Absolute configuration of the isomer of fluorocitrate that inhibits aconitase

Multiscale analysis of enantioselectivity in enzyme-catalysed 'lethal synthesis' using projector-based embedding

The action of fluoroacetate as a broad-spectrum mammalian pesticide depends on the 'lethal synthesis' of fluorocitrate by citrate synthase, through a subtle enantioselective enolization of fluoroacetyl-coenzyme A. In this work, we demonstrate how a projection-based embedding method can be applied to calculate coupled cluster (CCSD(T)) reaction profiles from quantum mechanics/molecular mechanics optimized pathways for this enzyme reaction. Comparison of pro-R and pro-S proton abstraction in citrate synthase at the CCSD(T)-in-DFT//MM level yields the correct enantioselectivity. We thus demonstrate the potential of projection-based embedding for determining stereoselectivity in enzymatic systems. We further show that the method is simple to apply, eliminates variability due to the choice of density functional theory functional and allows the efficient calculation of CCSD(T) quality enzyme reaction barriers.

Aconitate hydratase of mammals under oxidative stress

Data on the structure, functions, regulation of activity, and expression of cytosolic and mitochondrial aconitate hydratase isoenzymes of mammals are reviewed. The role of aconitate hydratase and structurally similar iron-regulatory protein in maintenance of homeostasis of cell iron is described. Information on modifications of the aconitate hydratase molecule and changes in expression under oxidative stress is generalized. The role of aconitate hydratase in the pathogenesis of some diseases is considered.

Fluorinated natural products: the biosynthesis of fluoroacetate and 4-fluorothreonine in Streptomyces cattleya

Organofluorine compounds are rare in Nature, with only a handful known to be produced by some species of plant and two microorganisms. Consequently, the mechanism of enzymatic carbon-fluorine bond formation is poorly understood. The bacterium Streptomyces cattleya biosynthesises fluoroacetate and 4-fluorothreonine as secondary metabolites and is a convenient system to study the biosynthesis and enzymology of fluorometabolite production. Using stable-isotope labelled precursors it has been shown that there is a common intermediate in the biosynthesis of the fluorometabolites, which has recently been identified as fluoroacetaldehyde. Studies with cell-free extracts of S. cattleya have identified two enzymes, an aldehyde dehydrogenase and a threonine transaldolase, that are involved in the biotransformation of fluoroacetaldehyde to fluoroacetate and 4-fluorothreonine.

Substrate and inhibitor specificity of mevalonate kinase determined with substrate analogues

Substrate analogues of mevalonate kinase were prepared and their kinetic properties determined with the enzyme from pig liver. The analogues are derived from mevalonate by substitutions of hydroxyl and/or methyl at C3(group 1), by systematic prolongation of C6 to yield 3-alkyl-3,5-dihydroxyvalerates (group 2) and by methylation at C2, C6 and C4 (group 3). Compounds of group 1 were inhibitors of mevalonate kinase, their Ki values ranging over 140 mM to 2.5 mM and decreasing in the series CH2 > C(CH3)2 > CH(OH) > CH(CH3) > C(OH)-CO2H > C(OH)CH2OH. Three of these analogues, the competitive inhibitors 3,5-dihydroxyvalerate, 3-methyl-5-hydroxy-valerate and 3,3-dimethyl-5-hydroxyvalerate are alos (poor) substrates of the kinase. Substances of group 2 contain an additional hydrophobic group and are noncompetitive inhibitors with Ki values decreasing systematically from 7.7 mM to 0.7 mM in the sequence 3-n-propyl, 3-n-butyl, 3-n-pentyl and 3-n-hexyl derivative of 3,5-dihydroxyvalerate. The methylated mevalonates of group 3 are competitive inhibitors as well as good substrates of the kinase, their Ki = Km values ranging over 4.3-0.2 mM and their maximal velocities related to mevalonate varying between 7-65%. All of these analogues have in common the carbon chain and 5-hydroxy group of the substrate and could, therefore, fit to the active site of mevalonate kinase and be phosphorylated by ATP. The results, however, indicate a strong dependence of such interaction on the ligands at C3 of the substrate. Enzymic activity as well as affinity are lost if both of these ligands are replaced by hydrogen; partial restoration occurs at a low level if only one of the ligands is concerned. In the presence of the ligands even monomethylation at C2 and C4 yields enzymically active products. Oxidation of C6 of the substrate yields inhibitors but methylation yields a reasonably good substrate. Thus 3,5-dihydroxy-3-ethylvalerate fits into the active site whereas higher C6 homologues are only inhibitory and probably bind to a hydrophobic region of the enzyme. Some of these results are shown to have parallels in the enzymic behaviour of fluorinated mevalonates.

Synthesis and evaluation of (+) and (-)-2,2-difluorocitrate as inhibitors of rat-liver ATP-citrate lyase and porcine-heart aconitase

The enantiomers (+) and (-)-2,2-difluorocitrate have been synthesized. Both are good inhibitors of ATP-citrate lyase, showing competitive inhibition against citrate, with Kis = 0.7 microM for (+)-2,2-difluorocitrate and 3.2 microM for (-)-2,2-difluorocitrate. The inhibition patterns with either ATP or CoA as the varied substrate were uncompetitive and mixed, respectively, but with much weaker inhibition constants. Neither isomer undergoes carbon-carbon bond cleavage as a substrate and there is no evidence of irreversible time-dependent inactivation. When ATP-citrate lyase is incubated with CoA and difluorocitrate, the maximal intrinsic ATPase rate is 10% of the citrate-induced rate for the (+)-enantiomer and 2% for the (-)-enantiomer. 19F-NMR studies confirm that only the (+)-enantiomer is chemically processed. The effects of the difluorocitrate enantiomers on the reaction catalysed by aconitase were examined. (-)-2,2-Difluorocitrate is a competitive inhibitor against citrate (Kis = 1.5 microM), whereas the (+)-enantiomer is a relatively poor mixed inhibitor (Ki greater than 300 microM). The (-)-enantiomer irreversibly inactivates aconitase at 1.1 min-1.mM-1 at 25 degrees C and pH 7.4, whereas no irreversible inhibition is seen with the (+)-enantiomer. Therefore, it would be expected that the (+)-enantiomer would slow the rate of acetyl-CoA synthesis in vivo, without inhibiting the citric acid cycle.

Fluoroacetate and fluorocitrate: mechanism of action

The concept of lethal synthesis as suggested by Peters is reviewed in the light of the more recent work in this area. It is suggested that fluorocitrate is a "suicide" substrate for aconitase rather than a competitive inhibitor as originally suggested. The use of these substances to study glial-neuronal relationships is considered.

Inhibition by aconitine of aconitase of pig heart

The present paper describes the in vitro inhibition type, inhibition constant and inhibition rate of pig heart aconitase by aconitine. The inhibition of aconitase activity by aconitine is totally non-competitive. The datum of inhibition constant, Ki = 0.11 +/- 0.01 mmol/l, indicates high affinity of aconitine by aconitase. The rate constant of inhibition kl = 49 min-1, the half-life for aconitase inhibition of 1 min and the 50% inhibition time of 8 min by 0.1 mmol/l aconitine show high specific inhibition of the enzyme by the inhibitor. These results suggest a possible molecular reason for the toxic and pharmacological actions produced on experimental animals by aconitine.