Dance Ian
School of Chemistry, UNSW Australia, Sydney, 2052, Australia.
J Inorg Biochem. 2017 Apr;169:32-43. doi: 10.1016/j.jinorgbio.2017.01.005. Epub 2017 Jan 5.
The active site of the enzyme nitrogenase is the FeMo-cofactor (FeMo-co), a C-centred FeMoS cluster, connected to the surrounding MoFe protein via ligands Cys and His. Density functional calculations, involving 14 additional surrounding amino acids, focus on His because its mutation causes important reactivity changes, including almost complete loss of ability to reduce N to NH. The Nε side-chain of His is capable of hydrogen bonding to S2B, bridging Fe2 and Fe6 of FeMo-co, believed to be the main reaction domain of nitrogenase. Details are presented for the possible ways in which protonated or deprotonated Nε of His interact with S2B or S2B-H or Fe2 or Fe2-H or Fe-(H). Movements of the His side-chain allow formation of a significant short dihydrogen bond between Nε of His and H on Fe2: Nε-H••H-Fe2, with H-H=1.39Å. It is shown that a 180° rotation of the imidazole ring of His is not able to facilitate transfer of protons from the protein surface to FeMo-co. His is able to move H atoms to and from S2B, and the characteristics of H transfer between S2B and Nε of His are described, together with their dependence on the protonation state of His and the redox state of FeMo-co. The water molecule on the posterior Nδ side of His can mediate proton transfer to and from the side-chain of Tyr. The accumulated results suggest that protonated His could be the agent for the first, most difficult, transfer of H to bound substrate N.
固氮酶的活性位点是铁钼辅因子（FeMo-co），它是以碳为中心的FeMoS簇，通过半胱氨酸（Cys）和组氨酸（His）配体与周围的钼铁蛋白相连。密度泛函计算涉及另外14个周围的氨基酸，重点关注组氨酸，因为其突变会导致重要的反应活性变化，包括将N还原为NH的能力几乎完全丧失。组氨酸的Nε侧链能够与S2B形成氢键，连接FeMo-co的Fe2和Fe6，这被认为是固氮酶的主要反应区域。文中详细介绍了组氨酸的质子化或去质子化的Nε与S2B或S2B-H或Fe2或Fe2-H或Fe-(H)相互作用的可能方式。组氨酸侧链的移动使得组氨酸的Nε与Fe2上的H之间形成了一个显著的短双氢键：Nε-H••H-Fe2，H-H键长为1.39Å。结果表明，组氨酸咪唑环180°的旋转并不能促进质子从蛋白质表面转移到FeMo-co。组氨酸能够将H原子在S2B之间移动，并描述了S2B与组氨酸的Nε之间H转移的特征，以及它们对组氨酸质子化状态和FeMo-co氧化还原状态的依赖性。组氨酸后侧Nδ侧的水分子可以介导质子与酪氨酸侧链之间的转移。累积的结果表明，质子化的组氨酸可能是将H首次转移到结合底物N上最困难步骤的媒介。
