First Half-Reaction Mechanism of Nitric Oxide Synthase: The Role of Proton and Oxygen Coupled Electron Transfer in the Reaction by Quantum Mechanics/Molecular Mechanics
First Half-Reaction Mechanism of Nitric Oxide Synthase: The Role of Proton and Oxygen Coupled Electron Transfer in the Reaction by Quantum Mechanics/Molecular Mechanics
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DOI：
10.1021/jp8073199
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发表时间：
2009-01-08
影响因子：
3.3
通讯作者：
Shaik, Sason
中科院分区：
文献类型：
--
作者：
Cho, Kyung-Bin;Carvajal, Maria Angels;Shaik, Sason
采用量子力学/分子力学（QM/MM）计算方法研究了一氧化氮合酶（NOS）的前半反应。只有当铁-氧配合物从外部来源接受一个质子时，才发现能量上可行的精氨酸羟基化途径。在血红素化学中没有考虑过这种形成的物种；它被描述为Por（+中心点）Fe(III)-OOH，其特征是与已知的铁-氢过氧化物化合物0具有相同的分子组成，但具有阳离子自由基卟啉部分。发现反应本身涉及质子偶联电子转移（PCET）和氧偶联电子转移（OCET）步骤，从而形成化合物I和精氨酸的最终单氧化。辅助因子H4B在这一机制中扮演着关键角色，作为电子供体（中性时）和电子吸收体（自由基-阳离子态时），从而在各种耦合质子和氧转移步骤中提供电子转移成分（见图4）。这种机制的各个部分占的许多实验观察,如以下几点:(a)第二个质子的起源提供血红素,(b)复合我的飘忽不定,(c)阴性peroxide-shunt通路的静止半反应,(d)的抑制H4B模拟4-amino-H4B N3由于质子化作用的位置,(e)的角色的188(伊诺编号)和水晶水的活性部位(W115),等等。对不同的机制假设进行了检验和排除，并提出了NOS第二半反应的新机制。
The first half-reaction of nitric oxide synthase (NOS) is investigated by means of quantum mechanical/molecular mechanical (QM/MM) calculations. An energetically feasible arginine hydroxylation path was found only when the iron-oxy complex accepted one proton from an external source. The so formed species has not been considered in heme chemistry; it is described as Por(+center dot)Fe(III)-OOH and is characterized by the same molecular constituency as the more known ferric-hydroperoxi de species, compound 0, but has a cation-radical porphyrin moiety. The reaction itself is found to involve proton coupled electron transfer (PCET) and oxygen coupled electron transfer (OCET) steps en route to the formation of compound I and the ultimate monooxygenation of arginine. The cofactor H4B turns out to be a key player in the mechanism acting alternatively as an electron donor (when neutral) and an electron sink (when in its radical-cation state) and, thereby, providing the electron transfer component in the various coupled proton and oxygen transfer steps (see Scheme 4). The various pieces of this mechanism account for many of the experimental observations, such as the following: (a) the origins of the second proton supplied to the heme, (b) the elusiveness of compound I, (c) the inactivity of peroxide-shunt pathways in NOS first half-reaction, (d) the inhibition of the H4B analogue 4-amino-H4B due to protonation at the N3 position, (e) the roles of Trp 188 (iNOS numbering) and the crystal water at the active site (W115), and so on. Alternative mechanistic hypotheses are tested and excluded, and a new mechanism for the NOS second half-reaction is proposed.