Transsulfuration pathway

Cystathionine is then cleaved by means of the β-elimination of the homocysteine portion of the molecule leaving behind an unstable imino acid, which is attacked by water to form pyruvate and ammonia (catalysed by the metC-encoded cystathionine β-lyase[4]).

The reverse pathway is present in several organisms, including humans, and involves the transfer of the thiol group from homocysteine to cysteine via a similar mechanism.

[5] Humans are auxotrophic for methionine, hence it is called an "essential amino acid" by nutritionists, but are not for cysteine due to the reverse trans-sulfurylation pathway.

All four transsulfuration enzymes require vitamin B6 in its active form (pyridoxal phosphate or PLP).

Members of this family belong to the type I and are:[6] Note: MetC, metB, metZ are closely related and have fuzzy boundaries so fall under the same NCBI orthologue cluster (COG0626).

The reverse transsulfuration pathway depicting the conversion of homocysteine to cysteine in reactions 5 and 6. Reaction 5 is catalyzed by cystathionine beta-synthase while reaction 6 is catalyzed by cystathionine gamma-lyase . The required homocysteine is synthesized from methionine in reactions 1, 2, and 3.