InterPro: IPR005856 Cysteine synthase K/M

This model discriminates cysteine synthases (both CysK and CysM) from cystathionine beta-synthase, a protein found primarily in eukaryotes and carrying a C-terminal CBS domain lacking from this protein. Bacterial proteins lacking the CBS domain but otherwise showing resemblance to cystathionine beta-synthases and considerable phylogenetic distance from known cysteine synthases were excluded.

Cysteine synthase (O-acetylserine (thiol)-lyase) is the enzyme responsible for the formation of cysteine from O-acetyl-serine and hydrogen sulphide with the concomitant release of acetic acid. In bacteria such two forms of the enzyme are known (genes cysK and cysM). CysM differs from CysK in that it can also use thiosulphate instead of sulphide, to produce cysteine thiosulphonate instead of cysteine.