Two structures of human Siah1 at 1.95 and 1.58 Å resolution provide more complete models for this protein and identify conformational variability in the subdomain organization.
Siah1 is an E3 ubiquitin ligase that contributes to proteasome-mediated degradation of multiple targets in key cellular processes and which shows promise as a therapeutic target in oncology. Structures of a truncated Siah1 bound to peptide-based inhibitors have been reported. Here, new crystallization conditions have allowed the determination of a construct encompassing dual zinc-finger subdomains and substrate-binding domains at significantly higher resolution. Although the crystals appear isomorphous, two structures present distinct states in which the spatial orientation of one zinc-finger subdomain differs with respect to the rest of the dimeric protein. Such a difference, which is indicative of conformational freedom, infers potential biological relevance related to recognition of binding partners. The crystallization conditions and improved models of Siah1 may aid future studies investigating Siah1–ligand complexes.
E3 ubiquitin ligase; seven-in-absentia homologue 1 (Siah1); zinc finger
The structure of a bifunctional deaminase/reductase involved in riboflavin biosynthesis in the pathogen A. baumannii has been determined in two crystal forms.
The bifunctional diaminohydroxyphosphoribosylaminopyrimidine deaminase/5-amino-6-(5-phosphoribosylamino)uracil reductase (RibD) represents a potential antibacterial drug target. The structure of recombinant Acinetobacter baumannii RibD is reported in orthorhombic and tetragonal crystal forms at 2.2 and 2.0 Å resolution, respectively. Comparisons with orthologous structures in the Protein Data Bank indicated close similarities. The tetragonal crystal form was obtained in the presence of guanosine monophosphate, which surprisingly was observed to occupy the adenine-binding site of the reductase domain.
bifunctional deaminase/reductase; Acinetobacter baumannii; RibD; riboflavin biosynthesis
Crystallographic analysis of the human SENP1 catalytic domain identified a well ordered Co2+ ion that contributes to intermolecular interactions relevant to crystallization of the enzyme. The presence of this ion was overlooked in previous studies.
Metal ions often stabilize intermolecular contacts between macromolecules, thereby promoting crystallization. When interpreting a medium-resolution electron-density map of the catalytic domain of human sentrin-specific protease 1 (SENP1), a strong feature indicative of an ordered divalent cation was noted. This was assigned as Co2+, an essential component of the crystallization mixture. The ion displays tetrahedral coordination by Glu430 and His640 from one molecule and the corresponding residues from a symmetry-related molecule. Analysis of the data derived from a previous structure of SENP1 suggested that Co2+ had been overlooked and re-refinement supported this conclusion. High-throughput automated re-refinement protocols also failed to mark the Co2+ position, supporting the requirement for the incorporation of as much information as possible to enhance the value of such protocols.
cobalt; sentrin-specific protease 1; SUMO
The crystal structure of inosine 5′-monophosphate dehydrogenase from P. aeruginosa has been determined to 2.25 Å resolution.
Inosine 5′-monophosphate dehydrogenase (IMPDH) represents a potential antimicrobial drug target. The crystal structure of recombinant Pseudomonas aeruginosa IMPDH has been determined to a resolution of 2.25 Å. The structure is a homotetramer of subunits dominated by a (β/α)8-barrel fold, consistent with other known structures of IMPDH. Also in common with previous work, the cystathionine β-synthase domains, residues 92–204, are not present in the model owing to disorder. However, unlike the majority of available structures, clearly defined electron density exists for a loop that creates part of the active site. This loop, composed of residues 297–315, links α8 and β9 and carries the catalytic Cys304. P. aeruginosa IMPDH shares a high level of sequence identity with bacterial and protozoan homologues, with residues involved in binding substrate and the NAD+ cofactor being conserved. Specific differences that have been proven to contribute to selectivity against the human enzyme in a study of Cryptosporidium parvum IMPDH are also conserved, highlighting the potential value of IMPDH as a drug target.
inosine 5′-monophosphate dehydrogenase; Pseudomonas aeruginosa; antimicrobial drug targets
A focused strategy has been directed towards the structural characterization of selected proteins from the bacterial pathogen P. aeruginosa. The objective is to exploit the resulting structural data, in combination with ligand-binding studies, and to assess the potential of these proteins for early-stage antimicrobial drug discovery.
Bacterial infections are increasingly difficult to treat owing to the spread of antibiotic resistance. A major concern is Gram-negative bacteria, for which the discovery of new antimicrobial drugs has been particularly scarce. In an effort to accelerate early steps in drug discovery, the EU-funded AEROPATH project aims to identify novel targets in the opportunistic pathogen Pseudomonas aeruginosa by applying a multidisciplinary approach encompassing target validation, structural characterization, assay development and hit identification from small-molecule libraries. Here, the strategies used for target selection are described and progress in protein production and structure analysis is reported. Of the 102 selected targets, 84 could be produced in soluble form and the de novo structures of 39 proteins have been determined. The crystal structures of eight of these targets, ranging from hypothetical unknown proteins to metabolic enzymes from different functional classes (PA1645, PA1648, PA2169, PA3770, PA4098, PA4485, PA4992 and PA5259), are reported here. The structural information is expected to provide a firm basis for the improvement of hit compounds identified from fragment-based and high-throughput screening campaigns.
protein structure; Gram-negative bacteria; Pseudomonas aeruginosa; infectious diseases; structure-based inhibitor design
A crystallographic and biochemical study of L. major cysteine synthase, which is a pyridoxyl phosphate-dependent enzyme, is reported. The structure was determined to 1.8 Å resolution and revealed that the cofactor has been lost and that a fragment of γ-poly-d-glutamic acid, a crystallization ingredient, was bound in the active site. The enzyme was inhibited by peptides.
Cysteine biosynthesis is a potential target for drug development against parasitic Leishmania species; these protozoa are responsible for a range of serious diseases. To improve understanding of this aspect of Leishmania biology, a crystallographic and biochemical study of L. major cysteine synthase has been undertaken, seeking to understand its structure, enzyme activity and modes of inhibition. Active enzyme was purified, assayed and crystallized in an orthorhombic form with a dimer in the asymmetric unit. Diffraction data extending to 1.8 Å resolution were measured and the structure was solved by molecular replacement. A fragment of γ-poly-d-glutamic acid, a constituent of the crystallization mixture, was bound in the enzyme active site. Although a d-glutamate tetrapeptide had insignificant inhibitory activity, the enzyme was competitively inhibited (K
i = 4 µM) by DYVI, a peptide based on the C-terminus of the partner serine acetyltransferase with which the enzyme forms a complex. The structure surprisingly revealed that the cofactor pyridoxal phosphate had been lost during crystallization.
Arabidopsis thaliana; cysteine synthase; Leishmania major
The structure of a triclinic crystal form of 4-diphosphocytidyl-2C-methyl-d-erythritol kinase has been determined. Comparisons with a previously reported monoclinic crystal form raise questions about our knowledge of the quaternary structure of this enzyme.
4-Diphosphocytidyl-2C-methyl-d-erythritol kinase (IspE; EC 220.127.116.11) contributes to the 1-deoxy-d-xylulose 5-phosphate or mevalonate-independent biosynthetic pathway that produces the isomers isopentenyl diphosphate and dimethylallyl diphosphate. These five-carbon compounds are the fundamental building blocks for the biosynthesis of isoprenoids. The mevalonate-independent pathway does not occur in humans, but is present and has been shown to be essential in many dangerous pathogens, i.e. Plasmodium species, which cause malaria, and Gram-negative bacteria. Thus, the enzymes involved in this pathway have attracted attention as potential drug targets. IspE produces 4-diphosphosphocytidyl-2C-methyl-d-erythritol 2-phosphate by ATP-dependent phosphorylation of 4-diphosphocytidyl-2C-methyl-d-erythritol. A triclinic crystal structure of the Escherichia coli IspE–ADP complex with two molecules in the asymmetric unit was determined at 2 Å resolution and compared with a monoclinic crystal form of a ternary complex of E. coli IspE also with two molecules in the asymmetric unit. The molecular packing is different in the two forms. In the asymmetric unit of the triclinic crystal form the substrate-binding sites of IspE are occluded by structural elements of the partner, suggesting that the ‘triclinic dimer’ is an artefact of the crystal lattice. The surface area of interaction in the triclinic form is almost double that observed in the monoclinic form, implying that the dimeric assembly in the monoclinic form may also be an artifact of crystallization.
mevalonate-independent pathway; isoprenoid biosynthesis; kinases
The structure of L. donovani pteridine reductase has been targeted to assist in a program of structure-based inhibitor research. Crystals that diffracted to 2.5 Å resolution were obtained and the structure has been solved. Unfortunately, the active site is disordered and this crystal form is unsuitable for use in characterizing enzyme–ligand interactions.
Pteridine reductase (PTR1) is a potential target for drug development against parasitic Trypanosoma and Leishmania species, protozoa that are responsible for a range of serious diseases found in tropical and subtropical parts of the world. As part of a structure-based approach to inhibitor development, specifically targeting Leishmania species, well ordered crystals of L. donovani PTR1 were sought to support the characterization of complexes formed with inhibitors. An efficient system for recombinant protein production was prepared and the enzyme was purified and crystallized in an orthorhombic form with ammonium sulfate as the precipitant. Diffraction data were measured to 2.5 Å resolution and the structure was solved by molecular replacement. However, a sulfate occupies a phosphate-binding site used by NADPH and occludes cofactor binding. The nicotinamide moiety is a critical component of the active site and without it this part of the structure is disordered. The crystal form obtained under these conditions is therefore unsuitable for the characterization of inhibitor complexes.
antifolates; pteridine reductase; Leishmania; pterins; Trypanosoma
The structure of S. aureus MenB, an enzyme in the biosynthetic pathway to vitamin K2, has been determined and compared with the enzyme derived from another important pathogen, M. tuberculosis.
Vitamin K2, or menaquinone, is an essential cofactor for many organisms and the enzymes involved in its biosynthesis are potential antimicrobial drug targets. One of these enzymes, 1,4-dihydroxy-2-naphthoyl-CoA synthase (MenB) from the pathogen Staphylococcus aureus, has been obtained in recombinant form and its quaternary structure has been analyzed in solution. Cubic crystals of the enzyme allowed a low-resolution structure (2.9 Å) to be determined. The asymmetric unit consists of two subunits and a crystallographic threefold axis of symmetry generates a hexamer consistent with size-exclusion chromatography. Analytical ultracentrifugation indicates the presence of six states in solution, monomeric through to hexameric, with the dimer noted as being particularly stable. MenB displays the crotonase-family fold with distinct N- and C-terminal domains and a flexible segment of structure around the active site. The smaller C-terminal domain plays an important role in oligomerization and also in substrate binding. The presence of acetoacetyl-CoA in one of the two active sites present in the asymmetric unit indicates how part of the substrate binds and facilitates comparisons with the structure of Mycobacterium tuberculosis MenB.
crotonase; synthase; vitamin biosynthesis; menaquinone; MenB
The structure of 6-phosphogluconate dehydrogenase from a moderate thermophile, G. stearothermophilus, is presented and compared with those of orthologous enzymes.
Two crystal structures of recombinant Geobacillus stearothermophilus 6-phosphogluconate dehydrogenase (Gs6PDH) in complex with the substrate 6-phosphogluconate have been determined at medium resolution. Gs6PDH shares significant sequence identity and structural similarity with the enzymes from Lactococcus lactis, sheep liver and the protozoan parasite Trypanosoma brucei, for which a range of structures have previously been reported. Comparisons indicate that amino-acid sequence conservation is more pronounced in the two domains that contribute to the architecture of the active site, namely the N-terminal and C-terminal domains, compared with the central domain, which is primarily involved in the subunit–subunit associations required to form a stable dimer. The active-site residues are highly conserved, as are the interactions with the 6-phosphogluconate. There is interest in 6PDH as a potential drug target for the protozoan parasite T. brucei, the pathogen responsible for African sleeping sickness. The recombinant T. brucei enzyme has proven to be recalcitrant to enzyme–ligand studies and a surrogate protein might offer new opportunities to investigate and characterize 6PDH inhibitors. The high degree of structural similarity, efficient level of expression and straightforward crystallization conditions mean that Gs6PDH may prove to be an appropriate model system for structure-based inhibitor design targeting the enzyme from Trypanosoma species.
pentose phosphate pathway; 6-phosphogluconate dehydrogenase; Geobacillus stearothermophilus
A double mutation designed to disrupt binding of isoprenoid diphosphate to an enzyme involved in isoprenoid biosynthesis was made and the structure determined. Despite the removal of six hydrogen-bonding interactions, the ligand, acquired during production in E. coli, remains bound. The reasons for this are discussed.
The essential enzyme 2C-methyl-d-erythritol-2,4-cyclodiphosphate (MECP) synthase, found in most eubacteria and the apicomplexan parasites, participates in isoprenoid-precursor biosynthesis and is a validated target for the development of broad-spectrum antimicrobial drugs. The structure and mechanism of the enzyme have been elucidated and the recent exciting finding that the enzyme actually binds diphosphate-containing isoprenoids at the interface formed by the three subunits that constitute the active protein suggests the possibility of feedback regulation of MECP synthase. To investigate such a possibility, a form of the enzyme was sought that did not bind these ligands but which would retain the quaternary structure necessary to create the active site. Two amino acids, Arg142 and Glu144, in Escherichia coli MECP synthase were identified as contributing to ligand binding. Glu144 interacts directly with Arg142 and positions the basic residue to form two hydrogen bonds with the terminal phosphate group of the isoprenoid diphosphate ligand. This association occurs at the trimer interface and three of these arginines interact with the ligand phosphate group. A dual mutation was designed (Arg142 to methionine and Glu144 to leucine) to disrupt the electrostatic attractions between the enzyme and the phosphate group to investigate whether an enzyme without isoprenoid diphosphate could be obtained. A low-resolution crystal structure of the mutated MECP synthase Met142/Leu144 revealed that geranyl diphosphate was retained despite the removal of six hydrogen bonds normally formed with the enzyme. This indicates that these two hydrophilic residues on the surface of the enzyme are not major determinants of isoprenoid binding at the trimer interface but rather that hydrophobic interactions between the hydrocarbon tail and the core of the enzyme trimer dominate ligand binding.
MECP synthase; site-directed mutagenesis; isoprenoid biosynthesis
The gene encoding the putative mevalonate diphosphate decarboxylase, an enzyme from the mevalonate pathway of isoprenoid precursor biosynthesis, has been cloned from T. brucei. Recombinant protein has been expressed, purified and highly ordered crystals obtained and characterized to aid the structure–function analysis of this enzyme.
Mevalonate diphosphate decarboxylase catalyses the last and least well characterized step in the mevalonate pathway for the biosynthesis of isopentenyl pyrophosphate, an isoprenoid precursor. A gene predicted to encode the enzyme from Trypanosoma brucei has been cloned, a highly efficient expression system established and a purification protocol determined. The enzyme gives monoclinic crystals in space group P21, with unit-cell parameters a = 51.5, b = 168.7, c = 54.9 Å, β = 118.8°. A Matthews coefficient V
M of 2.5 Å3 Da−1 corresponds to two monomers, each approximately 42 kDa (385 residues), in the asymmetric unit with 50% solvent content. These crystals are well ordered and data to high resolution have been recorded using synchrotron radiation.
decarboxylases; mevalonate biosynthesis; isoprenoids; Trypanosoma
The gene encoding the unusual metal-ion-dependent epoxidase involved in fosfomycin biosynthesis, S. wedmorensis (S)-2-hydroxypropylphosphonic acid epoxidase, has been cloned and the protein expressed, purified and crystallized. Two crystal forms have been obtained, one of which diffracts to high resolution.
The oxirane (1R,2S)-1,2-epoxypropylphosphonic acid (fosfomycin) is a natural product antibiotic produced in Streptomyces wedmorensis by the metal-ion-dependent (S)-2-hydroxypropylphosphonic acid epoxidase. This epoxidase is highly unusual since it has no requirement for a haem prosthetic group. The gene encoding the enzyme, fom4, has been cloned and a highly efficient recombinant source of the enzyme established. Two different crystal forms, tetragonal and hexagonal, have been obtained. The hexagonal form displays symmetry consistent with space group P61/522 and unit-cell parameters a = 86.44, c = 221.56 Å, γ = 120°. The Matthews coefficient, V
M, of 2.7 Å3 Da−1 corresponds to two subunits, each of approximate weight 21.4 kDa, in the asymmetric unit with 55% solvent content. These crystals diffract to high resolution and experimental phases are being sought to determine the structure.
antibiotics; epoxidases; metalloenzymes; Streptomyces
The structure of recombinant T. brucei UDP-galactose-4′-epimerase cocrystallized with NAD+ and the substrate analogue UDP-4-deoxy-4-fluoro-α-d-galactose has been determined at medium resolution. Comparisons with structures of human and E. coli UDP-galactose-4′-epimerase–ligand complexes reveal that the hexose moieties are able to adopt different orientations in the active site.
The structure of the NAD-dependent oxidoreductase UDP-galactose-4′-epimerase from Trypanosoma brucei in complex with cofactor and the substrate analogue UDP-4-deoxy-4-fluoro-α-d-galactose has been determined using diffraction data to 2.7 Å resolution. Despite the high level of sequence and structure conservation between the trypanosomatid enzyme and those from humans, yeast and bacteria, the binding of the 4-fluoro-α-d-galactose moiety is distinct from previously reported structures. Of particular note is the observation that when bound to the T. brucei enzyme, the galactose moiety of this fluoro-derivative is rotated approximately 180° with respect to the orientation of the hexose component of UDP-glucose when in complex with the human enzyme. The architecture of the catalytic centre is designed to effectively bind different orientations of the hexose, a finding that is consistent with a mechanism that requires the sugar to maintain a degree of flexibility within the active site.
short-chain dehydrogenase/reductases; Trypanosoma brucei; UDP-galactose-4′-epimerase; UDP-4-deoxy-4-fluoro-α-d-galactose
Attempts to crystallize a complex of papain (C. papaya) with a cysteine protease inhibitor from the parasitic pathogen T. brucei failed. However, over an extended period the mixture produced an ordered crystal of the protease carrying two peptide fragments in the active site. These correspond to dipeptides and tripeptides that are assigned as fragments of the inhibitor, which has presumably suffered proteolytic cleavage.
Attempts to cocrystallize the cysteine protease papain derived from the latex of Carica papaya with an inhibitor of cysteine proteases (ICP) from Trypanosoma brucei were unsuccessful. However, crystals of papain that diffracted to higher resolution, 1.5 Å, than other crystals of this archetypal cysteine protease were obtained, so the analysis was continued. Surprisingly, the substrate-binding cleft was occupied by two short peptide fragments which have been assigned as remnants of ICP. Comparisons reveal that these peptides bind in the active site in a manner similar to that of the human cysteine protease inhibitor stefin B when it is complexed to papain. The assignment of the fragment sequences is consistent with the specificity of the protease.
papain; cysteine protease; inhibitors; Trypanosoma brucei