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1.  Crystallographic analysis of human hemoglobin elucidates the structural basis of the potent and dual antisickling activity of pyridyl derivatives of vanillin. Corrigendum 
A correction to the paper by Abdulmalik et al. [(2011), Acta Cryst. D67, 920–928].
The affiliation of one of the authors of Abdulmalik et al. (2011) [Acta Cryst. D67, 920–928] is corrected.
doi:10.1107/S0907444911045860
PMCID: PMC3337008
hemoglobin; oxygen affinity; sickle-cell disease; polymerization; T state; R2 state; corrigendum
2.  Crystallographic analysis of human hemoglobin elucidates the structural basis of the potent and dual antisickling activity of pyridyl derivatives of vanillin 
Pyridyl derivatives of vanillin increase the fraction of the more soluble oxygenated sickle hemoglobin and/or directly increase the solubility of deoxygenated sickle hemoglobin. Crystallographic analysis reveals the structural basis of the potent and dual antisickling activity of these derivatives.
Vanillin has previously been studied clinically as an antisickling agent to treat sickle-cell disease. In vitro investigations with pyridyl derivatives of vanillin, including INN-312 and INN-298, showed as much as a 90-fold increase in antisickling activity compared with vanillin. The compounds preferentially bind to and modify sickle hemoglobin (Hb S) to increase the affinity of Hb for oxygen. INN-312 also led to a considerable increase in the solubility of deoxygenated Hb S under completely deoxygenated conditions. Crystallographic studies of normal human Hb with INN-312 and INN-298 showed that the compounds form Schiff-base adducts with the N-terminus of the α-subunits to constrain the liganded (or relaxed-state) Hb conformation relative to the unliganded (or tense-state) Hb conformation. Interestingly, while INN-298 binds and directs its meta-positioned pyridine-methoxy moiety (relative to the aldehyde moiety) further down the central water cavity of the protein, that of INN-312, which is ortho to the aldehyde, extends towards the surface of the protein. These studies suggest that these compounds may act to prevent sickling of SS cells by increasing the fraction of the soluble high-affinity Hb S and/or by stereospecific inhibition of deoxygenated Hb S polymerization.
doi:10.1107/S0907444911036353
PMCID: PMC3211971  PMID: 22101818
hemoglobin; oxygen affinity; sickle-cell disease; polymerization; T state; R2 state
3.  The use of trimethylamine N-oxide as a primary precipitating agent and related methylamine osmolytes as cryoprotective agents for macromolecular crystallography 
The stabilizing osmolyte trimethylamine N-oxide (TMAO) is shown to be an efficient primary precipitant for protein crystal growth. In addition to TMAO, two other methylamine osmolytes, sarcosine and betaine, are shown to be effective cryoprotective agents for protein crystal cooling.
Both crystallization and cryoprotection are often bottlenecks for high-resolution X-ray structure determination of macromolecules. Methylamine osmolytes are known stabilizers of protein structure. One such osmolyte, trimethylamine N-oxide (TMAO), has seen occasional use as an additive to improve macromolecular crystal quality and has recently been shown to be an effective cryoprotective agent for low-temperature data collection. Here, TMAO and the related osmolytes sarcosine and betaine are investigated as primary precipitating agents for protein crystal growth. Crystallization experiments were undertaken with 14 proteins. Using TMAO, seven proteins crystallized in a total of 13 crystal forms, including a new tetragonal crystal form of trypsin. The crystals diffracted well, and eight of the 13 crystal forms could be effectively cryo­­cooled as grown with TMAO as an in situ cryoprotective agent. Sarcosine and betaine produced crystals of four and two of the 14 proteins, respectively. In addition to TMAO, sarcosine and betaine were effective post-crystallization cryoprotective agents for two different crystal forms of thermolysin. Precipitation reactions of TMAO with several transition-metal ions (Fe3+, Co2+, Cu2+ and Zn2+) did not occur with sarcosine or betaine and were inhibited for TMAO at lower pH. Structures of proteins from TMAO-grown crystals and from crystals soaked in TMAO, sarcosine or betaine were determined, showing osmolyte binding in five of the 12 crystals tested. When an osmolyte was shown to bind, it did so near the protein surface, interacting with water molecules, side chains and backbone atoms, often at crystal contacts.
doi:10.1107/S0907444911050360
PMCID: PMC3245723  PMID: 22194335
crystallization; cryoprotection; osmolytes; trimethylamine N-oxide; sarcosine; betaine
4.  Deprotonated imidodiphosphate in AMPPNP-­containing protein structures 
In certain AMPPNP-containing protein structures, the nitrogen bridging the two terminal phosphate groups can be deprotonated.
Many different proteins utilize the chemical energy provided by the cofactor adenosine triphosphate (ATP) for their proper function. A number of structures in the Protein Data Bank (PDB) contain adenosine 5′-(β,γ-imido)triphosphate (AMPPNP), a nonhydrolysable analog of ATP in which the bridging O atom between the two terminal phosphate groups is substituted by the imido function. Under mild conditions imides do not have acidic properties and thus the imide nitrogen should be protonated. However, an analysis of protein structures containing AMPPNP reveals that the imide group is deprotonated in certain complexes if the negative charges of the phosphate moieties in AMPPNP are in part neutralized by coordinating divalent metals or a guanidinium group of an arginine.
doi:10.1107/S0907444911046105
PMCID: PMC3225179  PMID: 22120745
imidodiphosphate; adenosine 5′-(β,γ-methylene)triphosphate; AMPPNP
5.  Human Suv3 protein reveals unique features among SF2 helicases 
Crystal structures of the human mitochondrial helicase hSuv3 in complex with AMPPNP and with a short strand of RNA are presented.
Suv3 is a helicase that is involved in efficient turnover and surveillance of RNA in eukaryotes. In vitro studies show that human Suv3 (hSuv3) in complex with human polynucleotide phosphorylase has RNA degradosome activity. The enzyme is mainly localized in mitochondria, but small fractions are found in cell nuclei. Here, two X-ray crystallographic structures of human Suv3 in complex with AMPPNP, a nonhydrolysable analog of ATP, and with a short five-nucleotide strand of RNA are presented at resolutions of 2.08 and 2.9 Å, respectively. The structure of the enzyme is very similar in the two complexes and consists of four domains. Two RecA-like domains form the tandem typical of all helicases from the SF2 superfamily which together with the C-terminal all-helical domain makes a ring structure through which the nucleotide strand threads. The mostly helical N-terminal domain is positioned externally with respect to the core of the enzyme. Most of the typical helicase motifs are present in hSuv3, but the protein shows certain unique characteristics, suggesting that Suv3 enzymes may constitute a separate subfamily of helicases.
doi:10.1107/S0907444911040248
PMCID: PMC3211972  PMID: 22101826
mitochondrial helicases; human Suv3; SF2 helicases
6.  A novel mechanism of sulfur transfer catalyzed by O-acetylhomoserine sulfhydrylase in the methionine-biosynthetic pathway of Wolinella succinogenes  
MetY is the first reported structure of an O-acetylhomoserine sulfhydrylase that utilizes a protein thiocarboxylate intermediate as the sulfur source in a novel methionine-biosynthetic pathway instead of catalyzing a direct sulfhydrylation reaction.
O-Acetylhomoserine sulfhydrylase (OAHS) is a pyridoxal 5′-­phosphate (PLP) dependent sulfide-utilizing enzyme in the l-cysteine and l-methionine biosynthetic pathways of various enteric bacteria and fungi. OAHS catalyzes the conversion of O-acetylhomoserine to homocysteine using sulfide in a process known as direct sulfhydrylation. However, the source of the sulfur has not been identified and no structures of OAHS have been reported in the literature. Here, the crystal structure of Wolinella succinogenes OAHS (MetY) determined at 2.2 Å resolution is reported. MetY crystallized in space group C2 with two monomers in the asymmetric unit. Size-exclusion chromatography, dynamic light scattering and crystal packing indicate that the biological unit is a tetramer in solution. This is further supported by the crystal structure, in which a tetramer is formed using a combination of non­­crystallographic and crystallographic twofold axes. A search for structurally homologous proteins revealed that MetY has the same fold as cystathionine γ-lyase and methionine γ-lyase. The active sites of these enzymes, which are also PLP-dependent, share a high degree of structural similarity, suggesting that MetY belongs to the γ-elimination subclass of the Cys/Met metabolism PLP-dependent family of enzymes. The structure of MetY, together with biochemical data, provides insight into the mechanism of sulfur transfer to a small molecule via a protein thiocarboxylate intermediate.
doi:10.1107/S0907444911028010
PMCID: PMC3176619  PMID: 21931214
Wolinella succinogenes; O-acetylhomoserine sulfhydrylases; pyridoxal 5′-phosphate; γ-elimination; direct sulfhydrylation; Cys/Met metabolism; O-acetylhomoserine; protein thiocarboxylate
7.  Two-photon excited UV fluorescence for protein crystal detection 
Complementary measurements using SONICC and TPE-UVF allow the sensitive and selective detection of protein crystals.
Two-photon excited ultraviolet fluorescence (TPE-UVF) microscopy is explored for sensitive protein-crystal detection as a complement to second-order nonlinear optical imaging of chiral crystals (SONICC). Like conventional ultraviolet fluorescence (UVF), TPE-UVF generates image contrast based on the intrinsic fluorescence of aromatic residues, generally producing higher fluorescence emission within crystals than the mother liquor by nature of the higher local protein concentration. However, TPE-UVF has several advantages over conventional UVF, including (i) insensitivity to optical scattering, allowing imaging in turbid matrices, (ii) direct compatibility with conventional optical plates and windows by using visible light for excitation, (iii) elimination of potentially damaging out-of-plane UV excitation, (iv) improved signal to noise through background reduction from out-of-plane excitation and (v) relatively simple integration into instrumentation developed for SONICC.
doi:10.1107/S0907444911028253
PMCID: PMC3176620  PMID: 21931215
protein crystal detection; microscopy; second-harmonic generation; two-photon excited fluorescence; UV fluorescence
8.  Structural studies of human glioma pathogenesis-related protein 1 
Structural analysis of a truncated soluble domain of human glioma pathogenesis-related protein 1, a membrane protein implicated in the proliferation of aggressive brain cancer, is presented.
Human glioma pathogenesis-related protein 1 (GLIPR1) is a membrane protein that is highly upregulated in brain cancers but is barely detectable in normal brain tissue. GLIPR1 is composed of a signal peptide that directs its secretion, a conserved cysteine-rich CAP (cysteine-rich secretory proteins, antigen 5 and pathogenesis-related 1 proteins) domain and a transmembrane domain. GLIPR1 is currently being investigated as a candidate for prostate cancer gene therapy and for glioblastoma targeted therapy. Crystal structures of a truncated soluble domain of the human GLIPR1 protein (sGLIPR1) solved by molecular replacement using a truncated polyalanine search model of the CAP domain of stecrisp, a snake-venom cysteine-rich secretory protein (CRISP), are presented. The correct molecular-replacement solution could only be obtained by removing all loops from the search model. The native structure was refined to 1.85 Å resolution and that of a Zn2+ complex was refined to 2.2 Å resolution. The latter structure revealed that the putative binding cavity coordinates Zn2+ similarly to snake-venom CRISPs, which are involved in Zn2+-dependent mechanisms of inflammatory modulation. Both sGLIPR1 structures have extensive flexible loop/turn regions and unique charge distributions that were not observed in any of the previously reported CAP protein structures. A model is also proposed for the structure of full-length membrane-bound GLIPR1.
doi:10.1107/S0907444911028198
PMCID: PMC3176621  PMID: 21931216
glioblastoma; CAP; venom allergens; testis-specific protein; venom antigen 5; membrane protein; Ancylosoma secreted protein; sperm coating protein
9.  Structural analysis of human dihydrofolate reductase as a binary complex with the potent and selective inhibitor 2,4-diamino-6-{2′-O-(3-carboxypropyl)oxydibenz[b,f]-azepin-5-yl}methylpteridine reveals an unusual binding mode 
These data for the binary complex of PT684 with hDHFR reveal that the 3-carboxypropyl side chain occupies two alternate positions, neither of which interacts with the conserved Arg in the active site. These data also do not confirm the computational model for PT684 binding to hDHFR which was based on the structure of PT653 in Pneumocystis carinii DHFR.
In order to understand the structure–activity profile observed for a series of substituted dibenz[b,f]azepine antifolates, the crystal structure of the binary complex of human dihydro­folate reductase (hDHFR) with the potent and selective inhibitor 2,4-diamino-6-­{2′-O-(3-carboxypropyl)oxydibenz[b,f]-azepin-5-yl}methylpteridine (PT684) was determined to 1.8 Å resolution. These data revealed that the carboxylate side chain of PT684 occupies two alternate positions, neither of which interacts with the conserved Arg70 in the active-site pocket, which in turn hydrogen bonds to water. These observations are in contrast to those reported for the ternary complex of mouse DHFR (mDHFR) with NADPH [Cody et al. (2008 ▶), Acta Cryst. D64, 977–984], in which the 3-carboxypropyl side chain of PT684 was hydrolyzed to its hydroxyl derivative, PT684a. The crystallization conditions differed for the human and mouse DHFR crystals (100 mM K2HPO4 pH 6.9, 30% ammonium sulfate for hDHFR; 15 mM Tris pH 8.3, 75 mM sodium cacodylate, PEG 4K for mDHFR). Additionally, the side chains of Phe31 and Gln35 in the hDHFR complex have a single conformation, whereas in the mDHFR complex they occupied two alternative conformations. These data show that the hDHFR complex has a decreased active-site volume compared with the mDHFR complex, as reflected in a relative shift of helix C (residues 59–64) of 1.2 Å, and a shift of 1.5 Å compared with the ternary complex of Pneumocystis carinii DHFR (pcDHFR) with the parent dibenz[b,f]azepine PT653. These data suggest that the greater inhibitory potency of PT684 against pcDHFR is consistent with the larger active-site volume of pcDHFR and the predicted interactions of the carboxylate side chain with Arg75.
doi:10.1107/S0907444911030071
PMCID: PMC3176622  PMID: 21931219
dihydrofolate reductase; dibenz[b,f]azepine antifolates; inhibitors
10.  Can radiation damage to protein crystals be reduced using small-molecule compounds? 
Free-radical scavengers that are known to be effective protectors of proteins in solution are found to increase global radiation damage to protein crystals. Protective mechanisms may become deleterious in the protein-dense environment of a crystal.
Recent studies have defined a data-collection protocol and a metric that provide a robust measure of global radiation damage to protein crystals. Using this protocol and metric, 19 small-molecule compounds (introduced either by cocrystalliz­ation or soaking) were evaluated for their ability to protect lysozyme crystals from radiation damage. The compounds were selected based upon their ability to interact with radiolytic products (e.g. hydrated electrons, hydrogen, hydroxyl and perhydroxyl radicals) and/or their efficacy in protecting biological molecules from radiation damage in dilute aqueous solutions. At room temperature, 12 compounds had no effect and six had a sensitizing effect on global damage. Only one compound, sodium nitrate, appeared to extend crystal lifetimes, but not in all proteins and only by a factor of two or less. No compound provided protection at T = 100 K. Scavengers are ineffective in protecting protein crystals from global damage because a large fraction of primary X-ray-induced excitations are generated in and/or directly attack the protein and because the ratio of scavenger molecules to protein molecules is too small to provide appreciable competitive protection. The same reactivity that makes some scavengers effective radioprotectors in protein solutions may explain their sensitizing effect in the protein-dense environment of a crystal. A more productive focus for future efforts may be to identify and eliminate sensitizing compounds from crystallization solutions.
doi:10.1107/S0907444911032835
PMCID: PMC3176623  PMID: 21931220
radiation damage; scavengers; radioprotection; global damage; site-specific damage
11.  Structure of N 5-carboxyaminoimidazole ribonucleotide synthase (PurK) from Bacillus anthracis  
The crystal structure of N 5-carboxyaminoimidazole ribonucleotide synthase from Bacillus anthracis with only an Mg cation provides some insight into the catalytic mechanism of this enzyme and the role of a crucial loop during catalysis.
The apo structure of N 5-carboxyaminoimidazole ribonucleotide synthase (PurK) from Bacillus anthracis (baPurK) with Mg2+ in the active site is reported at 1.96 Å resolution. PurK is an enzyme in the purine-biosynthetic pathway, unique to prokaryotes, that converts 5-aminoimidazole ribonucleotide to N 5-carboxyaminoimidazole ribonucleotide and has been suggested as a potential antimicrobial drug target. Two interesting features of baPurK are a flexible B-loop (residues 149/150–157) that is in close contact with the active site and the binding of Mg2+ to the active site without additional ligands.
doi:10.1107/S0907444911029210
PMCID: PMC3270386  PMID: 21931218
N5-carboxy­aminoimidazole ribonucleotide synthase; PurK; Bacillus anthracis; purine biosynthesis
12.  Structure of trifunctional THI20 from yeast 
This study presents the crystal structure of the trifunctional THI20 from S. cerevisiae, an enzyme with an N-terminal HMP kinase (ThiD-like) domain and a C-terminal thiaminase II (TenA-like) domain. In addition, two distinct structural classes of TenA have been identified by comparison to other known structures.
In a recently characterized thiamin-salvage pathway, thiamin-degradation products are hydrolyzed by thiaminase II, yielding 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP). This compound is an intermediate in thiamin biosynthesis that, once phosphorylated by an HMP kinase, can be used to synthesize thiamin monophosphate. Here, the crystal structure of Saccharomyces cerevisiae THI20, a trifunctional enzyme containing an N-terminal HMP kinase/HMP-P kinase (ThiD-like) domain and a C-terminal thia­min­ase II (TenA-like) domain, is presented. Comparison to structures of the monofunctional enzymes reveals that while the ThiD-like dimer observed in THI20 resembles other ThiD structures, the TenA-like domain, which is tetrameric in all previously reported structures, forms a dimer. Similarly, the active site of the ThiD-like domain of THI20 is highly similar to other known ThiD enzymes, while the TenA-like active site shows unique features compared with previously structurally characterized TenAs. In addition, a survey of known TenA structures revealed two structural classes, both of which have distinct conserved features. The TenA domain of THI20 possesses some features of both classes, consistent with its ability to hydrolyze both thiamin and the thiamin-degradation product 2-methyl-4-amino-5-aminomethylpyrimidine.
doi:10.1107/S0907444911024814
PMCID: PMC3169313  PMID: 21904031
thiamin salvage; thiamin metabolism; TenA; ThiD; thiaminases; HMP kinases
13.  Dark progression reveals slow timescales for radiation damage between T = 180 and 240 K 
Between T = 180 and 240 K, radiation damage progresses on minute timescales when the X-rays are off, suggesting that a fraction of damage at higher temperatures may be outrun using currently available sources and detectors.
Can radiation damage to protein crystals be ‘outrun’ by collecting a structural data set before damage is manifested? Recent experiments using ultra-intense pulses from a free-electron laser show that the answer is yes. Here, evidence is presented that significant reductions in global damage at temperatures above 200 K may be possible using conventional X-ray sources and current or soon-to-be available detectors. Specifically, ‘dark progression’ (an increase in damage with time after the X-rays have been turned off) was observed at temperatures between 180 and 240 K and on timescales from 200 to 1200 s. This allowed estimation of the temperature-dependent timescale for damage. The rate of dark progression is consistent with an Arrhenius law with an activation energy of 14 kJ mol−1. This is comparable to the activation energy for the solvent-coupled diffusive damage processes responsible for the rapid increase in radiation sensitivity as crystals are warmed above the glass transition near 200 K. Analysis suggests that at T = 300 K data-collection times of the order of 1 s (and longer at lower temperatures) may allow significant reductions in global radiation damage, facilitating structure solution on crystals with liquid solvent. No dark progression was observed below T = 180 K, indicating that no important damage process is slowed through this timescale window in this temperature range.
doi:10.1107/S0907444911027600
PMCID: PMC3169314  PMID: 21904032
radiation damage; temperature dependence; glass transition; dark progression; dose rate
14.  Streptavidin and its biotin complex at atomic resolution 
Analysis of atomic resolution crystal structures of wild-type streptavidin (1.03 Å) and its biotin complex (0.95 Å) indicate the range of conformational states taken on by this protein in the solid state. Most of the structural variation is found in the polypeptide loops between the strands in this β-sandwich protein.
Atomic resolution crystallographic studies of streptavidin and its biotin complex have been carried out at 1.03 and 0.95 Å, respectively. The wild-type protein crystallized with a tetramer in the asymmetric unit, while the crystals of the biotin complex contained two subunits in the asymmetric unit. Comparison of the six subunits shows the various ways in which the protein accommodates ligand binding and different crystal-packing environments. Conformational variation is found in each of the polypeptide loops connecting the eight strands in the β-­sandwich subunit, but the largest differences are found in the flexible binding loop (residues 45–52). In three of the unliganded subunits the loop is in an ‘open’ conformation, while in the two subunits binding biotin, as well as in one of the unliganded subunits, this loop ‘closes’ over the biotin–binding site. The ‘closed’ loop contributes to the protein’s high affinity for biotin. Analysis of the anisotropic displacement parameters included in the crystallographic models is consistent with the variation found in the loop structures and the view that the dynamic nature of the protein structure contributes to the ability of the protein to bind biotin so tightly.
doi:10.1107/S0907444911027806
PMCID: PMC3169315  PMID: 21904034
streptavidin; biotin
15.  Structure of a tropomyosin N-terminal fragment at 0.98 Å resolution 
The crystal structure of the N-terminal fragment of the short nonmuscle α-tropomyosin has been determined at a resolution of 0.98 Å.
Tropomyosin (TM) is an elongated two-chain protein that binds along actin filaments. Important binding sites are localized in the N-terminus of tropo­myosin. The structure of the N-terminus of the long muscle α-TM has been solved by both NMR and X-ray crystallography. Only the NMR structure of the N-­terminus of the short nonmuscle α-TM is available. Here, the crystal structure of the N-terminus of the short nonmuscle α-TM (αTm1bZip) at a resolution of 0.98 Å is reported, which was solved from crystals belonging to space group P31 with unit-cell parameters a = b = 33.00, c = 52.03 Å, α = β = 90, γ = 120°. The first five N-­terminal residues are flexible and residues 6–35 form an α-helical coiled coil. The overall fold and the secondary structure of the crystal structure of αTM1bZip are highly similar to the NMR structure and the atomic coordinates of the corresponding Cα atoms between the two structures superimpose with a root-mean-square deviation of 0.60 Å. The crystal structure validates the NMR structure, with the positions of the side chains being determined precisely in our structure.
doi:10.1107/S090744491102645X
PMCID: PMC3169316  PMID: 21904035
coiled coils; actin-binding proteins; tropomodulin-binding proteins; cytoskeletal proteins
16.  Structural and kinetic insights into the mechanism of 5-hydroxyisourate hydrolase from Klebsiella pneumoniae  
The crystal structure of 5-hydroxyisourate hydrolase from K. pneumoniae and the steady-state kinetic parameters of the native enzyme as well as several mutants provide insights into the catalytic mechanism of this enzyme and the possible roles of the active-site residues.
The stereospecific oxidative degradation of uric acid to (S)-­allantoin has recently been demonstrated to proceed via two unstable intermediates and requires three separate enzymatic reactions. The second step of this reaction, the conversion of 5-­hydroxyisourate (HIU) to 2-oxo-4-hydroxy-4-­carboxy-5-ureidoimidazoline, is catalyzed by HIU hydrolase (HIUH). The high-resolution crystal structure of HIUH from the opportunistic pathogen Klebsiella pneumoniae (KpHIUH) has been determined. KpHIUH is a homotetrameric protein that, based on sequence and structural similarity, belongs to the transthyretin-related protein family. In addition, the steady-state kinetic parameters for this enzyme and four active-site mutants have been measured. These data provide valuable insight into the functional roles of the active-site residues. Based upon the structural and kinetic data, a mechanism is proposed for the KpHIUH-catalyzed reaction.
doi:10.1107/S090744491101746X
PMCID: PMC3144850  PMID: 21795808
purine catabolism; ureides; uric acid degradation; Hpx genes; transthyretin-like proteins
17.  Structure of 2-oxo-3-deoxygalactonate kinase from Klebsiella pneumoniae  
The crystal structure of 2-oxo-3-deoxygalactonate kinase from the De Ley–Doudoroff pathway of galactose metabolism has been determined at 2.1 Å resolution.
In most organisms, efficient d-galactose utilization requires the highly conserved Leloir pathway that converts d-galactose to d-glucose 1-phosphate. However, in some bacterial and fungal species alternative routes of d-galactose assimilation have been identified. In the so-called De Ley–Doudoroff pathway, d-galactose is metabolized into pyruvate and d-­glyceraldehyde 3-phosphate in five consecutive reactions carried out by specific enzymes. The penultimate step in this pathway involves the phosphorylation of 2-oxo-3-deoxygalactonate to 2-oxo-3-deoxygalactonate 6-phosphate catalyzed by 2-­oxo-3-deoxygalactonate kinase, with ATP serving as a phosphoryl-group donor. Here, a crystal structure of 2-oxo-3-deoxygalactonate kinase from Klebsiella pneumoniae determined at 2.1 Å resolution is reported, the first structure of an enzyme from the De Ley–Doudoroff pathway. Structural comparison indicates that the enzyme belongs to the ASKHA (acetate and sugar kinases/hsc70/actin) family of phosphotransferases. The protein is composed of two α/β domains, each of which contains a core common to all family members. Additional elements introduced between conserved structural motifs define the unique features of 2-oxo-3-deoxygalactonate kinase and possibly determine the biological function of the protein.
doi:10.1107/S0907444911021834
PMCID: PMC3144851  PMID: 21795809
2-oxo-3-deoxygalactonate kinase; galactose; De Ley–Doudoroff pathway; ASKHA family
18.  A conformation-dependent stereochemical library improves crystallographic refinement even at atomic resolution 
A script was created to allow SHELXL to use the new CDL v.1.2 stereochemical library which defines the target values for main-chain bond lengths and angles as a function of the residue’s ϕ/ψ angles. Test refinements using this script show that the refinement behavior of structures at resolutions even better than 1 Å is substantially enhanced by the use of the new conformation-dependent ideal geometry paradigm.
To utilize a new conformation-dependent backbone-geometry library (CDL) in protein refinements at atomic resolution, a script was written that creates a restraint file for the SHELXL refinement program. It was found that the use of this library allows models to be created that have a substantially better fit to main-chain bond angles and lengths without degrading their fit to the X-ray data even at resolutions near 1 Å. For models at much higher resolution (∼0.7 Å), the refined model for parts adopting single well occupied positions is largely independent of the restraints used, but these structures still showed much smaller r.m.s.d. residuals when assessed with the CDL. Examination of the refinement tests across a wide resolution range from 2.4 to 0.65 Å revealed consistent behavior supporting the use of the CDL as a next-generation restraint library to improve refinement. CDL restraints can be generated using the service at http://pgd.science.oregonstate.edu/cdl_shelxl/.
doi:10.1107/S090744491102292X
PMCID: PMC3144852  PMID: 21795811
stereochemical libraries; refinement; conformation-dependent library
19.  Structural and biochemical characterization of N 5-­carboxyaminoimidazole ribonucleotide synthetase and N 5-carboxyaminoimidazole ribonucleotide mutase from Staphylococcus aureus  
Purine biosynthesis is considered to be a promising new antibiotic target; in this article, the structural and biochemical characterization of S. aureus PurK and PurE are described.
With the rapid rise of methicillin-resistant Staphylococcus aureus infections, new strategies against S. aureus are urgently needed. De novo purine biosynthesis is a promising yet unexploited target, insofar as abundant evidence has shown that bacteria with compromised purine biosynthesis are attenuated. Fundamental differences exist within the process by which humans and bacteria convert 5-aminoimidazole ribonucleotide (AIR) to 4-carboxy-5-aminoimidazole ribo­nucleotide (CAIR). In bacteria, this transformation occurs through a two-step conversion catalyzed by PurK and PurE; in humans, it is mediated by a one-step conversion catalyzed by class II PurE. Thus, these bacterial enzymes are potential targets for selective antibiotic development. Here, the first comprehensive structural and biochemical characterization of PurK and PurE from S. aureus is presented. Structural analysis of S. aureus PurK reveals a nonconserved phenylalanine near the AIR-binding site that occupies the putative position of the imidazole ring of AIR. Mutation of this phenylalanine to isoleucine or tryptophan reduced the enzyme efficiency by around tenfold. The K m for bicarbonate was determined for the first time for a PurK enzyme and was found to be ∼18.8 mM. The structure of PurE is described in comparison to that of human class II PurE. It is confirmed biochemically that His38 is essential for function. These studies aim to provide foundations for future structure-based drug-discovery efforts against S. aureus purine biosynthesis.
doi:10.1107/S0907444911023821
PMCID: PMC3144853  PMID: 21795812
purine biosynthesis; Staphylococcus aureus; enzyme kinetics; structure-based drug discovery; antimicrobials
20.  Processing incommensurately modulated protein diffraction data with Eval15. Corrigendum 
A correction to the article by Porta et al. [(2011). Acta Cryst. D67, 628–638].
A correction is made to a figure in the article by Porta et al. [(2011). Acta Cryst. D67, 628–638].
doi:10.1107/S0907444911026631
PMCID: PMC3270385
modulation; incommensurate; Eval15; profilin–actin; corrigendum
21.  Processing incommensurately modulated protein diffraction data with Eval15 
Data processing of an incommensurately modulated profilin–actin crystal is described.
Recent challenges in biological X-ray crystallography include the processing of modulated diffraction data. A modulated crystal has lost its three-dimensional translational symmetry but retains long-range order that can be restored by refining a periodic modulation function. The presence of a crystal modulation is indicated by an X-ray diffraction pattern with periodic main reflections flanked by off-lattice satellite reflections. While the periodic main reflections can easily be indexed using three reciprocal-lattice vectors a*, b*, c*, the satellite reflections have a non-integral relationship to the main lattice and require a q vector for indexing. While methods for the processing of diffraction intensities from modulated small-molecule crystals are well developed, they have not been applied in protein crystallography. A recipe is presented here for processing incommensurately modulated data from a macromolecular crystal using the Eval program suite. The diffraction data are from an incommensurately modulated crystal of profilin–actin with single-order satellites parallel to b*. The steps taken in this report can be used as a guide for protein crystallographers when encountering crystal modulations. To our knowledge, this is the first report of the processing of data from an incommensurately modulated macromolecular crystal.
doi:10.1107/S0907444911017884
PMCID: PMC3121298  PMID: 21697601
modulation; incommensurate; Eval15; profilin–actin
22.  Isothiazolidinone (IZD) as a phosphoryl mimetic in inhibitors of the Yersinia pestis protein tyrosine phosphatase YopH 
The first X-ray crystal structure of the Y. pestis protein tyrosine phosphatase YopH in complex with an isothiazolidinone-based lead-fragment compound is reported.
Isothiazolidinone (IZD) heterocycles can act as effective components of protein tyrosine phosphatase (PTP) inhibitors by simultaneously replicating the binding interactions of both a phosphoryl group and a highly conserved water molecule, as exemplified by the structures of several PTP1B–inhibitor complexes. In the first unambiguous demonstration of IZD interactions with a PTP other than PTP1B, it is shown by X-ray crystallography that the IZD motif binds within the catalytic site of the Yersinia pestis PTP YopH by similarly displacing a highly conserved water molecule. It is also shown that IZD-based bidentate ligands can inhibit YopH in a nonpromiscuous fashion at low micromolar concentrations. Hence, the IZD moiety may represent a useful starting point for the development of YopH inhibitors.
doi:10.1107/S0907444911018610
PMCID: PMC3121299  PMID: 21697602
YopH; isothiazolidinone; Yersinia pestis; protein tyrosine phosphatases
23.  Development of high-performance X-ray transparent crystallization plates for in situ protein crystal screening and analysis 
An optically, UV and X-ray transparent crystallization plate suitable for in situ analysis has been developed. The plate uses contact line pinning rather than wells to confine the liquids.
X-ray transparent crystallization plates based upon a novel drop-pinning technology provide a flexible, simple and inexpensive approach to protein crystallization and screening. The plates consist of open cells sealed top and bottom by thin optically, UV and X-ray transparent films. The plates do not need wells or depressions to contain liquids. Instead, protein drops and reservoir solution are held in place by rings with micrometre dimensions that are patterned onto the bottom film. These rings strongly pin the liquid contact lines, thereby improving drop shape and position uniformity, and thus crystallization reproducibility, and simplifying automated image analysis of drop contents. The same rings effectively pin solutions containing salts, proteins, cryoprotectants, oils, alcohols and detergents. Strong pinning by rings allows the plates to be rotated without liquid mixing to 90° for X-ray data collection or to be inverted for hanging-drop crystallization. The plates have the standard SBS format and are compatible with standard liquid-handling robots.
doi:10.1107/S090744491101883X
PMCID: PMC3121300  PMID: 21697603
protein crystallization; crystallization plates; drop pinning; high-throughput screening
24.  Mycobacterium tuberculosis acyl carrier protein synthase adopts two different pH-dependent structural conformations 
Bacterial acyl carrier protein synthase plays an essential role in the synthesis of fatty acids, nonribosomal peptides and polyketides. In Mycobacterium tuberculosis, AcpS or group I phosphopentatheine transferase exhibits two different structural conformations depending upon the pH.
The crystal structures of acyl carrier protein synthase (AcpS) from Mycobacterium tuberculosis (Mtb) and Corynebacterium ammoniagenes determined at pH 5.3 and pH 6.5, respectively, are reported. Comparison of the Mtb apo-AcpS structure with the recently reported structure of the Mtb AcpS–ADP complex revealed that AcpS adopts two different conformations: the orthorhombic and trigonal space-group structures show structural differences in the α2 helix and in the conformation of the α3–α4 connecting loop, which is in a closed conformation. The apo-AcpS structure shows electron density for the entire model and was obtained at lower pH values (4.4–6.0). In contrast, at a higher pH value (6.5) AcpS undergoes significant conformational changes, resulting in disordered regions that show no electron density in the AcpS model. The solved structures also reveal that C. ammoniagenes AcpS undergoes structural rearrangement in two regions, similar to the recently reported Mtb AcpS–ADP complex structure. In vitro reconstitution experiments show that AcpS has a higher post-translational modification activity between pH 4.4 and 6.0 than at pH values above 6.5, where the activity drops owing to the change in conformation. The results show that apo-AcpS and AcpS–ADP adopt different conformations depending upon the pH conditions of the crystallization solution.
doi:10.1107/S0907444911020221
PMCID: PMC3270384  PMID: 21697604
acyl carrier protein synthases; Mycobacterium tuberculosis; Corynebacterium ammoniagenes; acyl carrier proteins; fatty-acid synthases
25.  Mutation of the His ligand in mitoNEET stabilizes the 2Fe–2S cluster despite conformational heterogeneity in the ligand environment 
The spectroscopic and stability properties and X-ray crystal structure of the H87C mutant of the 2Fe–2S ligand mitoNEET are reported. Strikingly, the single point mutation leads to changes in its absorbance and CD spectra and an increase of around sixfold in the stability of the 2Fe–2S clusters over the pH range 5–7. However, the crystal structure of the H87C mutant displays heterogeneity in a few key residues, including the introduced cysteine ligand. Nonetheless, the cluster is highly stabilized from release.
MitoNEET is the only identified Fe–S protein localized to the outer mitochondrial membrane and a 1.5 Å resolution X-ray analysis has revealed a unique structure [Paddock et al. (2007 ▶), Proc. Natl Acad. Sci. USA, 104, 14342–14347]. The 2Fe–2S cluster is bound with a 3Cys–1His coordination which defines a new class of 2Fe–2S proteins. The hallmark feature of this class is the single noncysteine ligand His87, which when replaced by Cys decreases the redox potential (E m) by ∼300 mV and increases the stability of the cluster by around sixfold. Unexpectedly, the pH dependence of the lifetime of the 2Fe–2S cluster remains the same as in the wild-type protein. Here, the crystal structure of H87C mitoNEET was determined to 1.7 Å resolution (R factor = 18%) to investigate the structural basis of the changes in the properties of the 2Fe–2S cluster. In comparison to the wild type, structural changes are localized to the immediate vicinity of the cluster-binding region. Despite the increased stability, Cys87 displays two distinct conformations, with distances of 2.3 and 3.2 Å between the Sγ and the outer Fe of the 2Fe–2S cluster. In addition, Lys55 exhibits multiple conformations in the H87C mutant protein. The structure and distinct characteristics of the H87C mutant provide a framework for further studies investigating the effects of mutation on the properties of the 2Fe–2S cluster in this new class of proteins.
doi:10.1107/S0907444911011577
PMCID: PMC3107049  PMID: 21636891
mitochondrial outer membrane; 2Fe–2S clusters; CISD1; diabetes; pioglitazone

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