallography; high-throughput; in-solution; screening; protein interaction
17.  Structures in solid state and solution of dimethoxy curcuminoids: regioselective bromination and chlorination 
Several papers described the structure of curcumin and some other derivatives in solid and in solution. In the crystal structure of curcumin, the enol H atom is located symmetrically between both oxygen atoms of the enolone fragment with an O···O distance of 2.455 Å, which is characteristic for symmetrical H-bonds. In the solution, the geometry of the enolone fragment is attributed to the inherent disorder of the local environment, which solvates one of the basic sites better than the other, stabilizing one tautomer over the other. In this paper, how the position of methoxy groups in dimethoxy curcuminoids influence the conformation of molecules and how the halogen atoms change it when they are bonded at α-position in keto-enol part of molecules is described.
Six isomers of dimethoxy curcuminoids were prepared. Conformations in solid state, which were determined by X-ray single crystallography and 1H MAS and 13C CPMAS NMR measurements, depend on the position of methoxy groups in curcuminoid molecules. In solution, a fast equilibrium between both keto-enol forms exists. A theoretical calculation finding shows that the position of methoxy groups changes the energy of HOMO and LUMO. An efficient protocol for the highly regioselective bromination and chlorination leading to α-halogenated product has been developed. All α-halogenated compounds are present mainly in cis keto-enol form.
The structures in solid state of dimethoxy curcuminoids depend on the position of methoxy groups. The NMR data of crystalline solid samples of 3,4-diOCH3 derivative, XRD measurements and X-ray structures lead us to the conclusion that polymorphism exists in solids. The same conclusion can be done for 3,5-diOCH3 derivative. In solution, dimethoxy curcuminoids are present in the forms that can be described as the coexistence of two equivalent tautomers being in fast equilibrium. The position of methoxy groups has a small influence on the enolic hydrogen bond. Theoretical calculations show that the energy gap between HOMO and LUMO depend on the position of methoxy groups and are lower in solution. Chlorination and bromination on α-position of 1,3-diketone moiety do not change the preferential form being cis keto-enol as in parent compounds.
PMCID: PMC3720230  PMID: 23800041
Dimethoxy curcuminoids; X-ray structures; NMR experiments of solids; Theoretical investigations; Regioselectivity; Enols; Halogenation; α-halodiketones
18.  High-Resolution 3D Structure Determination of Kaliotoxin by Solid-State NMR Spectroscopy 
PLoS ONE  2008;3(6):e2359.
High-resolution solid-state NMR spectroscopy can provide structural information of proteins that cannot be studied by X-ray crystallography or solution NMR spectroscopy. Here we demonstrate that it is possible to determine a protein structure by solid-state NMR to a resolution comparable to that by solution NMR. Using an iterative assignment and structure calculation protocol, a large number of distance restraints was extracted from 1H/1H mixing experiments recorded on a single uniformly labeled sample under magic angle spinning conditions. The calculated structure has a coordinate precision of 0.6 Å and 1.3 Å for the backbone and side chain heavy atoms, respectively, and deviates from the structure observed in solution. The approach is expected to be applicable to larger systems enabling the determination of high-resolution structures of amyloid or membrane proteins.
PMCID: PMC2387072  PMID: 18523586
19.  Comparison of NMR and crystal structures for the proteins TM1112 and TM1367 
Acta Crystallographica Section F  2010;66(Pt 10):1381-1392.
NMR structures of the proteins TM1112 and TM1367 solved by the JCSG in solution at 298 K could be superimposed with the corresponding crystal structures at 100 K with r.m.s.d. values of <1.0 Å for the backbone heavy atoms. For both proteins the structural differences between multiple molecules in the asymmetric unit of the crystals correlated with structural variations within the bundles of conformers used to represent the NMR solution structures. A recently introduced JCSG NMR structure-determination protocol, which makes use of the software package UNIO for extensive automation, was further evaluated by comparison of the TM1112 structure obtained using these automated methods with another NMR structure that was independently solved in another PSI center, where a largely interactive approach was applied.
The NMR structures of the TM1112 and TM1367 proteins from Thermotoga maritima in solution at 298 K were determined following a new protocol which uses the software package UNIO for extensive automation. The results obtained with this novel procedure were evaluated by comparison with the crystal structures solved by the JCSG at 100 K to 1.83 and 1.90 Å resolution, respectively. In addition, the TM1112 solution structure was compared with an NMR structure solved by the NESG using a conventional largely interactive methodology. For both proteins, the newly determined NMR structure could be superimposed with the crystal structure with r.m.s.d. values of <1.0 Å for the backbone heavy atoms, which provided a starting platform to investigate local structure variations, which may arise from either the methods used or from the different chemical environments in solution and in the crystal. Thereby, these comparative studies were further explored with the use of reference NMR and crystal structures, which were computed using the NMR software with input of upper-limit distance constraints derived from the molecular models that represent the results of structure determination by NMR and by X-ray diffraction, respectively. The results thus obtained show that NMR structure calculations with the new automated UNIO software used by the JCSG compare favorably with those from a more labor-intensive and time-intensive interactive procedure. An intriguing observation is that the ‘bundles’ of two TM1112 or three TM1367 molecules in the asymmetric unit of the crystal structures mimic the behavior of the bundles of 20 conformers used to represent the NMR solution structures when comparing global r.m.s.d. values calculated either for the polypeptide backbone, the core residues with solvent accessibility below 15% or all heavy atoms.
PMCID: PMC2954229  PMID: 20944235
NMR and crystal structure comparison; structure-determination software; reference structures; Thermotoga maritima
20.  Effects of Restrained Sampling Space and Nonplanar Amino Groups on Free-Energy Predictions for RNA with Imino and Sheared Tandem GA Base Pairs Flanked by GC, CG, iGiC or iCiG Base Pairs 
Guanine-adenine (GA) base pairs play important roles in determining the structure, dynamics, and stability of RNA. In RNA internal loops, GA base pairs often occur in tandem arrangements and their structure is context and sequence dependent. Calculations reported here test the thermodynamic integration (TI) approach with the amber99 force field by comparing computational predictions of free energy differences with the free energy differences expected on the basis of NMR determined structures of the RNA motifs (5′-GCGGACGC-3′)2, (5′-GCiGGAiCGC-3′)2, (5′-GGCGAGCC-3′)2, and (5′-GGiCGAiGCC-3′)2. Here, iG and iC denote isoguanosine and isocytidine, which have amino and carbonyl groups transposed relative to guanosine and cytidine. The NMR structures show that the GA base pairs adopt either imino (cis Watson−Crick/Watson−Crick A-G) or sheared (trans Hoogsteen/Sugar edge A-G) conformations depending on the identity and orientation of the adjacent base pair. A new mixing function for the TI method is developed that allows alchemical transitions in which atoms can disappear in both the initial and final states. Unrestrained calculations gave ΔG° values 2−4 kcal/mol different from expectations based on NMR data. Restraining the structures with hydrogen bond restraints did not improve the predictions. Agreement with NMR data was improved by 0.7 to 1.5 kcal/mol, however, when structures were restrained with weak positional restraints to sample around the experimentally determined NMR structures. The amber99 force field was modified to partially include pyramidalization effects of the unpaired amino group of guanosine in imino GA base pairs. This provided little or no improvement in comparisons with experiment. The marginal improvement is observed when the structure has potential cross-strand out-of-plane hydrogen bonding with the G amino group. The calculations using positional restraints and a nonplanar amino group reproduce the signs of ΔG° from the experimental results and are, thus, capable of providing useful qualitative insights complementing the NMR experiments. Decomposition of the terms in the calculations reveals that the dominant terms are from electrostatic and interstrand interactions other than hydrogen bonds in the base pairs. The results suggest that a better description of the backbone is key to reproducing the experimental free energy results with computational free energy predictions.
PMCID: PMC2807739  PMID: 20090924
21.  High-resolution membrane protein structure by joint calculations with solid-state NMR and X-ray experimental data 
Journal of biomolecular NMR  2011;51(3):227-233.
X-ray diffraction and Nuclear Magnetic Resonance spectroscopy (NMR) are the staple methods for revealing atomic structures of proteins. Since crystals of biomolecular assemblies and membrane proteins often diffract weakly and such large systems encroach upon the molecular tumbling limit of solution NMR, new methods are essential to extend structures of such systems to high resolution. Here we present a method that incorporates solid-state NMR restraints alongside of X-ray reflections into the conventional model building and refinement steps of structure calculations. Using the 3.7 Å crystal structure of the integral membrane protein complex DsbB-DsbA as a test case yielded a significantly improved backbone precision of 0.92 Å in the transmembrane region, a 58% enhancement from using X-ray reflections alone. Furthermore, addition of solid-state NMR restraints greatly improved the overall quality of the structure by promoting 22% of DsbB transmembrane residues into the most favored regions of Ramachandran space in comparison to the crystal structure. This method is widely applicable to any protein system where X-ray data are available, and is particularly useful for the study of weakly diffracting crystals.
PMCID: PMC3204959  PMID: 21938394
membrane protein; solid-state NMR; X-ray reflections; high resolution; joint calculation
22.  Membrane Protein Structure and Dynamics from NMR Spectroscopy 
We review the current state of membrane protein structure determination using solid-state nuclear magnetic resonance (NMR) spectroscopy. Multidimensional magic-angle-spinning correlation NMR combined with oriented-sample experiments has made it possible to measure a full panel of structural constraints of membrane proteins directly in lipid bilayers. These constraints include torsion angles, interatomic distances, oligomeric structure, protein dynamics, ligand structure and dynamics, and protein orientation and depth of insertion in the lipid bilayer. Using solid-state NMR, researchers have studied potassium channels, proton channels, Ca2+ pumps, G protein–coupled receptors, bacterial outer membrane proteins, and viral fusion proteins to elucidate their mechanisms of action. Many of these membrane proteins have also been investigated in detergent micelles using solution NMR. Comparison of the solid-state and solution NMR structures provides important insights into the effects of the solubilizing environment on membrane protein structure and dynamics.
PMCID: PMC4082981  PMID: 22136620
magic-angle spinning; multidimensional correlation; solid-state nuclear magnetic resonance; ion channels; GPCR
23.  Kissing G Domains of MnmE Monitored by X-Ray Crystallography and Pulse Electron Paramagnetic Resonance Spectroscopy 
PLoS Biology  2009;7(10):e1000212.
The authors of this research article demonstrate the nature of the conformational changes MnmE was previously suggested to undergo during its GTPase cycle, and show the nucleotide-dependent dynamic movements of the G domains around two swivel positions relative to the rest of the protein. These movements are of crucial importance for understanding the mechanistic principles of this GAD.
MnmE, which is involved in the modification of the wobble position of certain tRNAs, belongs to the expanding class of G proteins activated by nucleotide-dependent dimerization (GADs). Previous models suggested the protein to be a multidomain protein whose G domains contact each other in a nucleotide dependent manner. Here we employ a combined approach of X-ray crystallography and pulse electron paramagnetic resonance (EPR) spectroscopy to show that large domain movements are coupled to the G protein cycle of MnmE. The X-ray structures show MnmE to be a constitutive homodimer where the highly mobile G domains face each other in various orientations but are not in close contact as suggested by the GDP-AlFx structure of the isolated domains. Distance measurements by pulse double electron-electron resonance (DEER) spectroscopy show that the G domains adopt an open conformation in the nucleotide free/GDP-bound and an open/closed two-state equilibrium in the GTP-bound state, with maximal distance variations of 18 Å. With GDP and AlFx, which mimic the transition state of the phosphoryl transfer reaction, only the closed conformation is observed. Dimerization of the active sites with GDP-AlFx requires the presence of specific monovalent cations, thus reflecting the requirements for the GTPase reaction of MnmE. Our results directly demonstrate the nature of the conformational changes MnmE was previously suggested to undergo during its GTPase cycle. They show the nucleotide-dependent dynamic movements of the G domains around two swivel positions relative to the rest of the protein, and they are of crucial importance for understanding the mechanistic principles of this GAD.
Author Summary
MnmE is an evolutionary conserved G protein that is involved in modification of the wobble U position of certain tRNAs to suppress translational wobbling. Despite high homology between its G domain and the small G protein Ras, MnmE displays entirely different regulatory properties to that of many molecular switch-type G proteins of the Ras superfamily, as its GTPase is activated by nucleotide-dependent homodimerization across the nucleotide-binding site. Here we explore the unusual G domain cycle of the MnmE protein by combining X-ray crystallography with pulse electron paramagnetic resonance (EPR) spectroscopy, which enables distance determinations between spin markers introduced at specific sites within the G domain. We determined the structures of the full-length MnmE dimer in the diphosphate and triphosphate states, which represent distinct steps of the G domain cycle, and demonstrate that the G domain cycle of MnmE comprises large conformational changes and domain movements of up to 18 Å, in which the G domains of the dimeric protein traverse from a GDP-bound open state through an open/closed equilibrium in the triphosphate state to a closed conformation in the transition state, so as to assemble the catalytic machinery.
PMCID: PMC2749940  PMID: 19806182
24.  A Comparative NMR Study of the Polypeptide Backbone Dynamics of Hemoglobin in the Deoxy and Carbonmonoxy Forms† 
Biochemistry  2007;46(23):6795-6803.
Model-free based NMR dynamics studies have been undertaken for polypeptide backbone amide N-H bond vectors for both the deoxy- and carbonmonoxy forms of chain-specific, isotopically (15N and 2H) labeled tetrameric hemoglobin (Hb), using 15N-relaxation parameters [longitudinal relaxation rate (R1), transverse relaxation rate (R2), and heteronuclear nuclear Overhauser effect (NOE)] measured at two temperatures (29 and 34 °C) and two magnetic field strengths (11.7 and 14.1 Tesla). In both deoxy- and carbonmonoxy forms of human normal adult hemoglobin (Hb A), the amide N-H bonds of most amino acid residues are rigid on the fast time scale (ns-ps), except for the loop regions and certain helix-helix connections. Although rigid in deoxy-Hb A, β146His has been found to be free from restriction of its backbone motions in the CO form, presumably due to the rupture of its hydrogen-bond/salt-bridge network. We now have direct dynamics evidence for this structural transition of Hb in solution. While remarkably flexible in the deoxy state, α31Arg and β123Thr, neighbors in the intra-dimer (α1β1) interface, exhibit stiffening upon CO binding. These findings imply a role for α31Arg and β123Thr in the intra-dimer communication, but contradict the results from X-ray crystallography. We have also found that there is considerable flexibility in the intra-dimer (α1β1) interface (i.e., B, G, and H helices, and the GH corner), and possible involvement of several amino acid residues (e.g., α31Arg, β3Leu, β41Phe, β123Thr, and β146His) in the allosteric pathway. Several amino acid residues at the intra-dimer interfaces, such as β109Val, appear to be involved in possible conformational exchange processes. The dynamic picture derived from the present study provides new insights into the traditional description of the stereochemical mechanism for the cooperative oxygenation of Hb A based on X-ray crystallographic results.
PMCID: PMC2533159  PMID: 17497935
25.  Free-energy Landscapes of Ion-channel Gating Are Malleable: changes in the number of bound ligands are accompanied by changes in the location of the transition state in acetylcholine-receptor channels† 
Biochemistry  2003;42(50):14977-14987.
Acetylcholine-receptor channels (AChRs) are allosteric membrane proteins that mediate synaptic transmission by alternatively opening and closing (‘gating’) a cation-selective transmembrane pore. Although ligand binding is not required for the channel to open, the binding of agonists (for example, acetylcholine) increases the closed ⇌ open equilibrium constant because the ion-impermeable → ion-permeable transition of the ion pathway is accompanied by a low → high affinity change at the agonist-binding sites. The fact that the gating conformational change of muscle AChRs can be kinetically modeled as a two-state reaction has paved the way to the experimental characterization of the corresponding transition state, which represents a snapshot of the continuous sequence of molecular events separating the closed and open states. Previous studies of fully (di-) liganded AChRs, combining single-channel kinetic measurements, site-directed mutagenesis, and data analysis in the framework of the linear free-energy relationships of physical organic chemistry, have suggested a transition-state structure that is consistent with channel opening being an asynchronous conformational change that starts at the extracellular agonist-binding sites and propagates towards the intracellular end of the pore. In this paper, I characterize the gating transition state of unliganded AChRs, and report a remarkable difference: unlike that of diliganded gating, the unliganded transition state is not a hybrid of the closed- and open-state structures but, rather, is almost indistinguishable from the open state itself. This displacement of the transition state along the reaction coordinate obscures the mechanism underlying the unliganded closed ⇌ open reaction but brings to light the malleable nature of free-energy landscapes of ion-channel gating.
The muscle acetylcholine receptor channel (AChR)1 is the neurotransmitter-gated ion channel that mediates neuromuscular synaptic transmission in vertebrates (1). Although the structure of this large pentameric transmembrane protein (∼470 residues per subunit) is not known with atomic resolution, a wealth of structural information exists, mainly from mutational studies, affinity labeling, chemical modification of specific residues, electron microscopy, and crystallography (reviewed in ref. 2). As is the case of any other allosteric protein, the dynamic behavior of this receptor-channel can be understood in the framework of thermodynamic cycles, with conformational changes and ligand-binding events as the elementary steps (3-5). Thus, the AChR can adopt a variety of different conformations that can interconvert (closed, open, and desensitized ‘states’), and each conformation has a distinct ligand-binding affinity (low affinity in the closed state and high affinity in the open and desensitized states) and a particular ‘catalytic efficiency’ (ion-impermeable in the closed and desensitized states, and ion-permeable in the open state). To meet the physiological requirement of a small closed ⇌ open (‘gating’) equilibrium constant for the unliganded receptor, and a large gating equilibrium constant for the ACh-diliganded receptor, the affinity of the AChR for ACh must be higher in the open than in the closed conformation (4-6). This follows from the notion that the equilibrium constants governing the different reaction steps (ligand binding and gating) of these cyclic reaction schemes are constrained by the principle of detailed balance.
Hence, irrespective of whether the receptor is diliganded, monoliganded or unliganded, two changes must take place in going from the closed state (low ligand affinity and ion-impermeable) to the open state (high ligand affinity and ion-permeable): a) the pore becomes permeable to ions, and b) the transmitter-binding sites, some 50 Å away from the pore domain (7), increase their affinity for the ligand (with the reverse changes taking place during closing). The apparent lack of stable intermediates between the closed and open conformations, inferred from kinetic modeling of the diliganded-gating reaction (8), suggests that these two changes occur as a result of a one-step, global conformational change. The question, then, arises as to whether this concerted conformational change proceeds synchronously (i.e., every residue of the protein moves ‘in unison’) or asynchronously (i.e., following a sequence of events; ref. 9) and, if the latter were the case, whether multiple, few, or just one sequence of events is actually traversed by the channel to ‘connect’ the end states.
Analysis of the correlation between rate and equilibrium constants of gating in diliganded AChRs has allowed us to address some of these issues by probing the structure of the transition state (8, 10-12), that is, the intermediate species between the end states of a one-step reaction that can be most easily studied. Interpretation of these results in the framework of the classical rate-equilibrium free-energy relationships of physical organic chemistry (13, 14), revealed that AChR diliganded gating is a highly asynchronous reaction, and suggested that the transition-state ensemble is quite homogeneous, as if the crossing of the energy barrier were confined to a narrow pass at the top of the energy landscape. In the opening direction, the conformational rearrangement that leads to the low-to-high affinity change at the extracellular binding sites precedes the conformational rearrangement of the pore that renders the channel ion-permeable. This propagated global conformational change, which we have referred to as a ‘conformational wave’ (11), must reverse during channel closing so that closing starts at the pore and propagates all the way to the binding sites.
It is not at all obvious why the diliganded-gating conformational change starts at the binding sites when the channel opens, nor even why the conformational change propagates at all through the receptor, instead of taking place synchronously throughout the protein. Is there any correlation between the location of the domain that binds agonist and the location of the initiation site for the opening conformational change? Could the latter have started from the intracellular end of the pore, for example, and have propagated to the (extracellular) transmitter-binding sites? What difference does it make to be liganded or unliganded as far as the mechanism of the gating conformational change is concerned? To address these issues, I set out to explore the mechanism of gating in unliganded AChRs by probing the structure of the corresponding transition state using kinetic measurements, site-directed mutagenesis, and the concepts of rate-equilibrium free-energy relationships and Φ-value analysis.
Briefly, a Φ-value can be assigned to any position in the protein by estimating the slope of a ‘Brönsted plot’2 [log (gating rate constant) versus log (gating equilibrium constant)] where each point corresponds to a different amino-acid substitution at that given position. More coarsegrained Φ-values can also be obtained by using different agonists or different transmembrane potentials, for example, as a means of altering the rate and equilibrium constants of gating. Very often, rate-equilibrium plots are linear, and 0 < Φ < 1. A value of Φ = 0 suggests that the position in question (in the case of a mutation series) experiences a closed-state-like environment at the transition state whereas a value of Φ = 1 suggests an open-state-like environment. A fractional Φ-value suggests an environment that is intermediate between those experienced in the closed and open states (16).
Earlier results indicated that the Φ-values obtained by varying the transmembrane potential are different in diliganded and unliganded AChRs. These Φ-values, which are a measure of the closed-state-like versus open-state-like character of the channel’s voltage-sensing elements at the transition state, are 0.070 ± 0.060 in diliganded receptors (17), and 1.025 ± 0.053 in unliganded AChRs (11, 18). The present study reveals that residues at the transmitter-binding sites (Figure 1), the extracellular loop that links the second (M2) and third (M3) transmembrane segments (M2-M3 linker), and the upper and lower half of M2, which during diliganded gating have Φ-values of ∼1 (ref. 11), ∼0.7 (ref. 10), ∼0.35 (refs 8, 11, 12), and ∼0 (ref. 12), respectively, have also Φ-values very close to 1 during unliganded gating. This generalized shift in Φ-values suggests that the diliganded → unliganded perturbation deforms the energy landscape of gating in such a way that the ‘new’ transition state occurs very close to the open state, to such an extent that all tested positions experience an open-state-like environment at the transition state of unliganded gating. Thus, the transition state occurs so ‘late’ (i.e., so close to the open state) that its inferred structure does not provide any clues as to the intermediate stages of this reaction.
Hence, the mechanism of unliganded gating remains obscure. The change in the position of the transition state along a reaction coordinate, as a result of perturbations to the energy landscape, is a very well known phenomenon in organic chemistry (e.g., refs 20-26), and protein folding (e.g., refs 27-34). In this paper, I show that this phenomenon can also take place in the case of allosteric transitions and, therefore, that the structure of the transition state of a global conformational change need not be fixed; rather, it can change depending on the experimental conditions.
PMCID: PMC1463891  PMID: 14674774

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