Cysteine formation from homocysteine

Transsulfuration is the interconversion of homocysteine and cysteine, and it fully takes place in bacteria and some plants and fungi. Animals however have only one direction of this bidirectional path, the synthesis of cysteine from homocysteine via cystathionine. Because excess cysteine is degraded to hydrogen sulfide, which is now known as a neuromodulator and smooth muscle relaxant, this pathway is also the main source of its production, which takes place in the cytosol, as well as in extracellular space.

Summary from Reactome (R-HSA-1614603), read 2026-09-23.