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一氧化二氮还原酶的μ4-S四核铜锌簇还原N2O的机制。

Mechanism of N2O reduction by the mu4-S tetranuclear CuZ cluster of nitrous oxide reductase.

作者信息

Gorelsky Serge I, Ghosh Somdatta, Solomon Edward I

机构信息

Department of Chemistry, Stanford University, Stanford, CA 94305, USA.

出版信息

J Am Chem Soc. 2006 Jan 11;128(1):278-90. doi: 10.1021/ja055856o.

Abstract

Reaction thermodynamics and potential energy surfaces are calculated using density functional theory to investigate the mechanism of the reductive cleavage of the N-O bond by the mu(4)-sulfide-bridged tetranuclear Cu(Z) site of nitrous oxide reductase. The Cu(Z) cluster provides an exogenous ligand-binding site, and, in its fully reduced 4Cu(I) state, the cluster turns off binding of stronger donor ligands while enabling the formation of the Cu(Z)-N(2)O complex through enhanced Cu(Z) --> N(2)O back-donation. The two copper atoms (Cu(I) and Cu(IV)) at the ligand-binding site of the cluster play a crucial role in the enzymatic function, as these atoms are directly involved in bridged N(2)O binding, bending the ligand to a configuration that resembles the transition state (TS) and contributing the two electrons for N(2)O reduction. The other atoms of the Cu(Z) cluster are required for extensive back-bonding with minimal sigma ligand-to-metal donation for the N(2)O activation. The low reaction barrier (18 kcal mol(-)(1)) of the direct cleavage of the N-O bond in the Cu(Z)-N(2)O complex is due to the stabilization of the TS by a strong Cu(IV)(2+)-O(-) bond. Due to the charge transfer from the Cu(Z) cluster to the N(2)O ligand, noncovalent interactions with the protein environment stabilize the polar TS and reduce the activation energy to an extent dependent on the strength of proton donor. After the N-O bond cleavage, the catalytic cycle consists of a sequence of alternating protonation/one-electron reduction steps which return the Cu(Z) cluster to the fully reduced (4Cu(I)) state for future turnover.

摘要

利用密度泛函理论计算反应热力学和势能面,以研究一氧化二氮还原酶的μ(4)-硫化物桥连四核铜(Z)位点对N-O键进行还原裂解的机理。铜(Z)簇提供了一个外源配体结合位点,在其完全还原的4Cu(I)状态下,该簇关闭了较强供体配体的结合,同时通过增强的Cu(Z)→N₂O反馈给体作用促进了Cu(Z)-N₂O配合物的形成。簇的配体结合位点上的两个铜原子(Cu(I)和Cu(IV))在酶促功能中起关键作用,因为这些原子直接参与桥连N₂O的结合,将配体弯曲成类似于过渡态(TS)的构型,并为N₂O还原提供两个电子。Cu(Z)簇的其他原子对于N₂O活化所需的广泛反馈键合以及最小化的σ配体到金属的给体作用是必需的。Cu(Z)-N₂O配合物中N-O键直接裂解的低反应势垒(18 kcal mol⁻¹)是由于强Cu(IV)²⁺-O⁻键对过渡态的稳定作用。由于电荷从Cu(Z)簇转移到N₂O配体,与蛋白质环境的非共价相互作用稳定了极性过渡态,并将活化能降低到一定程度,该程度取决于质子供体的强度。N-O键裂解后,催化循环由一系列交替的质子化/单电子还原步骤组成,这些步骤使Cu(Z)簇回到完全还原的(4Cu(I))状态以进行后续周转。

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