Crystal structure of UDP-N-acetylmuramoyl-L-alanine:D-glutamate ligase from Escherichia coli.

UDP-N-acetylmuramoyl-L-alanine:D-glutamate ligase (MurD) is a cytoplasmic enzyme involved in the biosynthesis of peptidoglycan which catalyzes the addition of D-glutamate to the nucleotide precursor UDP-N-acetylmuramoyl-L-alanine (UMA). The crystal structure of MurD in the presence of its substrate UMA has been solved to 1.9 A resolution. Phase information was obtained from multiple anomalous dispersion using the K-shell edge of selenium in combination with multiple isomorphous replacement. The structure comprises three domains of topology each reminiscent of nucleotide-binding folds: the N- and C-terminal domains are consistent with the dinucleotide-binding fold called the Rossmann fold, and the central domain with the mononucleotide-binding fold also observed in the GTPase family. The structure reveals the binding site of the substrate UMA, and comparison with known NTP complexes allows the identification of residues interacting with ATP. The study describes the first structure of the UDP-N-acetylmuramoyl-peptide ligase family.     

Crystal structure of UDP-N-acetylmuramoyl-L-alanyl-D-glutamate: meso-diaminopimelate ligase from Escherichia coli

UDP-N-acetylmuramoyl-l-alanyl-d-glutamate:meso-diaminopimelate ligase is a cytoplasmic enzyme that catalyzes the addition of meso-diaminopimelic acid to nucleotide precursor UDP-N-acetylmuramoyl-l-alanyl-d-glutamate in the biosynthesis of bacterial cell-wall peptidoglycan. The crystal structure of the Escherichia coli enzyme in the presence of the final product of the enzymatic reaction, UDP-MurNAc-l-Ala-gamma-d-Glu-meso-A(2)pm, has been solved to 2.0 A resolution. Phase information was obtained by multiwavelength anomalous dispersion using the K shell edge of selenium. The protein consists of three domains, two of which have a topology reminiscent of the equivalent domain found in the already established three-dimensional structure of the UDP-N-acetylmuramoyl-l-alanine: D-glutamate-ligase (MurD) ligase, which catalyzes the immediate previous step of incorporation of d-glutamic acid in the biosynthesis of the peptidoglycan precursor. The refined model reveals the binding site for UDP-MurNAc-l-Ala-gamma-d-Glu-meso-A(2)pm, and comparison with the six known MurD structures allowed the identification of residues involved in the enzymatic mechanism. Interestingly, during refinement, an excess of electron density was observed, leading to the conclusion that, as in MurD, a carbamylated lysine residue is present in the active site. In addition, the structural determinant responsible for the selection of the amino acid to be added to the nucleotide precursor was identified.     

Crystal structure of lipoate-protein ligase A bound with the activated intermediate: insights into interaction with lipoyl domains

Lipoic acid is the covalently attached cofactor of several multi-component enzyme complexes that catalyze key metabolic reactions. Attachment of lipoic acid to the lipoyl-dependent enzymes is catalyzed by lipoate-protein ligases (LPLs). In Escherichia coli, two distinct enzymes lipoate-protein ligase A (LplA) and lipB-encoded lipoyltransferase (LipB) catalyze independent pathways for lipoylation of the target proteins. The reaction catalyzed by LplA occurs in two steps. First, LplA activates exogenously supplied lipoic acid at the expense of ATP to lipoyl-AMP. Next, it transfers the enzyme-bound lipoyl-AMP to the epsilon-amino group of a specific lysine residue of the lipoyl domain to give an amide linkage. To gain insight into the mechanism of action by LplA, we have determined the crystal structure of Thermoplasma acidophilum LplA in three forms: (i) the apo form; (ii) the ATP complex; and (iii) the lipoyl-AMP complex. The overall fold of LplA bears some resemblance to that of the biotinyl protein ligase module of the E. coli biotin holoenzyme synthetase/bio repressor (BirA). Lipoyl-AMP is bound deeply in the bifurcated pocket of LplA and adopts a U-shaped conformation. Only the phosphate group and part of the ribose sugar of lipoyl-AMP are accessible from the bulk solvent through a tunnel-like passage, whereas the rest of the activated intermediate is completely buried inside the active site pocket. This first view of the activated intermediate bound to LplA allowed us to propose a model of the complexes between Ta LplA and lipoyl domains, thus shedding light on the target protein/lysine residue specificity of LplA.     

Crystal structure of SecB from Escherichia coli

The chaperone SecB from Escherichia coli is primarily involved in passing precursor proteins into the Sec system via specific interactions with SecA. The crystal structure of SecB from E. coli has been solved to 2.35 A resolution. The structure shows flexibility in the crossover loop and the helix-connecting loop, regions that have been implicated to be part of the SecB substrate-binding site. Moreover conformational variability of Trp36 is observed as well as different loop conformations for the different monomers. Based on this, we speculate that SecB can regulate the access or extent of its hydrophobic substrate-binding site, by modulating the conformation of the crossover loop and the helix-connecting loop. The structure also clearly explains why the tetrameric equilibrium is shifted towards the dimeric state in the mutant SecBCys76Tyr. The buried cysteine residue is crucial for tight packing, and mutations are likely to disrupt the tetramer formation but not the dimer formation.     

Crystal structure of glutamine phosphoribosylpyrophosphate amidotransferase from Escherichia coli.

Crystal structures of glutamine phosphoribosylpyrophosphate (PRPP) amidotransferase from Escherichia coli have been determined to 2.0-A resolution in the absence of ligands, and to 2.5-A resolution with the feedback inhibitor AMP bound to the PRPP catalytic site. Glutamine PRPP amidotransferase (GPATase) employs separate catalytic domains to abstract nitrogen from the amide of glutamine and to transfer nitrogen to the acceptor substrate PRPP. The unliganded and AMP-bound structures, which are essentially identical, are interpreted as the inhibited form of the enzyme because the two active sites are disconnected and the PRPP active site is solvent exposed. The structures were compared with a previously reported 3.0-A structure of the homologous Bacillus subtilis enzyme (Smith JL et al., 1994, Science 264:1427-1433). The comparison indicates a pattern of conservation of peptide structures involved with catalysis and variability in enzyme regulatory functions. Control of glutaminase activity, communication between the active sites, and regulation by feedback inhibitors are addressed differently by E. coli and B. subtilis GPATases. The E. coli enzyme is a prototype for the metal-free GPATases, whereas the B. subtilis enzyme represents the metal-containing enzymes. The structure of the E. coli enzyme suggests that a common ancestor of the two enzyme subfamilies may have included an Fe-S cluster.     

Crystal structure at 1.2 A resolution and active site mapping of Escherichia coli peptidyl-tRNA hydrolase.

Peptidyl-tRNA hydrolase activity from Escherichia coli ensures the recycling of peptidyl-tRNAs produced through abortion of translation. This activity, which is essential for cell viability, is carried out by a monomeric protein of 193 residues. The structure of crystalline peptidyl-tRNA hydrolase could be solved at 1.2 A resolution. It indicates a single alpha/beta globular domain built around a twisted mixed beta-sheet, similar to the central core of an aminopeptidase from Aeromonas proteolytica. This similarity allowed the characterization by site-directed mutagenesis of several residues of the active site of peptidyl-tRNA hydrolase. These residues, strictly conserved among the known peptidyl-tRNA hydrolase sequences, delineate a channel which, in the crystal, is occupied by the C-end of a neighbouring peptidyl-tRNA hydrolase molecule. Hence, several main chain atoms of three residues belonging to one peptidyl-tRNA hydrolase polypeptide establish contacts inside the active site of another peptidyl-tRNA hydrolase molecule. Such an interaction is assumed to represent the formation of a complex between the enzyme and one product of the catalysed reaction.     

Crystal structure of the PdxY Protein from Escherichia coli

The crystal structure of Escherichia coli PdxY, the protein product of the pdxY gene, has been determined to a 2.2-A resolution. PdxY is a member of the ribokinase superfamily of enzymes and has sequence homology with pyridoxal kinases that phosphorylate pyridoxal at the C-5' hydroxyl. The protein is a homodimer with an active site on each monomer composed of residues that come exclusively from each respective subunit. The active site is filled with a density that fits that of pyridoxal. In monomer A, the ligand appears to be covalently attached to Cys122 as a thiohemiacetal, while in monomer B it is not covalently attached but appears to be partially present as pyridoxal 5'-phosphate. The presence of pyridoxal phosphate and pyridoxal as ligands was confirmed by the activation of aposerine hydroxymethyltransferase after release of the ligand by the denaturation of PdxY. The ligand, which appears to be covalently attached to Cys122, does not dissociate after denaturation of the protein. A detailed comparison (of functional properties, sequence homology, active site and ATP-binding-site residues, and active site flap types) of PdxY with other pyridoxal kinases as well as the ribokinase superfamily in general suggested that PdxY is a member of a new subclass of the ribokinase superfamily. The structure of PdxY also permitted an interpretation of work that was previously published about this enzyme.     

Crystal structure of D-serine dehydratase from Escherichia coli

D-Serine dehydratase from Escherichia coli is a member of the β-family (fold-type II) of the pyridoxal 5'-phosphate-dependent enzymes, catalyzing the conversion of D-serine to pyruvate and ammonia. The crystal structure of monomeric D-serine dehydratase has been solved to 1.97?-resolution for an orthorhombic data set by molecular replacement. In addition, the structure was refined in a monoclinic data set to 1.55? resolution. The structure of DSD reveals a larger pyridoxal 5'-phosphate-binding domain and a smaller domain. The active site of DSD is very similar to those of the other members of the β-family. Lys118 forms the Schiff base to PLP, the cofactor phosphate group is liganded to a tetraglycine cluster Gly279-Gly283, and the 3-hydroxyl group of PLP is liganded to Asn170 and N1 to Thr424, respectively. In the closed conformation the movement of the small domain blocks the entrance to active site of DSD. The domain movement plays an important role in the formation of the substrate recognition site and the catalysis of the enzyme. Modeling of D-serine into the active site of DSD suggests that the hydroxyl group of D-serine is coordinated to the carboxyl group of Asp238. The carboxyl oxygen of D-serine is coordinated to the hydroxyl group of Ser167 and the amide group of Leu171 (O1), whereas the O2 of the carboxyl group of D-serine is hydrogen-bonded to the hydroxyl group of Ser167 and the amide group of Thr168. A catalytic mechanism very similar to that proposed for L-serine dehydratase is discussed.     

Crystal structure of unliganded phosphofructokinase from Escherichia coli

In an attempt to characterize the mechanism of co-operativity in the allosteric enzyme phosphofructokinase from Escherichia coli, crystals were grown in the absence of activating ligands. The crystal structure was determined to a resolution of 2.4 A by the method of molecular replacement, using the known structure of the liganded active state as a starting model, and has been refined to a crystallographic R-factor of 0.168 for all data. Although the crystallization solution would be expected to contain the enzyme in its inactive conformation, with a low affinity for the co-operative substrate fructose 6-phosphate, the structure in these crystals does not show the change in quaternary structure seen in the inactive form of the Bacillus stearothermophilus enzyme (previously determined at low resolution), nor does it show any substantial change in the fructose 6-phosphate site from the structure seen in the liganded form. Compared to the liganded form, there are considerable changes around the allosteric effector site, including the disordering of the last 19 residues of the chain. It seems likely that the observed conformation corresponds an active unliganded form, in which the absence of ligand in the effector site induces structural changes that spread through much of the subunit, but cause only minor changes in the active site. It is not clear why the crystals should contain the enzyme in a high-affinity conformation, which presumably represents only a small fraction of the molecules in the crystallizing solution. However, this structure does identify the conformational changes involved in binding of the allosteric effectors.     

Crystal structure of the hydrogenase maturating endopeptidase HYBD from Escherichia coli.

The maturation of [NiFe] hydrogenases includes formation of the nickel metallocenter, proteolytic processing of the metal center carrying large subunit, and its assembling with other hydrogenase subunits. The hydrogenase maturating enzyme HYBD from Escherichia coli, a protease of molecular mass 17.5 kDa, specifically cleaves off a 15 amino acid peptide from the C terminus of the precursor of the large subunit of hydrogenase 2 in a nickel-dependent manner. Here we report the crystal structure of HYBD at 2.2 A resolution. It consists of a twisted five-stranded beta-sheet surrounded by four and three helices, respectively, on each side. A cadmium ion from the crystallization buffer binds to the proposed nickel-binding site and is penta-coordinated by Glu16, Asp62, His93, and a water molecule in a pseudo-tetragonal arrangement. HYBD is topologically related to members of the metzincins superfamily of zinc endoproteinases, sharing the central beta-sheet and three helices. In contrast to the metzincins, the metal-binding site of HYBD is localized at the C-terminal end of the beta-sheet. Three helical insertions unique to HYBD pack against one side of the sheet, build up the active site cleft, and provide His93 as ligand to the metal. From this structure, we derive molecular clues into how the protease HYBD is involved in the hydrogenase maturation process.     

Crystal structure of thioredoxin from Escherichia coli at 1.68 A resolution

The crystal structure of thioredoxin from Escherichia coli has been refined by the stereochemically restrained least-squares procedure to a crystallographic R-factor of 0.165 at 1.68 A resolution. In the final model, the root-mean-square deviation from ideality for bond distances is 0.015 A and for angle distances 0.035 A. The structure contains 1644 protein atoms from two independent molecules, two Cu2+, 140 water molecules and seven methylpentanediol molecules. Ten residues have been modeled in two alternative conformations. E. coli thioredoxin is a compact molecule with 90% of its residues in helices, beta-strands or reverse turns. The molecule consists of two conformational domains, beta alpha beta alpha beta and beta beta alpha, connected by a single-turn alpha-helix and a 3(10) helix. The beta-sheet forms the core of the molecule packed on either side by clusters of hydrophobic residues. Helices form the external surface. The active site disulfide bridge between Cys32 and Cys35 is located at the amino terminus of the second alpha-helix. The positive electrostatic field due to the helical dipole is probably important for stabilizing the anionic intermediate during the disulfide reductase function of the protein. The more reactive cysteine, Cys32, has its sulfur atom exposed to solvent and also involved in a hydrogen bond with a backbone amide group. Residues 29 to 37, which include the active site cysteine residues, form a protrusion on the surface of the protein and make relatively fewer interactions with the rest of the structure. The disulfide bridge exhibits a right-handed conformation with a torsion angle of 81 degrees and 72 degrees about the S-S bond in the two molecules. Twenty-five pairs of water molecules obey the noncrystallographic symmetry. Most of them are involved in establishing intramolecular hydrogen-bonding interactions between protein atoms and thus serve as integral parts of the folded protein structure. Methylpentanediol molecules often pack against the loops and stabilize their structure. Cu2+ used for crystallization exhibit a distorted octahedral square bipyramid co-ordination and provide essential packing interactions in the crystal. The two independent protein molecules are very similar in conformation but distinctly different in atomic detail (root-mean-square = 0.94 A). The differences, which may be related to the crystal contacts, are localized mostly to regions far from the active site.     

Crystal structure of the outer membrane protease OmpT from Escherichia coli suggests a novel catalytic site

OmpT from Escherichia coli belongs to a family of highly homologous outer membrane proteases, known as omptins, which are implicated in the virulence of several pathogenic Gram-negative bacteria. Here we present the crystal structure of OmpT, which shows a 10-stranded antiparallel beta-barrel that protrudes far from the lipid bilayer into the extracellular space. We identified a putative binding site for lipopolysaccharide, a molecule that is essential for OmpT activity. The proteolytic site is located in a groove at the extracellular top of the vase-shaped beta-barrel. Based on the constellation of active site residues, we propose a novel proteolytic mechanism, involving a His-Asp dyad and an Asp-Asp couple that activate a putative nucleophilic water molecule. The active site is fully conserved within the omptin family. Therefore, the structure described here provides a sound basis for the design of drugs against omptin-mediated bacterial pathogenesis. Coordinates are in the Protein Data Bank (accession No. 1I78)     

Crystal structure of the ribosome recycling factor from Escherichia coli

We have determined the crystal structure of the Escherichia coli ribosome recycling factor (RRF), which catalyzes the disassembly of the termination complex in protein synthesis. The L-shaped molecule consists of two domains: a triple-stranded antiparallel coiled-coil and an alpha/beta domain. The coil domain has a cylindrical shape and negatively charged surface, which are reminiscent of the anticodon arm of tRNA and domain IV of elongation factor EF-G. We suggest that RRF binds to the ribosomal A-site through its coil domain, which is a tRNA mimic. The relative position of the two domains is changed about an axis along the hydrophobic cleft in the hinge where the alkyl chain of a detergent molecule is bound. The tRNA mimicry and the domain movement observed in RRF provide a structural basis for understanding the role of RRF in protein synthesis.     

Crystal structure of histidyl-tRNA synthetase from Escherichia coli complexed with histidyl-adenylate.

The crystal structure at 2.6 A of the histidyl-tRNA synthetase from Escherichia coli complexed with histidyl-adenylate has been determined. The enzyme is a homodimer with a molecular weight of 94 kDa and belongs to the class II of aminoacyl-tRNA synthetases (aaRS). The asymmetric unit is composed of two homodimers. Each monomer consists of two domains. The N-terminal catalytic core domain contains a six-stranded antiparallel beta-sheet sitting on two alpha-helices, which can be superposed with the catalytic domains of yeast AspRS, and GlyRS and SerRS from Thermus thermophilus with a root-mean-square difference on the C alpha atoms of 1.7-1.9 A. The active sites of all four monomers are occupied by histidyl-adenylate, which apparently forms during crystallization. The 100 residue C-terminal alpha/beta domain resembles half of a beta-barrel, and provides an independent domain oriented to contact the anticodon stem and part of the anticodon loop of tRNA(His). The modular domain organization of histidyl-tRNA synthetase reiterates a repeated theme in aaRS, and its structure should provide insight into the ability of certain aaRS to aminoacylate minihelices and other non-tRNA molecules.     

Crystal structure of Escherichia coli cystathionine gamma-synthase at 1.5 A resolution.

The transsulfuration enzyme cystathionine gamma-synthase (CGS) catalyses the pyridoxal 5'-phosphate (PLP)-dependent gamma-replacement of O-succinyl-L-homoserine and L-cysteine, yielding L-cystathionine. The crystal structure of the Escherichia coli enzyme has been solved by molecular replacement with the known structure of cystathionine beta-lyase (CBL), and refined at 1.5 A resolution to a crystallographic R-factor of 20.0%. The enzyme crystallizes as an alpha4 tetramer with the subunits related by non-crystallographic 222 symmetry. The spatial fold of the subunits, with three functionally distinct domains and their quaternary arrangement, is similar to that of CBL. Previously proposed reaction mechanisms for CGS can be checked against the structural model, allowing interpretation of the catalytic and substrate-binding functions of individual active site residues. Enzyme-substrate models pinpoint specific residues responsible for the substrate specificity, in agreement with structural comparisons with CBL. Both steric and electrostatic designs of the active site seem to achieve proper substrate selection and productive orientation. Amino acid sequence and structural alignments of CGS and CBL suggest that differences in the substrate-binding characteristics are responsible for the different reaction chemistries. Because CGS catalyses the only known PLP-dependent replacement reaction at Cgamma of certain amino acids, the results will help in our understanding of the chemical versatility of PLP.     

Crystal structure of chondroitin polymerase from Escherichia coli K4

Elongation of glycosaminoglycan chains, such as heparan and chondroitin, is catalyzed by bi-functional glycosyltransferases, for which both 3-dimensional structures and reaction mechanisms remain unknown. The bacterial chondroitin polymerase K4CP catalyzes elongation of the chondroitin chain by alternatively transferring the GlcUA and GalNAc moiety from UDP-GlcUA and UDP-GalNAc to the non-reducing ends of the chondroitin chain. Here, we have determined the crystal structure of K4CP in the presence of UDP and UDP-GalNAc as well as with UDP and UDP-GlcUA. The structures consisted of two GT-A fold domains in which the two active sites were 60 Å apart. UDP-GalNAc and UDP-GlcUA were found at the active sites of the N-terminal and C-terminal domains, respectively. The present K4CPstructures have provided the structural basis for further investigating the molecular mechanism of biosynthesis of chondroitin chain.