Phylogenetics and sequence analysis--some problems for the unwary.
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Animals↗
Biomedical subjects
Christopher J Schofield
Publications and source records attributed to Christopher J Schofield.
The chemical biology of branched-chain lipid metabolism.
Mammalian metabolism of some lipids including 3-methyl and 2-methyl branched-chain fatty acids occurs within peroxisomes. Such lipids, including phytanic and pristanic acids, are commonly found within the human diet and may be derived from chlorophyll in plant extracts. Due to the presence of a methyl group at its beta-carbon, the well-characterised beta-oxidation pathway cannot degrade phytanic acid. Instead its alpha-methylene group is oxidatively excised to give pristanic acid, which can be metabolised by the beta-oxidation pathway. Many defects in the alpha-oxidation pathway result in an accumulation of phytanic acid, leading to neurological distress, deterioration of vision, deafness, loss of coordination and eventual death. Details of the alpha-oxidation pathway have only recently been elucidated, and considerable progress has been made in understanding the detailed enzymology of one of the oxidative steps within this pathway. This review summarises these recent advances and considers the roles and likely mechanisms of the enzymes within the alpha-oxidation pathway.
Fatty Acids↗
Trypanosomiasis control: surmounting diminishing returns.
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Africa↗
The kinetic properties of various R258 mutants of deacetoxycephalosporin C synthase.
Site-directed mutagenesis was used to investigate the control of 2-oxoacid cosubstrate selectivity by deacetoxycephalosporin C synthase. The wild-type enzyme has a requirement for 2-oxoglutarate and cannot efficiently use hydrophobic 2-oxoacids (e.g. 2-oxohexanoic acid, 2-oxo-4-methyl-pentanoic acid) as the cosubstrate. The following mutant enzymes were produced: R258A, R258L, R258F, R258H and R258K. All of the mutants have broadened cosubstrate selectivity and were able to utilize hydrophobic 2-oxoacids. The efficiency of 2-oxoglutarate utilization by all mutants was decreased as compared to the wild-type enzyme, and in some cases activity was abolished with the natural cosubstrate.
Ampicillin↗
Structure of factor-inhibiting hypoxia-inducible factor (HIF) reveals mechanism of oxidative modification of HIF-1 alpha.
The activity of the transcription factor hypoxia-inducible factor (HIF) is regulated by oxygen-dependent hydroxylation. Under normoxic conditions, hydroxylation of proline residues triggers destruction of its alpha-subunit while hydroxylation of Asn(803) in the C-terminal transactivation domain of HIF-1 alpha (CAD) prevents its interaction with p300. Here we report crystal structures of the asparagine hydroxylase (factor-inhibiting HIF, FIH) complexed with Fe((II)), 2-oxoglutarate cosubstrate, and CAD fragments, which reveal the structural basis of HIF modification. CAD binding to FIH occurs via an induced fit process at two distinct interaction sites. At the hydroxylation site CAD adopts a loop conformation, contrasting with a helical conformation for the same residues when bound to p300. Asn(803) of CAD is buried and precisely orientated in the active site such that hydroxylation occurs at its beta-carbon. Together with structures with the inhibitors Zn((II)) and N-oxaloylglycine, analysis of the FIH-CAD complexes will assist design of hydroxylase inhibitors with proangiogenic properties. Conserved structural motifs within FIH imply it is one of an extended family of Fe((II)) oxygenases involved in gene regulation.
Amino Acid Sequence↗
Hypoxia-inducible factor asparaginyl hydroxylase (FIH-1) catalyses hydroxylation at the beta-carbon of asparagine-803.
Asparagine-803 in the C-terminal transactivation domain of human hypoxia-inducible factor (HIF)-1 alpha-subunit is hydroxylated by factor inhibiting HIF-1 (FIH-1) under normoxic conditions causing abrogation of the HIF-1alpha/p300 interaction. NMR and other analyses of a hydroxylated HIF fragment produced in vitro demonstrate that hydroxylation occurs at the beta-carbon of Asn-803 and imply production of the threo -isomer, in contrast with other known aspartic acid/asparagine hydroxylases that produce the erythro -isomer.
Amino Acid Sequence↗
Oligomeric structure of proclavaminic acid amidino hydrolase: evolution of a hydrolytic enzyme in clavulanic acid biosynthesis.
During biosynthesis of the clinically used beta-lactamase inhibitor clavulanic acid, one of the three steps catalysed by clavaminic acid synthase is separated from the other two by a step catalysed by proclavaminic acid amidino hydrolase (PAH), in which the guanidino group of an intermediate is hydrolysed to give proclavaminic acid and urea. PAH shows considerable sequence homology with the primary metabolic arginases, which hydrolyse arginine to ornithine and urea, but does not accept arginine as a substrate. Like other members of the bacterial sub-family of arginases, PAH is hexameric in solution and requires Mn2+ ions for activity. Other metal ions, including Co2+, can substitute for Mn2+. Two new substrates for PAH were identified, N-acetyl-(L)-arginine and (3R)-hydroxy-N-acetyl-(L)-arginine. Crystal structures of PAH from Streptomyces clavuligerus (at 1.75 A and 2.45 A resolution, where 1 A=0.1 nm) imply how it binds beta-lactams rather than the amino acid substrate of the arginases from which it evolved. The structures also suggest how PAH selects for a particular alcohol intermediate in the clavam biosynthesis pathway. As observed for the arginases, each PAH monomer consists of a core of beta-strands surrounded by alpha-helices, and its active site contains a di-Mn2+ centre with a bridging water molecule responsible for hydrolytic attack on to the guanidino group of the substrate. Comparison of structures obtained under different conditions reveals different conformations of a flexible loop, which must move to allow substrate binding.
Amino Acid Sequence↗
5,5-Fused thiophene gamma-lactams as templates for serine protease inhibition.
Novel 5,5-fused thiophene lactams are potent inhibitors and acylating agents of HNE and PPE.
Alkylating Agents↗
The use of dioxygen by HIF prolyl hydroxylase (PHD1).
The hypoxic response in animals is mediated by hydroxylation of proline residues in the alpha-subunit of hypoxia inducible factor (HIF). Hydroxylation is catalysed by prolyl-4-hydroxylases (PHD isozymes in humans) which are iron(II) and 2-oxoglutarate dependent oxygenases. Mutation of the arginine proposed to bind 2-oxoglutarate and of the 2His-1-carboxylate iron(II) binding motif in PHD1 dramatically reduces its activity. The source of the oxygen of the product alcohol is (>95%) dioxygen.
Amino Acid Sequence↗
Structural basis for the recognition of hydroxyproline in HIF-1 alpha by pVHL.
Hypoxia-inducible factor-1 (HIF-1) is a transcriptional complex that controls cellular and systemic homeostatic responses to oxygen availability. HIF-1 alpha is the oxygen-regulated subunit of HIF-1, an alpha beta heterodimeric complex. HIF-1 alpha is stable in hypoxia, but in the presence of oxygen it is targeted for proteasomal degradation by the ubiquitination complex pVHL, the protein of the von Hippel Lindau (VHL) tumour suppressor gene and a component of an E3 ubiquitin ligase complex. Capture of HIF-1 alpha by pVHL is regulated by hydroxylation of specific prolyl residues in two functionally independent regions of HIF-1 alpha. The crystal structure of a hydroxylated HIF-1 alpha peptide bound to VCB (pVHL, elongins C and B) and solution binding assays reveal a single, conserved hydroxyproline-binding pocket in pVHL. Optimized hydrogen bonding to the buried hydroxyprolyl group confers precise discrimination between hydroxylated and unmodified prolyl residues. This mechanism provides a new focus for development of therapeutic agents to modulate cellular responses to hypoxia.
Amino Acid Motifs↗
Hypoxia-inducible factor (HIF) asparagine hydroxylase is identical to factor inhibiting HIF (FIH) and is related to the cupin structural family.
Activity of the hypoxia-inducible factor (HIF) complex is controlled by oxygen-dependent hydroxylation of prolyl and asparaginyl residues. Hydroxylation of specific prolyl residues by 2-oxoglutarate (2-OG)-dependent oxygenases mediates ubiquitinylation and proteasomal destruction of HIF-alpha. Hydroxylation of an asparagine residue in the C-terminal transactivation domain (CAD) of HIF-alpha abrogates interaction with p300, preventing transcriptional activation. Yeast two-hybrid assays recently identified factor inhibiting HIF (FIH) as a protein that associates with the CAD region of HIF-alpha. Since FIH contains certain motifs present in iron- and 2-OG-dependent oxygenases we investigated whether FIH was the HIF asparaginyl hydroxylase. Assays using recombinant FIH and HIF-alpha fragments revealed that FIH is the enzyme that hydroxylates the CAD asparagine residue, that the activity is directly inhibited by cobalt(II) and limited by hypoxia, and that the oxygen in the alcohol of the hydroxyasparagine residue is directly derived from dioxygen. Sequence analyses involving FIH link the 2-OG oxygenases with members of the cupin superfamily, including Zn(II)-utilizing phosphomannose isomerase, revealing structural and evolutionary links between these metal-binding proteins that share common motifs.
Amino Acid Sequence↗
Molecular dynamics simulations of the acyl-enzyme and the tetrahedral intermediate in the deacylation step of serine proteases.
Despite the availability of many experimental data and some modeling studies, questions remain as to the precise mechanism of the serine proteases. Here we report molecular dynamics simulations on the acyl-enzyme complex and the tetrahedral intermediate during the deacylation step in elastase catalyzed hydrolysis of a simple peptide. The models are based on recent crystallographic data for an acyl-enzyme intermediate at pH 5 and a time-resolved study on the deacylation step. Simulations were carried out on the acyl enzyme complex with His-57 in protonated (as for the pH 5 crystallographic work) and deprotonated forms. In both cases, a water molecule that could provide the nucleophilic hydroxide ion to attack the ester carbonyl was located between the imidazole ring of His-57 and the carbonyl carbon, close to the hydrolytic position assigned in the crystal structure. In the "neutral pH" simulations of the acyl-enzyme complex, the hydrolytic water oxygen was hydrogen bonded to the imidazole ring and the side chain of Arg-61. Alternative stable locations for water in the active site were also observed. Movement of the His-57 side-chain from that observed in the crystal structure allowed more solvent waters to enter the active site, suggesting that an alternative hydrolytic process directly involving two water molecules may be possible. At the acyl-enzyme stage, the ester carbonyl was found to flip easily in and out of the oxyanion hole. In contrast, simulations on the tetrahedral intermediate showed no significant movement of His-57 and the ester carbonyl was constantly located in the oxyanion hole. A comparison between the simulated tetrahedral intermediate and a time-resolved crystallographic structure assigned as predominantly reflecting the tetrahedral intermediate suggests that the experimental structure may not precisely represent an optimal arrangement for catalysis in solution. Movement of loop residues 216-223 and P3 residue, seen both in the tetrahedral simulation and the experimental analysis, could be related to product release. Furthermore, an analysis of the geometric data obtained from the simulations and the pH 5 crystal structure of the acyl-enzyme suggests that since His-57 is protonated, in some aspects, this crystal structure resembles the tetrahedral intermediate.
Acylation↗
Crystal structure of a clavaminate synthase-Fe(II)-2-oxoglutarate-substrate-NO complex: evidence for metal centered rearrangements.
Clavaminate synthase (CAS), a 2-oxoglutarate (2OG) dependent dioxygenase, catalyses three steps in the biosynthesis of clavulanic acid. Crystals of CAS complexed with Fe(II), 2OG and deoxyguanidinoproclavaminate were exposed to nitric oxide (NO) acting as a dioxygen analogue. Prior to exposure with NO, the active site Fe(II) is octahedrally coordinated by a water molecule, the 2-oxo and 1-carboxylate groups of 2OG, and the side-chains of an aspartyl and two histidinyl residues. NO binds to the position previously occupied by the 2OG 1-carboxylate concomitant with rearrangement of the latter to the position previously occupied by the displaced water.
Binding Sites↗
Active site mutations of recombinant deacetoxycephalosporin C synthase.
Site-directed mutagenesis of active site residues of deacetoxycephalosporin C synthase active site residues was carried out to investigate their role in catalysis. The following mutations were made and their effects on the conversion of 2-oxoglutarate and the oxidation of penicillin N or G were assessed: M180F, G299N, G300N, Y302S, Y302F/G300A, Y302E, Y302H, and N304A. The Y302S, Y302E, and Y302H mutations reduced 2-oxoglutarate conversions and abolished (<2%) penicillin G oxidation. The Y302F/G300A mutation caused partial uncoupling of penicillin G oxidation from 2-oxoglutarate conversion, but did not uncouple penicillin N oxidation from 2-oxoglutarate conversion. Met-180 is involved in binding 2-oxoglutarate, and the M180F mutation caused uncoupling of 2-oxoglutarate from penicillin oxidation. The N304A mutation apparently enhanced in vitro conversion of penicillin N but had little effect on the oxidation of penicillin G, under standard assay conditions.
Binding Sites↗
X-ray structure of a serine protease acyl-enzyme complex at 0.95-A resolution.
Kinetic analyses led to the discovery that N-acetylated tripeptides with polar residues at P3 are inhibitors of porcine pancreatic elastase (PPE) that form unusually stable acyl-enzyme complexes. Peptides terminating in a C-terminal carboxylate were more potent than those terminating in a C-terminal amide, suggesting recognition by the oxy-anion hole is important in binding. X-ray diffraction data were recorded to 0.95-A resolution for an acyl-enzyme complex formed between PPE and N-acetyl-Asn-Pro-Ile-CO2H at approximately pH 5. The accuracy of the crystallographic coordinates allows structural issues concerning the mechanism of serine proteases to be addressed. Significantly, the ester bond of the acyl-enzyme showed a high level of planarity, suggesting geometric strain of the ester link is not important during catalysis. Several hydrogen atoms could be clearly identified and were included within the model. In keeping with a recent x-ray structure of subtilisin at 0.78 A (1), limited electron density is visible consistent with the putative location of a hydrogen atom approximately equidistant between the histidine and aspartate residues of the catalytic triad. Comparison of this high resolution crystal structure of the acyl-enzyme complex with that of native elastase at 1.1 A (2) showed that binding of the N-terminal part of the substrate can be accommodated with negligible structural rearrangements. In contrast, comparison with structures obtained as part of "time-resolved" studies on the reacting acyl-enzyme complex at >pH 7 (3) indicate small but significant structural differences, consistent with the proposed synchronization of ester hydrolysis and substrate release.
Animals↗
Enzymatic synthesis of monocyclic beta-lactams.
An Mg2+ and ATP dependent beta-lactam synthetase (BLS) catalyses formation of a beta-lactam ring during the biosynthesis of clavulanic acid, an important beta-lactamase inhibitor. An epimeric mixture of a 2-methylated derivative of the natural BLS substrate N2-(2-carboxyethyl)-L-arginine was synthesised and found to be a substrate for the enzyme. The epimeric products were characterised by 1H NMR and mass spectrometric analyses. The results suggest that a modified version of BLS might be used to catalyse the preparation of intermediates useful for the synthesis of beta-lactam antibiotics.
Amidohydrolases↗
Binding of D- and L-captopril inhibitors to metallo-beta-lactamase studied by polarizable molecular mechanics and quantum mechanics.
The bacterial Zn2+ metallo-beta-lactamase from B. fragilis is a zinc-enzyme with two potential metal ion binding sites. It cleaves the lactam ring of antibiotics, thus contributing to the acquired resistance of bacteria against antibiotics. The present study bears on the binuclear form of the enzyme. We compare several possible binding modes of captopril, a mercaptocarboxamide inhibitor of several zinc-metalloenzymes. Two diastereoisomers of captopril were considered, with either a D- or an L-proline residue. We have used the polarizable molecular mechanics procedure SIBFA (Sum of Interactions Between Fragments ab initio computed). Two beta-lactamase models were considered, encompassing 104 and 188 residues, respectively. The energy balances included the inter and intramolecular interaction energies as well as the contribution from solvation computed using a continuum reaction field procedure. The thiolate ion of the inhibitor is binding to both metal ions, expelling the bridging solvent molecule from the uncomplexed enzyme. Different competing binding modes of captopril were considered, either where the inhibitor binds in a monodentate mode to the zinc cations only with its thiolate ion, or in bidentate modes involving additional zinc binding by its carboxylate or ketone carbonyl groups. The additional coordination by the inhibitor's carboxylate or carbonyl group always occurs at the zinc ion, which is bound by a histidine, a cysteine, and an aspartate side chain. For both diastereomers, the energy balances favor monodentate binding of captopril via S-. The preference over bidentate binding is small. The interaction energies were recomputed in model sites restricted to captopril, the Zn2+ cations, and their coordinating end side chains from beta-lactamase (98 atoms). The interaction energies and their ranking among competing arrangements were consistent with those computed by ab initio HF and DFT procedures.
Algorithms↗
Structure and mechanism of anthocyanidin synthase from Arabidopsis thaliana.
Flavonoids are common colorants in plants and have long-established biomedicinal properties. Anthocyanidin synthase (ANS), a 2-oxoglutarate iron-dependent oxygenase, catalyzes the penultimate step in the biosynthesis of the anthocyanin class of flavonoids. The crystal structure of ANS reveals a multicomponent active site containing metal, cosubstrate, and two molecules of a substrate analog (dihydroquercetin). An additional structure obtained after 30 min exposure to dioxygen is consistent with the oxidation of the dihydroquercetin to quercetin and the concomitant decarboxylation of 2-oxoglutarate to succinate. Together with in vitro studies, the crystal structures suggest a mechanism for ANS-catalyzed anthocyanidin formation from the natural leucoanthocyanidin substrates involving stereoselective C-3 hydroxylation. The structure of ANS provides a template for the ubiquitous family of plant nonhaem oxygenases for future engineering and inhibition studies.
Amino Acid Sequence↗