Kinetic and crystallographic studies on deacetoxycephalosporin C synthase (DAOCS)

C-terminus mutations of Acremonium chrysogenum deacetoxy/deacetylcephalosporin C synthase with improved activity toward penicillin analogs

Deacetoxy/deacetylcephalosporin C synthase (acDAOC/DACS) from Acremonium chrysogenum is a bifunctional enzyme that catalyzes both the ring-expansion of penicillin N to deacetoxycephalosporin C (DAOC) and the hydroxylation of the latter to deacetylcephalosporin C (DAC). Three residues N305, R307 and R308 located in close proximity to the C-terminus of acDAOC/DACS were each mutated to leucine. The N305L and R308L mutant acDAOC/DACSs showed significant improvement in their ability to convert penicillin analogs. R308 was identified for the first time as a critical residue for DAOC/DACS activity. Kinetic analyses of purified R308L enzyme indicated its improved catalytic efficiency is due to combined improvements of K(m) and k(cat). Comparative modeling of acDAOC/DACS supports the involvement of R308 in the formation of substrate-binding pocket.

Structure of human phytanoyl-CoA 2-hydroxylase identifies molecular mechanisms of Refsum disease

Refsum disease (RD), a neurological syndrome characterized by adult onset retinitis pigmentosa, anosmia, sensory neuropathy, and phytanic acidaemia, is caused by elevated levels of phytanic acid. Many cases of RD are associated with mutations in phytanoyl-CoA 2-hydroxylase (PAHX), an Fe(II) and 2-oxoglutarate (2OG)-dependent oxygenase that catalyzes the initial alpha-oxidation step in the degradation of phytenic acid in peroxisomes. We describe the x-ray crystallographic structure of PAHX to 2.5 A resolution complexed with Fe(II) and 2OG and predict the molecular consequences of mutations causing RD. Like other 2OG oxygenases, PAHX possesses a double-stranded beta-helix core, which supports three iron binding ligands (His(175), Asp(177), and His(264)); the 2-oxoacid group of 2OG binds to the Fe(II) in a bidentate manner. The manner in which PAHX binds to Fe(II) and 2OG together with the presence of a cysteine residue (Cys(191)) 6.7 A from the Fe(II) and two further histidine residues (His(155) and His(281)) at its active site distinguishes it from that of the other human 2OG oxygenase for which structures are available, factor inhibiting hypoxia-inducible factor. Of the 15 PAHX residues observed to be mutated in RD patients, 11 cluster in two distinct groups around the Fe(II) (Pro(173), His(175), Gln(176), Asp(177), and His(220)) and 2OG binding sites (Trp(193), Glu(197), Ile(199), Gly(204), Asn(269), and Arg(275)). PAHX may be the first of a new subfamily of coenzyme A-binding 2OG oxygenases.

The enzymology of clavam and carbapenem biosynthesis

The enzyme-catalysed reactions involved in formation of the bicyclic clavam and carbapenem nuclei, including beta-amino acid and beta-lactam formation, are discussed and compared with those involved in penicillin and cephalosporin biosynthesis. The common role of unusual oxidation reactions in the biosynthetic pathways and the lack of synthetic reagents available to effect them are highlighted.

Structure–function correlations in oxygen activating non-heme iron enzymes

A large group of mononuclear non-heme iron enzymes exist which activate dioxygen to catalyze key biochemical transformations, including many of medical, pharmaceutical and environmental significance. These enzymes utilize high-spin Fe(II) active sites and additional reducing equivalents from cofactors or substrates to react with O2 to yield iron-oxygen intermediates competent to transform substrate to product. While Fe(II) sites have been difficult to study due to the lack of dominant spectroscopic features, a spectroscopic methodology has been developed which allows the elucidation of the geometric and electronic structures of these active sites and provides molecular level insight into the mechanisms of catalysis. This review provides a summary of this methodology with emphasis on its application to the determination of important active site structure-function correlations in mononuclear non-heme iron enzymes. These studies provide key insight into the mechanisms of oxygen activation, active site features that contribute to differences in reactivity and, combined with theoretical calculations and model studies, the nature of oxygen intermediates active in catalysis.

4-Hydroxyphenylpyruvate dioxygenase

4-Hydroxyphenylpyruvate dioxygenase (HPPD) is an Fe(II)-dependent, non-heme oxygenase that catalyzes the conversion of 4-hydroxyphenylpyruvate to homogentisate. This reaction involves decarboxylation, substituent migration and aromatic oxygenation in a single catalytic cycle. HPPD is a member of the alpha-keto acid dependent oxygenases that typically require an alpha-keto acid (almost exclusively alpha-ketoglutarate) and molecular oxygen to either oxygenate or oxidize a third molecule. As an exception in this class of enzymes HPPD has only two substrates, does not use alpha-ketoglutarate, and incorporates both atoms of dioxygen into the aromatic product, homogentisate. The tertiary structure of the enzyme would suggest that its mechanism converged with that of other alpha-keto acid enzymes from an extradiol dioxygenase progenitor. The transformation catalyzed by HPPD has both agricultural and therapeutic significance. HPPD catalyzes the second step in the pathway for the catabolism of tyrosine, that is common to essentially all aerobic forms of life. In plants this pathway has an anabolic branch from homogentisate that forms essential isoprenoid redox cofactors such as plastoquinone and tocopherol. Naturally occurring multi-ketone molecules act as allelopathic agents by inhibiting HPPD and preventing the production of homogentisate and hence required redox cofactors. This has been the basis for the development of a range of very effective herbicides that are currently used commercially. In humans, deficiencies of specific enzymes of the tyrosine catabolism pathway give rise to a number of severe metabolic disorders. Interestingly, HPPD inhibitor/herbicide molecules act also as therapeutic agents for a number of debilitating and lethal inborn defects in tyrosine catabolism by preventing the accumulation of toxic metabolites.

Family shuffling of expandase genes to enhance substrate specificity for penicillin G

Deacetoxycephalosporin C synthase (expandase) from Streptomyces clavuligerus, encoded by cefE, is an important industrial enzyme for the production of 7-aminodeacetoxycephalosporanic acid from penicillin G. To improve the substrate specificity for penicillin G, eight cefE-homologous genes were directly evolved by using the DNA shuffling technique. After the first round of shuffling and screening, using an Escherichia coli ESS bioassay, four chimeras with higher activity were subjected to a second round. Subsequently, 20 clones were found with significantly enhanced activity. The kinetic parameters of two isolates that lack substrate inhibition showed 8.5- and 118-fold increases in the k(cat)/K(m) ratio compared to the S. clavuligerus expandase. The evolved enzyme with the 118-fold increase is the most active obtained to date anywhere. Our shuffling results also indicate the remarkable plasticity of the expandase, suggesting that more-active chimeras might be achievable with further rounds.

The active site and substrate-binding mode of 1-aminocyclopropane-1-carboxylate oxidase determined by site-directed mutagenesis and comparative modelling studies

The active site and substrate-binding mode of MD-ACO1 (Malus domestica Borkh. 1-aminocyclopropane-1-carboxylate oxidase) have been determined using site-directed mutagenesis and comparative modelling methods. The MD-ACO1 protein folds into a compact jelly-roll motif comprised of eight a-helices, 12 b-strands and several long loops. The active site is well defined as a wide cleft near the C-terminus. The co-substrate ascorbate is located in cofactor Fe2+-binding pocket, the so-called '2-His-1-carboxylate facial triad'. In addition, our results reveal that Arg244 and Ser246 are involved in generating the reaction product during enzyme catalysis. The structure agrees well with the biochemical and site-directed mutagenesis results. The three-dimensional structure together with the steady-state kinetics of both the wild-type and mutant MD-ACO1 proteins reveal how the substrate specificity of MD-ACO1 is involved in the catalytic mechanism, providing insights into understanding the fruit ripening process at atomic resolution.

Conformational Flexibility of the C Terminus with Implications for Substrate Binding and Catalysis Revealed in a New Crystal Form of Deacetoxycephalosporin C Synthase

Deacetoxycephalosporin C synthase (DAOCS) from Streptomyces clavuligerus catalyses the oxidative ring expansion of the penicillin nucleus into the nucleus of cephalosporins. The reaction requires dioxygen and 2-oxoglutarate as co-substrates to create a reactive iron-oxygen intermediate from a ferrous iron in the active site. The active enzyme is monomeric in solution. The structure of DAOCS was determined earlier from merohedrally twinned crystals where the last four C-terminal residues (308-311) of one molecule penetrate the active site of a neighbouring molecule, creating a cyclic trimeric structure in the crystal. Shortening the polypeptide chain from the C terminus by more than four residues diminishes activity. Here, we describe a new crystal form of DAOCS in which trimer formation is broken and the C-terminal arm is free. These crystals show no signs of twinning, and were obtained from DAOCS labelled with an N-terminal His-tag. The modified DAOCS is catalytically active. The free C-terminal arm protrudes into the solvent, and the C-terminal domain (residues 268-299) is rotated by about 16 degrees towards the active site. The last 12 residues (300-311) are disordered. Structures for various enzyme-substrate and enzyme-product complexes in the new crystal form confirm overlapping binding sites for penicillin and 2-oxoglutarate. The results support the notion that 2-oxoglutarate and dioxygen need to react first to produce an oxidizing iron species, followed by reaction with the penicillin substrate. The position of the penicillin nucleus is topologically similar in the two crystal forms, but the penicillin side-chain in the new non-twinned crystals overlaps with the position of residues 304-306 of the C-terminal arm in the twinned crystals. An analysis of the interactions between the C-terminal region and residues in the active site indicates that DAOCS could also accept polypeptide chains as ligands, and these could bind near the iron.

A linear, O-coordinated eta1-CO2 bound to uranium

The electron-rich, six-coordinate tris-aryloxide uranium(III) complex [((AdArO)3tacn)U(III)] [where (AdArOH)3tacn = 1,4,7-tris(3-adamantyl-5-tert-butyl-2-hydroxybenzyl)1,4,7-triazacyclononane] reacts rapidly with CO2 to yield [((AdArO)3tacn)U(IV)(CO2)], a complex in which the CO(2) ligand is linearly coordinated to the metal through its oxygen atom (eta1-OCO). The latter complex has been crystallographically and spectroscopically characterized. The inequivalent O-C-O bond lengths [1.122 angstroms (A) for the O-C bond adjacent to uranium and 1.277 A for the other], considered together with magnetization data and electronic and vibrational spectra, support the following bonding model: U(IV)=O=C*-O- <--> U(IV)-OC-O-. In these charge-separated resonance structures, the uranium center is oxidized to uranium(IV) and the CO2 ligand reduced by one electron.

A complete library of amino acid alterations at N304 in Streptomyces clavuligerus deacetoxycephalosporin C synthase elucidates the basis for enhanced penicillin analogue conversion

N304 of Streptomyces clavuligerus deacetoxycephalosporin C synthase was mutagenized to alter its catalytic ability. Given that N304A, N304K, N304L, and N304R mutant enzymes exhibited significant improvements in penicillin analogue conversions, we advocate that replacement of N304 with residues with aliphatic or basic side chains is preferable for engineering of a hypercatalytic enzyme.

Controlling the Substrate Selectivity of Deacetoxycephalosporin/deacetylcephalosporin C Synthase

Deacetoxycephalosporin/deacetylcephalosporin C synthase (DAOC/DACS) is an iron(II) and 2-oxoglutarate-dependent oxygenase involved in the biosynthesis of cephalosporin C in Cephalosporium acremonium. It catalyzes two oxidative reactions, oxidative ring-expansion of penicillin N to deacetoxycephalosporin C, and hydroxylation of the latter to give deacetylcephalosporin C. The enzyme is closely related to deacetoxycephalosporin C synthase (DAOCS) and DACS from Streptomyces clavuligerus, which selectively catalyze ring-expansion or hydroxylation reactions, respectively. In this study, structural models based on DAOCS coupled with site-directed mutagenesis were used to identify residues within DAOC/DACS that are responsible for controlling substrate and reaction selectivity. The M306I mutation abolished hydroxylation of deacetylcephalosporin C, whereas the W82A mutant reduced ring-expansion of penicillin G (an "unnatural" substrate). Truncation of the C terminus of DAOC/DACS to residue 310 (Delta310 mutant) enhanced ring-expansion of penicillin G by approximately 2-fold. A double mutant, Delta310/M306I, selectively catalyzed the ring-expansion reaction and had similar kinetic parameters to the wild-type DAOC/DACS. The Delta310/N305L/M306I triple mutant selectively catalyzed ring-expansion of penicillin G and had improved kinetic parameters (K(m) = 2.00 +/- 0.47 compared with 6.02 +/- 0.97 mm for the wild-type enzyme). This work demonstrates that a single amino acid residue side chain within the DAOC/DACS active site can control whether the enzyme catalyzes ring-expansion, hydroxylation, or both reactions. The catalytic efficiency of mutant enzymes can be improved by combining active site mutations with other modifications including C-terminal truncation and modification of Asn-305.

The structural basis of cephalosporin formation in a mononuclear ferrous enzyme

Deacetoxycephalosporin-C synthase (DAOCS) is a mononuclear ferrous enzyme that transforms penicillins into cephalosporins by inserting a carbon atom into the penicillin nucleus. In the first half-reaction, dioxygen and 2-oxoglutarate produce a reactive iron-oxygen species, succinate and CO2. The oxidizing iron species subsequently reacts with penicillin to give cephalosporin and water. Here we describe high-resolution structures for ferrous DAOCS in complex with penicillins, the cephalosporin product, the cosubstrate and the coproduct. Steady-state kinetic data, quantum-chemical calculations and the new structures indicate a reaction sequence in which a 'booby-trapped' oxidizing species is formed. This species is stabilized by the negative charge of succinate on the iron. The binding sites of succinate and penicillin overlap, and when penicillin replaces succinate, it removes the stabilizing charge, eliciting oxidative attack on itself. Requisite groups of penicillin are within 1 A of the expected position of a ferryl oxygen in the enzyme-penicillin complex.

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.

Engineering Streptomyces clavuligerus deacetoxycephalosporin C synthase for optimal ring expansion activity toward penicillin G

The deacetoxycephalosporin C synthase (DAOCS) from Streptomyces clavuligerus was engineered with the aim of enhancing the conversion of penicillin G into phenylacetyl-7-aminodeacetoxycephalosporanic acid, a precursor of 7-aminodeacetoxycephalosporanic acid, for industrial application. A single round of random mutagenesis followed by the screening of 5,500 clones identified three mutants, G79E, V275I, and C281Y, that showed a two- to sixfold increase in the k(cat)/K(m) ratio compared to the wild-type enzyme. Site-directed mutagenesis to modify residues surrounding the substrate resulted in three mutants, N304K, I305L, and I305M, with 6- to 14-fold-increased k(cat)/K(m) values. When mutants containing all possible combinations of these six sites were generated to optimize the ring expansion activity for penicillin G, the double mutant, YS67 (V275I, I305M), showed a significant 32-fold increase in the k(cat)/K(m) ratio and a 5-fold increase in relative activity for penicillin G, while the triple mutant, YS81 (V275I, C281Y, I305M), showed an even greater 13-fold increase in relative activity toward penicillin G. Our results demonstrate that this is a robust approach to the modification of DAOCS for an optimized DAOCS-penicillin G reaction.

Interconversion of two oxidized forms of taurine/alpha-ketoglutarate dioxygenase, a non-heme iron hydroxylase: evidence for bicarbonate binding

Taurinealpha-ketoglutarate (alphaKG) dioxygenase, or TauD, is a mononuclear non-heme iron hydroxylase that couples the oxidative decarboxylation of alphaKG to the decomposition of taurine, forming sulfite and aminoacetaldehyde. Prior studies revealed that taurine-free TauD catalyzes an O(2)- and alphaKG-dependent self-hydroxylation reaction involving Tyr-73, yielding an Fe(III)-catecholate chromophore with a lambda(max) of 550 nm. Here, a chromophore (lambda(max) 720 nm) is described and shown to arise from O(2)-dependent self-hydroxylation of TauD in the absence of alphaKG, but requiring the product succinate. A similar chromophore rapidly develops with the alternative oxidant H(2)O(2). Resonance Raman spectra indicate that the approximately 700-nm chromophore also arises from an Fe(III)-catecholate species, and site-directed mutagenesis studies again demonstrate Tyr-73 involvement. The approximately 700-nm and 550-nm species are shown to interconvert by the addition or removal of bicarbonate, consistent with the alphaKG-derived CO(2) remaining tightly bound to the oxidized metal site as bicarbonate. The relevance of the metal-bound bicarbonate in TauD to reactions of other members of this enzyme family is discussed.

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.

Reversal of DNA alkylation damage by two human dioxygenases

The Escherichia coli AlkB protein protects against the cytotoxicity of methylating agents by repair of the DNA lesions 1-methyladenine and 3-methylcytosine, which are generated in single-stranded stretches of DNA. AlkB is an alpha-ketoglutarate- and Fe(II)-dependent dioxygenase that oxidizes the relevant methyl groups and releases them as formaldehyde. Here, we identify two human AlkB homologs, ABH2 and ABH3, by sequence and fold similarity, functional assays, and complementation of the E. coli alkB mutant phenotype. The levels of their mRNAs do not appear to correlate with cell proliferation but tissue distributions are different. Both enzymes remove 1-methyladenine and 3-methylcytosine from methylated polynucleotides in an alpha-ketoglutarate-dependent reaction, and act by direct damage reversal with the regeneration of the unsubstituted bases. AlkB, ABH2, and ABH3 can also repair 1-ethyladenine residues in DNA with the release of acetaldehyde.

C-terminus modification of Streptomyces clavuligerus deacetoxycephalosporin C synthase improves catalysis with an expanded substrate specificity

The biosynthesis of cephalosporins is catalyzed by deacetoxycephalosporin C synthase (DAOCS). Based on computational, biochemical, and structural analyses, it has been proposed that modification of the C-terminus of DAOCS might be a constructive strategy for engineering improvement in enzyme activity. Therefore, five hydrophilic residues namely N301, Y302, N304, R306, and R307 located in proximity to the C-terminus of Streptomyces clavuligerus DAOCS (scDAOCS) were selected and each substituted with a hydrophobic leucine residue. Substitutions at positions 304, 306, and 307 created mutant scDAOCSs with improved efficiencies in penicillin analog conversion up to 397%. And since it has been previously advocated that the C-terminus is crucial for guiding substrate entry, a truncated mutant DAOCS was constructed to assess its involvement. The truncation of the C-terminus at position 310 in the wild-type scDAOCS resulted in reduction of indiscriminate conversion of penicillin analog but this defect was compensated by the replacement of asparagine with leucine at position 304.

The role of arginine residues in substrate binding and catalysis by deacetoxycephalosporin C synthase

Deacetoxycephalosporin C synthase (DAOCS) catalyses the oxidative ring expansion of penicillin N, the committed step in the biosynthesis of cephamycin C by Streptomyces clavuligerus. Site-directed mutagenesis was used to investigate the seven Arg residues for activity (74, 75, 160, 162, 266, 306 and 307), selected on the basis of the DAOCS crystal structure. Greater than 95% of activity was lost upon mutation of Arg-160 and Arg266 to glutamine or other residues. These results are consistent with the proposed roles for these residues in binding the carboxylate linked to the nucleus of penicillin N (Arg160 and Arg162) and the carboxylate of the alpha-aminoadipoyl side-chain (Arg266). The results for mutation of Arg74 and Arg75 indicate that these residues play a less important role in catalysis/binding. Together with previous work, the mutation results for Arg306 and Arg307 indicate that modification of the C-terminus may be profitable with respect to altering the penicillin side-chain selectivity of DAOCS.

Utilization of sterol carrier protein-2 by phytanoyl-CoA 2-hydroxylase in the peroxisomal alpha oxidation of phytanic acid

Since it possesses a 3-methyl group, phytanic acid is degraded by a peroxisomal alpha-oxidation pathway, the first step of which is catalyzed by phytanoyl-CoA 2-hydroxylase (PAHX). Mutations in human PAHX cause phytanic acid accumulations leading to Adult Refsum's Disease (ARD), which is also observed in a sterol carrier protein 2 (SCP-2)-deficient mouse model. Phytanoyl-CoA is efficiently 2-hydroxylated by PAHX in vitro in the presence of mature SCP-2. Other straight-chain fatty acyl-CoA esters were also 2-hydroxylated and the products isolated and characterized. Use of SCP-2 increases discrimination between straight-chain (e.g., hexadecanoyl-CoA) and branched-chain (e.g., phytanoyl-CoA) substrates by PAHX. The results explain the phytanic acid accumulation in the SCP-2-deficient mouse model and suggest that some of the common symptoms of ARD and other peroxisomal diseases may arise in part due to defects in SCP-2 function caused by increased phytanic acid levels.

Non-heme iron oxygenases

Our understanding of the biological significance and chemical properties of non-heme iron oxygenases has increased dramatically in recent years. New group members have emerged from genome sequences and biochemical analyses. Spectroscopic and crystallographic studies have provided critical insights into catalysis. Self-hydroxylation reactions, commonplace in these proteins, reveal important features of metallocenter reactivity.

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.

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.

Mutation of N304 to leucine in Streptomyces clavuligerus deacetoxycephalosporin C synthase creates an enzyme with increased penicillin analogue conversion

Superimposition of deacetoxycephalosporin C synthase (DAOCS) and isopenicillin N synthase (IPNS) structures revealed that R74, R160, R266 and N304 are strategically located in the catalytic cavity of Streptomyces clavuligerus DAOCS (scDAOCS) and are crucial for orchestrating different substrates. Substitutions at these sites to a hydrophobic leucine residue were expected to stabilize the hydrophobic substrate bound state. Substantial improvements in the biotransformation of penicillin G, ampicillin and amoxicillin to their respective cephalosporin moieties were observed using the N304L mutant scDAOCS. Thus, our results have demonstrated the enhancement of scDAOCS activity via critical computational analysis and site-directed mutagenesis of endogenous ligands.

Contrasting fates for 6-alpha-methylpenicillin N upon oxidation by deacetoxycephalosporin C synthase (DAOCS) and deacetoxy/deacetylcephalosporin C synthase (DAOC/DACS)

6-alpha-methylpenicillin N was synthesised via known routes from 6-aminopenicillanic acid, and tested as a substrate for recombinant DAOCS and DAOC/DACS. Incubation with DAOCS resulted in conversion of 2-oxoglutarate without oxidation of the penicillin substrate ('uncoupled turnover'). Incubation with DAOC/DACS resulted in oxidation to the cephem aldehyde. This is the first example of substrate-induced 'uncoupled turnover', which has been proposed to be an editing mechanism for these enzymes.

Alteration of the co-substrate selectivity of deacetoxycephalosporin C synthase. The role of arginine 258

Deacetoxycephalosporin C synthase is an iron(II) 2-oxoglutaratedependent oxygenase that catalyzes the oxidative ring-expansion of penicillin N to deacetoxycephalosporin C. The wild-type enzyme is only able to efficiently utilize 2-oxoglutarate and 2-oxoadipate as a 2-oxoacid co-substrate. Mutation of arginine 258, the side chain of which forms an electrostatic interaction with the 5-carboxylate of the 2-oxoglutarate co-substrate, to a glutamine residue reduced activity to about 5% of the wild-type enzyme with 2-oxoglutarate. However, other aliphatic 2-oxoacids, which were not co-substrates for the wild-type enzyme, were utilized by the R258Q mutant. These 2-oxoacids "rescued" catalytic activity to the level observed for the wild-type enzyme as judged by penicillin N and G conversion. These co-substrates underwent oxidative decarboxylation as observed for 2-oxoglutarate in the normal reaction with the wild-type enzyme. Crystal structures of the iron(II)- 2-oxo-3-methylbutanoate (1.5 A), and iron(II)-2-oxo-4-methylpentanoate (1.6 A) enzyme complexes were obtained, which reveal the molecular basis for this "chemical co-substrate rescue" and help to rationalize the co-substrate selectivity of 2-oxoglutaratedependent oxygenases.