Rabeh Wael M, Cook Paul F
Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, USA.
J Biol Chem. 2004 Jun 25;279(26):26803-6. doi: 10.1074/jbc.R400001200. Epub 2004 Apr 8.
The O-acetylserine sulfhydrylase (OASS) from Salmonella typhimurium catalyzes a beta-replacement reaction in which the beta-acetoxy group of O-acetyl-L-serine (OAS) is replaced by bisulfide to give L-cysteine and acetate. The kinetic mechanism of OASS is ping-pong with a stable alpha-aminoacrylate intermediate. The enzyme is a homodimer with one pyridoxal 5'-phosphate (PLP) bound per subunit deep within the protein in a cleft between the N- and C-terminal domains of each of the monomers. All of the active site residues are contributed by a single subunit. The enzyme cycles through open and closed conformations as it catalyzes its reaction with structural changes largely limited to a subdomain of the N-terminal domain. The elimination of acetic acid from OAS is thought to proceed via an anti-E2 mechanism, and the only catalytic group identified to date is lysine 41, which originally participates in Schiff base linkage to PLP. The transition state for the elimination of acetic acid is thought to be asynchronous and earlier for Cbeta-O bond cleavage than for Calpha-H bond cleavage.
鼠伤寒沙门氏菌的O-乙酰丝氨酸巯基酶（OASS）催化一种β-取代反应，其中O-乙酰-L-丝氨酸（OAS）的β-乙酰氧基被二硫化物取代，生成L-半胱氨酸和乙酸盐。OASS的动力学机制为乒乓机制，具有稳定的α-氨基丙烯酸酯中间体。该酶是一种同型二聚体，每个亚基在每个单体的N端和C端结构域之间的裂隙中，于蛋白质内部深处结合一个磷酸吡哆醛（PLP）。所有活性位点残基均由单个亚基提供。该酶在催化反应时会经历开放和闭合构象的循环，其结构变化主要局限于N端结构域的一个亚结构域。OAS中乙酸的消除被认为是通过反E2机制进行的，迄今为止鉴定出的唯一催化基团是赖氨酸41，它最初参与与PLP的席夫碱连接。乙酸消除的过渡态被认为是异步的，且Cβ-O键断裂比Cα-H键断裂更早。
