Figure 4.

A schematic overview of the association between homocysteine and H2S is represented. Homocysteine is biosynthesized from methionine by S-adenosyl methionine (SAM) synthetase, methyltransferase (MT), and s-adenosyl-L-homocysteine (SAH) hydrolase. Hcy can be either remethylated to methionine (see Figure 2) or trans-sulfurated to cysteine under the catalysis of cystathionine beta-synthase (CBS) and cystathionine gamma-lyase (CSE) that requires vitamin B6 as a cofactor. Hcy and cysteine are substrates for H2S production, and the generation of H2S is catalyzed by CBS, CSE, and 3-mercaptopyruvate sulfurtransferase (MST).