Threonine and 2-Oxobutanoate Degradation
As an essential amino acid, threonine is not synthesized in humans and must be obtained from either the diet or from intestinal microflora. In humans, threonine is converted to 2-oxobutanoate via serine dehydratase, and thereby enters the pathway leading to succinyl-CoA. This is the major route for threonine degradation in humans.
2-oxobutanoate, also known as 2-Ketobutyric acid, is a 2-keto acid that is commonly produced in the metabolism of amino acids such as methionine and threonine. Like other 2-keto acids, degradation of 2-oxobutanoate occurs in the mitochondrial matrix and begins with oxidative decarboxylation to its acyl coenzyme A derivative, propionyl-CoA. This reaction is mediated by a class of large, multienzyme complexes called 2-oxo acid dehydrogenase complexes. While no 2-oxo acid dehydrogenase complex is specific to 2-oxobutanoate, numerous complexes can catalyze its reaction. In this pathway the branched-chain alpha-keto acid dehydrogenase complex is depicted. All 2-oxo acid dehydrogenase complexes consist of three main components: a 2-oxo acid dehydrogenase (E1) with a thiamine pyrophosphate cofactor, a dihydrolipoamide acyltransferase (E2) with a lipoate cofactor, and a dihydrolipoamide dehydrogenase (E3) with a flavin cofactor. E1 binds the 2-oxobutanoate to the lipoate on E2, which then transfers the propionyl group to coenzyme A, producing propionyl-CoA and reducing the lipoate. E3 then transfers protons to NAD in order to restore the lipoate. Propionyl-CoA carboxylase transforms the propionyl-CoA to S-methylmalonyl-CoA, which is then converted to R-methylmalonyl-CoA via methylmalonyl-CoA epimerase. In the final step, methylmalonyl-CoA mutase acts on the R-methylmalonyl-CoA to produce succinyl-CoA.
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