InterPro: IPR001216 Cysteine synthase/cystathionine beta-synthase P-phosphate-binding site

Cysteine synthase (EC:4.2.99.8) (CSase) is the enzyme responsible for the formation of cysteine from O-acetyl-serine and hydrogen sulphide with the concomitant release of acetic acid [1]. In bacteria two such forms of the enzyme are known (genes cysK and cysM). In plants there are also two forms, one located in the cytoplasm and the other in chloroplasts.

Cystathionine beta-synthase (EC:4.2.1.22) that catalyses the first irreversible step in homocysteine transulphuration; the conjugation of homocysteine and serine forming cystathionine, also belongs to this group of sequences [2]. Both synthases are pyridoxal-phosphate dependent and are evolutionary related. The pyridoxal-phosphate attaches to the lysine residue which is part of this signature and which is found in the N-terminal section of these enzymes; the sequence around the lysine residue is highly conserved.