cobamamide and 3-butene-1-2-diol

The reactions of diol dehydratase with 3-unsaturated 1,2-diols and thioglycerol were investigated. Holodiol dehydratase underwent rapid and irreversible inactivation by either 3-butene-1,2-diol, 3-butyne-1,2-diol or thioglycerol without catalytic turnovers. In the inactivation, the Co-C bond of adenosylcobalamin underwent irreversible cleavage forming unidentified radicals and cob(II)alamin that resisted oxidation even in the presence of oxygen. Two moles of 5'-deoxyadenosine per mol of enzyme was formed as an inactivation product from the coenzyme adenosyl group. Inactivated holoenzymes underwent reactivation by diol dehydratase-reactivating factor in the presence of ATP, Mg(2+) and adenosylcobalamin. It was thus concluded that these substrate analogues served as mechanism-based inactivators or pseudosubstrates, and that the coenzyme was damaged in the inactivation, whereas apoenzyme was not damaged. In the inactivation by 3-unsaturated 1,2-diols, product radicals stabilized by neighbouring unsaturated bonds might be unable to back-abstract the hydrogen atom from 5'-deoxyadenosine and then converted to unidentified products. In the inactivation by thioglycerol, a product radical may be lost by the elimination of sulphydryl group producing acrolein and unidentified sulphur compound(s). H(2)S or sulphide ion was not formed. The loss or stabilization of product radicals would result in the inactivation of holoenzyme, because the regeneration of the coenzyme becomes impossible.

Topics: Butylene Glycols; Cobamides; Electron Spin Resonance Spectroscopy; Enzyme Inhibitors; Escherichia coli; Glycerol; Glycols; Kinetics; Models, Molecular; Propanediol Dehydratase

Coenzyme B(12)-dependent glycerol dehydratase is a radical enzyme that catalyses the conversion of glycerol into 3-hydroxypropanal and propane-1,2-diol into propanal via enzyme-bound intermediate radicals. The substrate analogue but-3-ene-1,2-diol was studied in the expectation that it would lead to the 4,4-dihydroxylbut-2-en-1-yl radical, which is stabilised (allylic) and not reactive enough to retrieve a hydrogen atom from 5'-deoxyadenosine, thereby interrupting the catalytic cycle. Racemic and enantiomerically pure but-3-ene-1,2-diols and their [1,1-(2)H(2)], [2-(2)H] and [4,4-(2)H(2)] isotopomers were synthesised and characterised by NMR spectroscopy. (S)-[4-(14)C]but-3-ene-1,2-diol was also prepared. Kinetic measurements showed but-3-ene-1,2-diol to be a competitive inhibitor of glycerol dehydratase (K(i)=0.21 mM, k(i)=5.0x10(-2) s(-1)). With [4-(14)C]but-3-ene-1,2-diol it was demonstrated that species derived from the diol become tightly bound to the enzyme's active site, but not covalently bound, because the radioactivity could be removed upon denaturation of the enzyme. EPR measurements with propane-1,2-diol as substrate generated sharp signals after 10 s that disappeared after about 1 min. In contrast, EPR resonances appeared and disappeared more slowly when but-3-ene-1,2-diol was incubated with the enzyme. Among the deuterated isotopomers, only [1,1-(2)H(2)]but-3-ene-1,2-diol showed a significantly different EPR spectrum from that of the unlabelled diol; this indicated that coupling between the unpaired electron and a deuterium at C-1 was stronger than with deuterium at C-2 or C-4. The experiments suggest the formation of the 1,2-dihydroxybut-3-en-1-yl radical, which decomposes to unidentified product(s).

Topics: Catalytic Domain; Citrobacter freundii; Cobamides; Electron Spin Resonance Spectroscopy; Enzyme Inhibitors; Glycols; Kinetics; Recombinant Proteins; Staining and Labeling; Stereoisomerism; Sugar Alcohol Dehydrogenases; Time Factors