PXLP-CBS tetramers condenses HCYS and L-Ser to form L-Cystathionine

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.

甲硫氨酸转化为半胱氨酸的第一步反应由胱硫醚-β-裂合酶催化，该酶向底物添加一个丝氨酸分子。此酶为四聚体，并以两个血红素分子作为辅因子（Janosik 等人，2001年）。CBS 还利用吡哆醛-5'-磷酸盐（PXLP）作为辅因子，在进化上保守的赖氨酸119残基处（Kery 等人，1999年）。胱硫醚-β-合酶样蛋白（CBSL）被认为与CBS具有相似的功能。