半胱氨酸脱硫酶的生化特性及其脱硫作用机制

Biochemical Properties of Cysteine Desulfurase and Its Mechanism for the Desulfuration of L-Cysteine

半胱氨酸脱硫酶是一类依赖磷酸吡哆醛的酶,为同质二聚体,能催化L-半胱氨酸脱硫生成L-丙氨酸和硫。NifS首先在棕色固氮菌中发现,并被认为在固氮酶铁硫簇的形成中起重要作用。NifS同系物也存在于许多非固氮的原核及真核生物中,它们在分子质量、分光光度特性、底物选择性、氨基酸序列和生物学功能等方面非常相似。根据氨基酸序列的相似性,将NifS同系物分成二组,Ⅰ组包括NifS和IscS等,Ⅱ组包括CSD和CsdB等。半胱氨酸脱硫酶均具有保守的赖氨酸和半胱氨酸残基,前者与PLP形成Schiff碱,后者参与半胱氨酸过硫化物中间物形成。半胱氨酸脱硫酶的作用机理涉及半胱氨酸残基亲核攻击底物L-半胱氨酸上的巯基,形成与酶结合的半胱氨酸过硫化物中间物,该过硫化物中间物作为供硫体,参与生物素、硫胺素、钼碟呤以及铁硫簇、硫代核苷等含硫分子的生物合成。

Cysteine desulfurases are pyridoxal phosphate dependent homodimeric enzymes that catalyze the desulfuration of L-cyste- ine to yield L-alanine and free sulfur. The enzyme Nips was first identified in Azotobacter vinelandii, indicating that it might serve a general role in the formation of Fe-S clusters in nitrogenase. Nips homologs also occur in many nondiazotrophic prokaryotes and eukaryotes, and they are very similar in molecular weight, spectrophotometric property, substrate specificity, amino acid sequence, and function to the A. vinelandii NIPS. On the basis of sequence similarity relationships, the NiPS homologs are divided into groups Ⅰ and Ⅱ. NiPS and IscS are group Ⅰ enzymes, whereas CSD and CsdB are members of group Ⅱ. All cysteine desulfurases contain a conserved lyslne that forms a Schlff base with the PLP cofactor in the resting state and a conserved catalytic cysteine involved in transient persulfide formation. The mechanism for desulfuration of L-cysteine catalyzed by cysteine desulfurase involves the cysteine residue acting as a nucleophile to attack the sulfhydryl group of substrate L-cysteine, and resulting in the formation of an enzymebound cysteine persulfide intermediate. The persulfide intermediate served as sulfur donor is subsequently incorporated into the biosynthetic pathways of sulfur-containing biomolecules such as biotin, thiamine, and molybdopterin, as well as Fe-S clusters in proteins and thionucleosides in tRNA.