BHMT2 tetramer transfers CH3 group from SMM to LHCYS

L-homocysteine (LHCYS) is derived from L-methionine (L-Met) and can either be remethylated to reform L-Met or take part in cysteine biosynthesis via the trans-sulfuration pathway. LHCYS remethylation can occur by the action of two enzymes; cobalamin-dependent methionine synthase and betaine-homocysteine methyltransferase, using methyltetrahydrofolate and betaine respectively as methyl donors. A third enzyme, S-methylmethionine-homocysteine S-methyltransferase (BHMT2), can use S-methylmethionine (SMM) as the methyl donor to methylate LHCYS and reform L-Met. BHMT2 is a tetrameric, cytosolic enzyme that requires one Zn2+ ion per subunit as cofactor (Szegedi et al. 2008).

L-型高半胱氨酸（LHCYS）源自L-型蛋氨酸（L-Met），能够被重新甲基化为L-Met，或通过转硫化途径参与半胱氨酸的生物合成。LHCYS的甲基化作用可通过两种酶的催化进行；即钴胺素依赖性蛋氨酸合成酶和胆碱高半胱氨酸甲基转移酶，分别以甲基四氢叶酸和胆碱作为甲基供体。第三种酶，S-甲基蛋氨酸高半胱氨酸S-甲基转移酶（BHMT2），能以S-甲基蛋氨酸（SMM）为甲基供体，对LHCYS进行甲基化并重新合成L-Met。BHMT2是一种四聚体、细胞质酶，每个亚基需要一价锌离子作为辅因子（Szegedi等，2008年）。