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Christopher Davies

Publications and source records attributed to Christopher Davies.

29 records · Page 2Linked to original sources
PubMed2004

A novel phosphoglucose isomerase (PGI)/phosphomannose isomerase from the crenarchaeon Pyrobaculum aerophilum is a member of the PGI superfamily: structural evidence at 1.16-A resolution.

The crystal structure of a dual specificity phosphoglucose isomerase (PGI)/phosphomannose isomerase from Pyrobaculum aerophilum (PaPGI/PMI) has been determined in native form at 1.16-A resolution and in complex with the enzyme inhibitor 5-phosphoarabinonate at 1.45-A resolution. The similarity of its fold, with the inner core structure of PGIs from eubacterial and eukaryotic sources, confirms this enzyme as a member of the PGI superfamily. The almost total conservation of amino acids in the active site, including the glutamate base catalyst, shows that PaPGI/PMI uses the same catalytic mechanisms for both ring opening and isomerization for the interconversion of glucose 6-phosphate (Glc-6-P) to fructose 6-phosphate (Fru-6-P). The lack of structural differences between native and inhibitor-bound enzymes suggests this activity occurs without any of the conformational changes that are the hallmark of the well characterized PGI family. The lack of a suitable second base in the active site of PaPGI/PMI argues against a PMI mechanism involving a trans-enediol intermediate. Instead, PMI activity may be the result of additional space in the active site imparted by a threonine, in place of a glutamine in other PGI enzymes, which could permit rotation of the C-2-C-3 bond of mannose 6-phosphate.

Amino Acid Sequence↗
PubMed2004

The crystal structure of the hydrolase domain of 10-formyltetrahydrofolate dehydrogenase: mechanism of hydrolysis and its interplay with the dehydrogenase domain.

10-Formyltetrahydrofolate dehydrogenase (FDH) converts 10-formyltetrahydrofolate, a precursor for nucleotide biosynthesis, to tetrahydrofolate. The protein comprises two functional domains: a hydrolase domain that removes a formyl group from 10-formyltetrahydrofolate and a NADP(+)-dependent dehydrogenase domain that reduces the formyl to carbon dioxide. As a first step toward deciphering the catalytic mechanism of the enzyme, we have determined the crystal structure of the hydrolase domain of FDH from rat, solved to 2.3-A resolution. The structure comprises two domains. As expected, domain 1 shares the same Rossmann fold as the related enzymes, methionyl-tRNA-formyltransferase and glycinamide ribonucleotide formyltransferase, but, unexpectedly, the structural similarity between the amino-terminal domain of 10-formyltetrahydrofolate dehydrogenase and methionyl-tRNA-formyltransferase extends to the C terminus of both proteins. The active site contains a molecule of beta-mercaptoethanol that is positioned between His-106 and Asp-142 and that appears to mimic the formate product. We propose a catalytic mechanism for the hydrolase reaction in which Asp-142 polarizes the catalytic water molecule and His-106 orients the carbonyl group of formyl. The structure also provides clues as to how, in the native enzyme, the hydrolase domain transfers its product to the dehydrogenase domain.

Amino Acid Sequence↗
PubMed2004

Overexpression and enzymatic characterization of Neisseria gonorrhoeae penicillin-binding protein 4.

The penicillin-binding proteins (PBPs) are ubiquitous bacterial enzymes involved in cell wall biosynthesis, and are the targets of the beta-lactam antibiotics. The low molecular mass Neisseria gonorrhoeae PBP 4 (NG PBP 4) is the fourth PBP revealed in the gonococcal genome. NG PBP 4 was cloned, overexpressed, purified, and characterized for beta-lactam binding, DD-carboxypeptidase activity, acyl-donor substrate specificity, transpeptidase activity, inhibition by a number of active site directed reagents, and pH profile. NG PBP 4 was efficiently acylated by penicillin (30,000 m-1.s-1). Against a set of five alpha- and epsilon-substituted l-Lys-D-Ala-D-Ala substrates, NG PBP 4 exhibited wide variation in specificity with a preference for N epsilon-acylated substrates, suggesting a possible preference for crosslinked pentapeptide substrates in the cell wall. Substrates with an N epsilon-Cbz group demonstrated pronounced substrate inhibition. NG PBP 4 showed 30-fold higher activity against the depsipeptide Lac-ester substrate than against the analogous peptide substrate, an indication that k2 (acylation) is rate determining for carboxypeptidase activity. No transpeptidase activity was apparent in a model transpeptidase reaction. Among a number of active site-directed agents, N-chlorosuccinimide, elastinal, iodoacetamide, iodoacetic acid, and phenylglyoxal gave substantial inhibition, and methyl boronic acid gave modest inhibition. The pH profile for activity against Ac2-l-Lys-D-Ala-d-Ala (kcat/Km) was bell-shaped, with pKa values at 6.9 and 10.1. Comparison of the enzymatic properties of NG PBP 4 with other DD-carboxypeptidases highlights both similarities and differences within these enzymes, and suggests the possibility of common mechanistic roles for the two highly conserved active site lysines in Class A and C low molecular mass PBPs.

Acylation↗
PubMed2003

Crystal structure of wild-type penicillin-binding protein 5 from Escherichia coli: implications for deacylation of the acyl-enzyme complex.

Penicillin-binding protein 5 (PBP 5) of Escherichia coli functions as a d-alanine carboxypeptidase (CPase), cleaving d-alanine from the C terminus of cell wall peptides. Like all PBPs, PBP 5 forms a covalent acyl-enzyme complex with beta-lactam antibiotics; however, PBP 5 is distinguished by its high rate of deacylation of the acylenzyme complex (t(1/2) approximately 10 min). A Gly105 --> Asp mutation in PBP 5 markedly impairs deacylation with only minor effects on acylation, and abolishes CPase activity. We have determined the three-dimensional structure of a soluble form of wild-type PBP 5 at 1.85-A resolution and have also refined the structure of the G105D mutant form of PBP 5 to 1.9-A resolution. Comparison of the two structures reveals that the major effect of the mutation is to disorder a loop comprising residues 74-90 that sits atop the SXN motif of the active site. Deletion of the 74-90 loop in wild-type PBP 5 markedly diminished the deacylation rate of penicillin G with a minimal impact on acylation, and abolished CPase activity. These effects were very similar to those observed in the G105D mutant, reinforcing the idea that this mutation causes disordering of the 74-90 loop. Mutation of two consecutive serines within this loop, which hydrogen bond to Ser110 and Asn112 in the SXN motif, had marked effects on CPase activity, but not beta-lactam antibiotic binding or hydrolysis. These data suggest a direct role for the SXN motif in deacylation of the acyl-enzyme complex and imply that the functioning of this motif is modulated by the 74-90 loop.

Amino Acid Motifs↗
PubMed2003

Structural evidence for a hydride transfer mechanism of catalysis in phosphoglucose isomerase from Pyrococcus furiosus.

In the Euryarchaeota species Pyrococcus furiosus and Thermococcus litoralis, phosphoglucose isomerase (PGI) activity is catalyzed by an enzyme unrelated to the well known family of PGI enzymes found in prokaryotes, eukaryotes, and some archaea. We have determined the crystal structure of PGI from Pyrococcus furiosus in native form and in complex with two active site ligands, 5-phosphoarabinonate and gluconate 6-phosphate. In these structures, the metal ion, which in vivo is presumed to be Fe2+, is located in the core of the cupin fold and is immediately adjacent to the C1-C2 region of the ligands, suggesting that Fe2+ is involved in catalysis rather than serving a structural role. The active site contains a glutamate residue that contacts the substrate, but, because it is also coordinated to the metal ion, it is highly unlikely to mediate proton transfer in a cis-enediol mechanism. Consequently, we propose a hydride shift mechanism of catalysis. In this mechanism, Fe2+ is responsible for proton transfer between O1 and O2, and the hydride shift between C1 and C2 is favored by a markedly hydrophobic environment in the active site. The absence of any obvious enzymatic machinery for catalyzing ring opening of the sugar substrates suggests that pyrococcal PGI has a preference for straight chain substrates and that metabolism in extreme thermophiles may use sugars in both ring and straight chain forms.

Binding Sites↗
PubMed2003

The structure of human phosphoglucose isomerase complexed with a transition-state analogue.

Phosphoglucose isomerase (PGI) is a workhorse enzyme of carbohydrate metabolism that interconverts glucose 6-phosphate and fructose 6-phosphate. Outside the cell, however, the protein appears to function as a cytokine. A crystal structure of human PGI bound with 5-phosphoarabinonate, a strong inhibitor that mimics the cis-enediol(ate) intermediate of the reaction, has been determined at 2.5 A resolution. The structure helps to confirm the assignment of Glu357 as the base catalyst in the isomerase reaction.

Crystallization↗
PubMed2003

Structure of native phosphoglucose isomerase from rabbit: conformational changes associated with catalytic function.

Phosphoglucose isomerase (PGI) is a housekeeping enzyme of metabolism that catalyses the interconversion of glucose 6-phosphate and fructose 6-phosphate, with roles in the glycolytic and gluconeogenic pathways. PGI is also a multifunctional protein that manifests the properties of a cytokine in a wide array of cellular processes, including the production of immunoglobulin by B cells and tumour-cell differentiation. The crystal structure of PGI in the native form from rabbit muscle has been solved at a resolution of 2.5 A by a combination of multiple isomorphous replacement and multi-crystal averaging techniques. Comparison with published structures of rabbit PGI in complex with three inhibitors and with the substrate fructose 6-phosphate reveals a number of conformational changes that may be associated with catalytic function. These occur in the small domain around the sugar phosphate-binding site, in a small helix carrying His388 and in a helix near the C-terminal end. One of these may be the structural rearrangement that has been postulated to be the rate-limiting step for catalysis.

Amino Acid Sequence↗
PubMed2003

Crystallization and preliminary X-ray diffraction analysis of phosphoglucose isomerase from Pyrococcus furiosus.

In several euryarchaeota, phosphoglucose isomerase (PGI) activity is catalyzed by an enzyme unrelated to the well-known family of PGI enzymes found in prokaryotes, eukaryotes and some archaea. In order to understand the mechanistic differences between the two families of enzymes we have crystallized PGI from the archaeon Pyrococcus furiosus. The crystals belong to the space group P2(1) and a complete dataset extending to 1.9 A resolution has been collected.

Crystallization↗
PubMed2002

Crystallization and preliminary X-ray diffraction analysis of recombinant hydrolase domain of 10-formyltetrahydrofolate dehydrogenase.

10-Formyltetrahydrofolate dehydrogenase (FDH) is an abundant enzyme in liver cytosol. It is important for the regulation of 10-formyltetrahydrofolate/tetrahydrofolate pools, for de novo purine biosynthesis and for the removal of formate in the form of CO(2). The enzyme is a natural fusion of two unrelated genes and consists of two functional catalytic domains. Here, the crystallization of the N-terminal domain of FDH is reported. This domain binds folate and functions as a 10-formyltetrahydrofolate hydrolase. The crystals grow as either spear-shaped needles or large plates, with the largest crystals reaching dimensions of 1.2 x 0.2 x 0.05 mm. Diffraction analysis revealed the space group to be P2(1)2(1)2, with unit-cell parameters a = 100.00, b = 64.63, c = 64.59 A. Based on the estimated solvent content, there is one 34 kDa molecule in the asymmetric unit. A native data set extending to 2.3 A resolution has been collected with good merging statistics.

Animals↗
PubMed2002

pH, inhibitor, and substrate specificity studies on Escherichia coli penicillin-binding protein 5.

The recent structural determination of Escherichia coli penicillin-binding protein 5 (PBP 5) provides the opportunity for detailed structure-function studies of this enzyme. PBP 5 was investigated in terms of its stability, linear reaction kinetics, acyl-donor substrate specificity, inhibition by a number of active site-directed reagents, and pH profile. PBP 5 demonstrated linear reaction kinetics for up to several hours. Dilution of PBP 5 generally resulted in substantial loss of activity, unless BSA or a BSA derivative was added to the diluting buffer. PBP 5 did not demonstrate a significant preference against a simple set of five alpha- and epsilon-substituted L-Lys-D-Ala-D-Ala derivatives, suggesting that PBP 5 lacks specificity for the cross-linked state of cell wall substrates. Among a number of active site-directed reagents, only some thiol-directed reagents gave substantial inhibition. Notably, serine-directed reagents, organic phosphates, and simple boronic acids were ineffective as inhibitors. PBP 5 was stable over the pH range 4.6-12.3, and the k(cat)/K(m) vs. pH profile for activity against Ac(2)-L-Lys-D-Ala-D-Ala was bell-shaped, with pK(a)s at 8.2 and 11.1. This is the first complete pH profile, including both acidic and basic limbs, for a PBP-catalyzed DD-carboxypeptidase (CPase) reaction. Based on its structure, similarity to Class A beta-lactamases, and results from mutagenesis studies, the acidic and basic limbs of the pH profile of PBP 5 are assigned to Lys-47 and Lys-213, respectively. This assignment supports a role for Lys-47 as the general base for acylation and deacylation reactions.

Animals↗