Homocysteine is an amino acid and homologue of cysteine that appears in the body as a result of the degradation of methionine. In mammals, homocysteine is used to biosynthesize cysteine via the following pathway. First the enzyme cystathionine beta-synthetase irreversibly condenses homocysteine with L-serine, forming L-cystathionine. The L-cystathionine is then cleaved by cystathionine gamma-lyase, producing 2-oxobutanoate, L-cysteine, and ammonia. The 2-oxobutanoate is further broken down via the 2-oxobutanoate degradation pathway, producing citric acid cycle intermediates, while the L-cysteine goes to the cysteine metabolism pathway. The homocysteine degradation pathway composes a part of the larger methionine metabolism pathway.

Homocysteine Degradation References