Adenosine 5′-Triphosphate-Yielding Pathways of Branched-Chain Amino Acid Fermentation by a Marine Spirochete

Abstract

The metabolic pathways utilized by an obligately anaerobic marine spirochete (strain MA-2) to ferment branched-chain amino acids were studied. The spirochete catabolized l -leucine to isovaleric acid, l -isoleucine to 2-methylbutyric acid, and l -valine to isobutyric acid, with accompanying CO 2 production in each fermentation. Cell extracts of spirochete MA-2 converted l -leucine, l -isoleucine, and l -valine to 2-ketoisocaproic, 2-keto-3-methylvaleric, and 2-ketoisovaleric acids, respectively, through mediation of 2-ketoglutarate-dependent aminotransferase activities. The branched-chain keto acids were decarboxylated and oxidized to form isovaleryl coenzyme A, 2-methylbutyryl coenzyme A, and isobutyryl coenzyme A, respectively, in the presence of sulfhydryl coenzyme A and benzyl viologen. The acyl coenzyme A's were converted to acyl phosphates by phosphate branched-chain acyltransferase enzymatic activities. Branched-chain fatty acid kinase activities catalyzed formation of isovaleric, 2-methylbutyric, and isobutyric acids from isovaleryl phosphate, 2-methylbutyryl phosphate, and isobutyryl phosphate, respectively. Adenosine 5′-triphosphate was formed during conversion of branched-chain acyl phosphates to branched-chain fatty acids. The results indicate that conversion of l -leucine, l -isoleucine, and l -valine to branched-chain fatty acids by spirochete MA-2 results in adenosine 5′-triphosphate generation. The metabolic pathways utilized for this conversion involve amino acid amino-transferase, 2-keto acid oxidoreductase, phosphate acyltransferase, and fatty acid kinase activities.