2-oxobutanoic acid

α-Ketobutyric acid is a product of the lysis of cystathionine. It is also one of the degradation products of threonine, produced by the catabolism of the amino acid by threonine dehydratase. It is also produced by the degradation of homocysteine and the metabolism of methionine.
α-Ketobutyric acid is transported into the mitochondrial matrix, where it is converted to propionyl-CoA by Branched-chain alpha-keto acid dehydrogenase complex. Further mitochondrial reactions produce succinyl CoA. This is first through the enzyme mitochondria Propionyl-CoA carboxylase with biotin as a cofactor to produce S-methylmalonyl-CoA. This is subsequently converted to R-methylmalonyl-CoA by mitochondrial methylmalonyl-CoA epimerase. Finally, mitochondrial methylmalonyl-CoA mutase with cofactor adenosylcobalamin produces succinyl-CoA which enters the citric acid cycle.