THEORETICAL MODELING OF COMPRESSION EFFECTS IN ENZYMIC METHYL TRANSFER
THEORETICAL MODELING OF COMPRESSION EFFECTS IN ENZYMIC METHYL TRANSFER
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DOI：
10.1021/ja00335a058
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发表时间：
1984-01-01
影响因子：
15
通讯作者：
WILLIAMS, IH
中科院分区：
文献类型：
--
作者：
WILLIAMS, IH
利用SCF-MO理论在4-31G水平上对催化和非催化过程进行了从头计算，研究了氨通过对称SN2过渡结构取代甲基铵离子并并过程中甲基转移的催化热力学。模型催化剂包括一对He原子，它们位于N.cntdot. cntdot. cntdot. cntdot. c.cntdot .cntdot. cntdot. n轴上，相距固定距离，通过排斥（破坏稳定）相互作用压缩反应体系，以及一个点电荷笼，通过吸引相互作用稳定反应物离子-分子络合物和过渡结构。Schowen关于压缩在甲基转移酶催化中的可能作用的假设被检验。有压缩的模型允许通过底物与催化剂的优先过渡态结合进行催化，但在没有压缩的情况下存在反催化。催化和非催化模型反应的动力学同位素效应与酶和非酶甲基转移的实验同位素效应趋势一致。
The energetics of catalysis of methyl transfer in a degenerate displacement of methylammonium ion by ammonia via a symmetric SN2 transition structure were investigated by ab initio calculations at the 4-31G levels of SCF-MO theory for catalyzed and uncatalyzed processes. The model catalyst comprises a pair of He atoms located a fixed distance apart on the N.cntdot..cntdot..cntdot.C.cntdot..cntdot..cntdot.N axis so as to compress the reacting system by repulsive (destabilizing) interactions and a cage of point charges serving to stabilize both the reactant ion-molecule complex and the transition structure by attractive interactions. Schowen''s hypothesis concerning the possible role of compression in enzymic catalysis of methyl transfer is examined. The model with compression permits catalysis by preferential transition-state binding of the substrate to the catalyst, but in the absence of compression there is anti-catalysis. Kinetic isotope effects calculated for catalyzed and uncatalyzed model reactions are in accord with trends in experimental isotope effects for enzymic and non-enzymic methyl transfers.