Mechanism of N2O reduction by the μ4-S tetranuclear Cuz cluster of nitrous oxide reductase
Mechanism of N2O reduction by the μ4-S tetranuclear Cuz cluster of nitrous oxide reductase
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DOI：
10.1021/ja055856o
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发表时间：
2006-01-11
影响因子：
15
通讯作者：
Solomon, EI
中科院分区：
文献类型：
--
作者：
Gorelsky, SI;Ghosh, S;Solomon, EI
利用密度泛函理论计算了反应热力学和势能面，探讨了氧化亚氮还原酶的mu(4)-硫化物桥接四核Cu-Z位点还原裂解N-O键的机理。Cu-Z簇提供了一个外源配体结合位点，在完全还原的4Cu(I)状态下，簇关闭了与更强的供体配体的结合，同时通过增强Cu-Z- > N2O的回给使Cu-Z-N2O络合物的形成成为可能。在配体结合位点的两个铜原子（Cu-I和Cu-IV）在酶促功能中起着至关重要的作用，因为这些原子直接参与桥接N2O结合，使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簇的其他原子需要进行广泛的背键，并且对于N2(O)活化的配体对金属的捐赠最小。Cu-Z-N2O配合物中N-O键直接断裂的低反应势垒（18 kcal mol(-1\)）是由于强Cu-IV(2+)- o -键稳定了TS。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TS，并在一定程度上降低了活化能，这取决于质子供体的强度。在N-O键断裂后，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簇返回到完全还原的（4Cu(I)）状态，以备将来的周转。
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-N2O complex through enhanced Cu-Z -> N2O 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 N2O binding, bending the ligand to a configuration that resembles the transition state (TS) and contributing the two electrons for N2O reduction. The other atoms of the Cu-Z cluster are required for extensive back-bonding with minimal a ligand-to-metal donation for the N2(O) activation. The low reaction barrier (18 kcal mol(-1\)) of the direct cleavage of the N-O bond in the Cu-Z-N2O 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 N2O 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.