The first step of homocysteine conversion to cysteine is catalyzed by cystathionine beta-lyase, which adds a serine molecule to the substrate. The enzyme is a tetramer with two heme molecules as cofactor (Janosik et al. 2001). CBS also utilises pyridoxal 5'-phosphate (PXLP) as cofactor, at the evolutionarily conserved Lys119 residue (Kery et al. 1999). The cystathionine beta-synthase-like protein (CBSL) is thought to function like CBS.