S-adenosyl-L-homocysteine水解酶催化的策略:过渡态稳定和避免失败的反应。
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胡杨X, Y,阴DH,特纳妈,王,波哈特RT,豪厄尔PL, Kuczera K, Schowen RL
S-adenosyl-L-homocysteine水解酶催化的策略:过渡态稳定和避免失败的反应。
生物化学,2003年2月25日,42 (7):1900 - 9。
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12590576 (在PubMed]
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S-Adenosylhomocysteine水解酶(AdoHcy水解酶)结晶与模拟解决方案包含中间模拟neplanocin束缚在其3 '酮形式活跃的站点所有的四子单元和四个紧密地绑定代数余子式的减少(NADH)状态。酶在封闭的构象,这对应于结构的催化化学反应发生。检查可用的结构的,非常详细的动力学研究[d·J·波特。博伊德,f . l .(1991)生物。266年化学,21616 - 21625。波特·d·J。博伊德,f . l .(1992)生物。267年化学,3205 - 3213。波特,d . j .(1998)生物。化学。268年,66 - 73]表明催化元素的策略AdoHcy水解酶对加速度AdoHcy可逆转换的腺苷(Ado)和同型半胱氨酸(Hcy)。每个亚基的酶,具有substrate-binding域,没有衬底在快速运动相对于四聚物的酶的核心,首先结合底物和停止运动。 Probably concurrently with oxidation of the substrate to its 3'-keto form, the closed active site is "sealed off" from the environment, as indicated by a large (10(8)(-)(9)-fold) reduction in the rate of departure of ligands, a feature that prevents exposure of the labile 3'-keto intermediates to the aqueous environment. Elimination of the 5'-substituent (Hcy in the hydrolytic direction, water in the synthetic direction) generates the central intermediate 4',5'-didehydro-5'-deoxy-3'-ketoadenosine. Abortive 3'-reduction of the central intermediate is prevented by a temporary suspension of all or part of the redox catalytic power of the enzyme during the existence of the central intermediate. The abortive reduction is 10(4)-fold slower than the productive reductions at the ends of the catalytic cycle and has a rate constant similar to those of nonenzymic intramolecular model reactions. The mechanism for suspending the redox catalytic power appears to be a conformationally induced increase in the distance across which hydride transfer must occur between cofactor and substrate, the responsible conformational change again being that which "seals" the active site. The crystal structure reveals a well-defined chain of three water molecules leading from the active site to the subunit surface, which may serve as a relay for proton exchange between solvent and active site in the closed form of the enzyme, permitting maintenance of active-site functional groups in catalytically suitable protonation states.