Synthesis of the 1-monoester of 2-ketoalkanedioic acids, for example, octyl α-ketoglutarate

Oxidative cleavage of cycloalkene-1-carboxylates, made from the corresponding carboxylic acids, and subsequent oxidation of the resulting ketoaldehyde afforded the important 1-monoesters of 2-ketoalkanedioic acids. Thus ozonolysis of octyl cyclobutene-1-carboxylate followed by sodium chlorite oxidation afforded the 1-monooctyl 2-ketoglutarate. This is a cell-permeable prodrug form of α-ketoglutarate, an important intermediate in the tricarboxylic acid (TCA, Krebs) cycle and a promising therapeutic agent in its own right.

Structural studies on 2-oxoglutarate oxygenases and related double-stranded β-helix fold proteins

Mononuclear non-heme ferrous iron dependent oxygenases and oxidases constitute an extended enzyme family that catalyze a wide range of oxidation reactions. The largest known sub-group employs 2-oxoglutarate as a cosubstrate and catalysis by these and closely related enzymes is proposed to proceed via a ferryl intermediate coordinated to the active site via a conserved HXD/E...H motif. Crystallographic studies on the 2-oxoglutarate oxygenases and related enzymes have revealed a common double-stranded beta-helix core fold that supports the residues coordinating the iron. This fold is common to proteins of the cupin and the JmjC transcription factor families. The crystallographic studies on 2-oxoglutarate oxygenases and closely related enzymes are reviewed and compared with other metallo-enzymes/related proteins containing a double-stranded beta-helix fold. Proposals regarding the suitability of the active sites and folds of the 2-oxoglutarate oxygenases to catalyze reactions involving reactive oxidizing species are described.

Oxidation by 2-oxoglutarate oxygenases: non-haem iron systems in catalysis and signalling

The 2-oxoglutarate (2OG) and ferrous iron dependent oxygenases are a superfamily of enzymes that catalyse a wide range of reactions including hydroxylations, desaturations and oxidative ring closures. Recently, it has been discovered that they act as sensors in the hypoxic response in humans and other animals. Substrate oxidation is coupled to conversion of 2OG to succinate and carbon dioxide. Kinetic, spectroscopic and structural studies are consistent with a consensus mechanism in which ordered binding of (co)substrates enables control of reactive intermediates. Binding of the substrate to the active site triggers the enzyme for ligation of dioxygen to the metal. Oxidative decarboxylation of 2OG then generates the ferryl species thought to mediate substrate oxidation. Structural studies reveal a conserved double-stranded beta-helix core responsible for binding the iron, via a 2His-1carboxylate motif and the 2OG side chain. The rigidity of this core contrasts with the conformational flexibility of surrounding regions that are involved in binding the substrate. Here we discuss the roles of 2OG oxygenases in terms of the generic structural and mechanistic features that render the 2OG oxygenases suited for their functions.

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.

Purification and characterization of clavaminate synthase from Streptomyces clavuligerus: an unusual oxidative enzyme in natural product biosynthesis

A pivotal step in the biosynthetic pathway to the beta-lactamase inhibitor clavulanic acid is the conversion of proclavaminic acid to clavaminic acid in a reaction requiring Fe2+, alpha-ketoglutarate, and oxygen [Elson, S. W., Baggaley, K. H., Gillett, J., Holland, S., Nicholson, N. H., Sime, J. T., & Woroniecki, S. R. (1987) J. Chem. Soc., Chem. Commun., 1736-1738]. Clavaminate synthase, the enzyme that catalyzes this oxidative cyclization/desaturation, has been purified to homogeneity from clavulanic acid producing cells of Streptomyces clavuligerus (ATCC 27064). The enzyme behaved as a monomer during gel filtration and migrated with Mr 47,000 during denaturing gel electrophoresis. After ion-exchange FPLC two active forms of the protein were resolved that differed slightly in kinetic constants and apparent molecular weight. Kinetic comparisons with the four possible diastereomers of proclavaminate confirmed the absolute configuration of the substrate to be 2S,3R. The stoichiometry of the overall transformation was determined to be proclavaminate + 2(alpha-ketoglutarate) + 2O2----clavaminate + 2(succinate) + 2CO2 + 2H2O. In the absence of proclavaminate a slow decarboxylation of alpha-ketoglutarate to succinate and CO2 was observed in an uncoupled reaction which resulted in enzyme inactivation. Steady-state kinetic studies were undertaken for an initial description of the enzyme's catalytic cycle. The double-reciprocal plot with alpha-ketoglutarate as the variable substrate was linear; this supports the proposal that two stepwise oxidations of proclavaminate occur, each with the consumption of alpha-ketoglutarate and oxygen and the release of succinate, CO2, and H2O. The intersecting initial velocity plots obtained from pairwise variation of substrate concentrations were consistent with a sequential kinetic mechanism for the first oxidation. Similarities observed between clavaminate synthase and alpha-ketoglutarate-dependent dioxygenases argue for a common mechanism of oxygen activation. However, the nature of the interactions of the substrates in the active site of clavaminate synthase apparently redirects the conventional hydroxylase activity of dioxygenases to the construction of a strained bicyclic skeleton driven by the overall reduction of dioxygen.