title: Dihydrolipoyl transacetylase of Escherichia coli. Formation of 8-S-acetyldihydrolipoamide.
abstract: The dihydrolipoyl transacetylase component (E2) of the pyruvate dehydrogenase complex catalyzes the reaction of acetyl coenzyme A (acetyl-CoA) with dihydrolipoamide, producing coenzyme A and S-acetyldihydrolipoamide. The acetyl group is shown by experiments reported herein to be bonded to S8 in the enzymatic product. 1H NMR analysis of synthetic samples of both structural isomers of S-acetyl-S-(phenylmercurio)dihydrolipoamide enabled structural assignments to be made. Reaction of 8-S-acetyl-6-S-(phenylmercurio)dihydrolipoamide with 3-mercaptopropionic acid in chloroform produced 8-S-acetyldihydrolipoamide which contained a small amount (5%) of the 6-S isomer. Reaction of 6,8-di-S-acetyldihydrolipoamide with NH2OH produced a 4:1 mixture of 6-S-acetyldihydrolipoamide and the 8-S isomer. These compounds did not isomerize at significant rates in chloroform but rapidly isomerized to the equilibrium mixture in aqueous solution (Keq = 3.4). The second-order rate constants for the hydroxide-catalyzed isomerization were found to be kf = (1.15 +/- 0.07) X 10(6) M-1 X s-1 and kr = (3.36 +/- 0.20) X 10(5) M-1 X s-1 in the direction of the formation of the 8-S isomer. The enzymatic product was trapped by addition of phenylmercuric hydroxide within 15 s-30 min after starting the reaction. 1H NMR analysis of the products obtained at various times showed that the enzymatic product was 8-S-acetyldihydrolipoamide, which underwent progressive isomerization to the mixture of isomers within a few minutes. In the reaction of acetyl-CoA with dihydrolipoamide, the latter substrate reacts in place of enzyme-bound dihydrolipoyl moieties. Therefore, acetylation occurs at the 8-S position of bound lipoyl groups.
mesh_terms: D000123:Acetyltransferases / Q000378:metabolism*; D051047:Dihydrolipoyllysine-Residue Acetyltransferase; D004926:Escherichia coli / Q000201:enzymology*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D011768:Pyruvate Dehydrogenase Complex / Q000378:metabolism; D013053:Spectrophotometry; D008063:Thioctic Acid / Q000031:analogs & derivatives* / Q000378:metabolism
chemical_list: D011768:Pyruvate Dehydrogenase Complex; C051702:8-acetyldihydrolipoamide; D008063:Thioctic Acid; D000123:Acetyltransferases; D051047:Dihydrolipoyllysine-Residue Acetyltransferase