The biosynthesis of cysteine begins with aspartate being phosphorylated into L-aspartyl-4-phosphate through an ATP driven aspartate kinase. L-aspartyl-4-phosphate is then catabolized through an NADPH dependent Aspartic Beta-Semiladehyde dehydrogenase resulting in the release of L-aspartate semialdehyde which is transformed into a homoserine through a Homoserine dehydrogenase. Homeserine in turn is acetylated through a homoserine O-trans-acetylase resulting in the release of O-acetyl-L-homoserine. The latter compound interacts with hydrogen sulfide through a O-acetylhomoserine (thiol)-lyase resulting in the release of L-homocysteine. L-homocysteine reacts with serine through a cystathionine beta synthase resulting in the release of water and L-cystathionine. This compound in turn can be turned into cysteine by reacting with water through a cystathionine gama-lyase. Cysteine can be turned back to L-cystathionine by reacting with a acetyl-L-homoserine spontaneously, thus resulting in L-cystathionine. Cysteine can also be degraded by reacting with a cystathionine gamma lyase resulting in the release of hydrogen sulfide, a hydrogen ion and 2-aminoprop-2-enoate which can spontaneously be converted into 2-iminopropanoate and further degraded into pyruvic acid.

Cysteine Metabolism References