US EPA

63966 

Journal Article 

Coenzyme A thiol esters of isobutyric, methacrylic, and "beta"-hydroxyisobutyric acids as intermediates in the enzymatic degradation of valine 

Robinson, WG; Nagle, R; Bachhawat, BK; Kupiecki, FP; Coon, MJ 

1957 

Yes 

Journal of Biological Chemistry
ISSN: 0021-9258
EISSN: 1083-351X 

224 

1-11 

Evidence is presented for the occurrence of the following reactions in heart and liver enzyme extracts: (1) the dehydrogenation of isobutyryl coenzyme A (CoA), an intermediate in valine metabolism, to furnish methacrylyl CoA; (2) the hydration of methacrylyl CoA by alpha,beta-unsaturated thiol ester of coenzyme A (acyl CoA) hydrase (crotonase) to yield beta-hydroxyisobutyryl CoA; (3) the loss -of CoA from the latter compound, presumably by the action of a specific deacylase; and (4) the oxidized diphosphopyridine nucleotide-dependent dehydrogenation of j3-hydroxyisobutyrate to yield methylmalonate semialdehyde. 2. In contrast to other alpha,beta-unsaturated CoA thiol esters, methacrylyl CoA readily undergoes spontaneous hydration. After brief incubation of the synthetically prepared compound in the absence of enzyme, the formation of beta-hydroxyisobutyryl CoA was demonstrated by paper chromatography of the thiol esters and of the corresponding hydroxamic acids. The spontaneous hydration is less rapid, however, than that catalyzed by crotonase. 3. The finding that the thiol ester linkage is cleaved by Reaction 3 may account for the ready loss of the resulting free carboxyl group as carbon dioxide at some subsequent metabolic step, as indicated by isotopic studies on valine and isobutyrate metabolism in other laboratories. 4. Beta-Hydroxyisobutyryl CoA is not dehydrogenated by enzyme preparations which attack free beta-hydroxyisobutyrate. The reversibility of Reaction 4 was demonstrated spectrophotometrically by the oxidation of reduced diphosphopyridine nucleotide in the presence of synthetically prepared methyhnalonate semialdehyde.