S-2-Bromo-Acyl-CoA Analogs Are Affinity Labels for the Medium-Chain Acyl-CoA Dehydrogenase from Pig-Kidney

S-2-Br-hexanoyl-CoA and the branched chain isomer S-2-Br-4-methyl-pentanoyl-CoA are affinity labels of the medium-chain acyl-CoA dehydrogenase from pig kidney. The straight chain thioester is both a substrate and an irreversible inhibitor of the dehydrogenase. Inactivation of the enzyme is biphasic and is half-complete in 4 min at pH 6.5, 25°C. Although S-2-Br-hexanoyl-CoA can partially reduce the FAD prosthetic group of the dehydrogenase, inactivation results from attachment of one molecule of inhibitor per subunit of the oxidized enzyme. The branched chain analogue is a very weak substrate of the dehydrogenase (0.1% that of octanoyl-CoA), but is almost as effective an inhibitor of the dehydrogenase. Incubation experiments with [14C]S-2-Br-methyl-pentanoyl-CoA followed by the isolation of radiolabeled peptide show that modification of the active site base, GLU376, is responsible for enzyme inactivation. The data are compatible with a simple nucleophilic attack of the carboxylate base on the C-2 atom of these 2-Br-analogues.