Hydride Transfer Reaction Catalyzed by Hyperthermophilic Dihydrofolate Reductase Is Dominated by Quantum Mechanical Tunneling and Is Promoted by Both Inter- and Intramonomeric Correlated Motions
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- 作者：Jiayun Pang ; Jingzhi Pu ; Jiali Gao ; Donald G. Truhlar ; and Rudolf K. Allemann
- 刊名：Journal of the American Chemical Society
- 出版年：2006
- 出版时间：June 21, 2006
- 年：2006
- 卷：128
- 期：24
- 页码：8015 - 8023
- 全文大小：467K
- 年卷期：v.128,no.24(June 21, 2006)
- ISSN：1520-5126
文摘
Simulations of hydride and deuteride transfer catalyzed by dihydrofolate reductase from thehyperthermophile Thermotoga maritima (TmDHFR) are presented. TmDHFR was modeled with its activehomodimeric quaternary structure, where each monomer has three subdomains. The potential energyfunction was a combined quantum mechanical and molecular mechanical potential (69 atoms were treatedquantum mechanically, and 35 287, by molecular mechanics). The calculations of the rate constants byensemble-averaged variational transition state theory with multidimensional tunneling predicted that hydrideand deuteride transfer at 278 K proceeded with 81 and 80% by tunneling. These percentages decreasedto 50 and 49% at 338 K. The kinetic isotope effect was dominated by contributions of bound vibrations anddecreased from 3.0 to 2.2 over the temperature range. The calculated rates for hydride and deuteridetransfer catalyzed by the hypothetical monomer were smaller by approximately 2 orders of magnitude. At298 K tunneling contributed 73 and 66% to hydride and deuteride transfer in the monomer. The decreasedcatalytic efficiency of the monomer was therefore not the result of a decrease of the tunneling contributionbut an increase in the quasi-classical activation free energy. The catalytic effect was associated in thedimer with correlated motions between domains as well as within and between subunits. The intrasubunitcorrelated motions were decreased in the monomer when compared to both native dimeric TmDHFR andmonomeric E. coli enzyme. TmDHFR and its E. coli homologue involve similar patterns of correlatedinteractions that affect the free energy barrier of hydride transfer despite only 27% sequence identity anddifferent quaternary structures.