The controlled relay of multiple protons required at the active site of nitrogenase
The controlled relay of multiple protons required at the active site of nitrogenase
复制标题
DOI：
10.1039/c2dt30518f
复制
发表时间：
2012-01-01
影响因子：
4
通讯作者：
Dance, Ian
中科院分区：
文献类型：
--
作者：
Dance, Ian
固氮酶在还原天然和非天然底物时，每个化学催化循环需要大量的质子。催化位点的活性表面(FEMO-辅因子，FEMO-co)位于蛋白质结构域中，该结构域主要是疏水和无水的，不能连续提供多个质子。通过对已有的高质量蛋白质晶体结构的详细分析，描述了从蛋白质表面到FeMo-co的关键硫原子(S3B)的水分子链的特征。从表面向内的水链的前半部分是分支的，略有变化，能够容纳外来的小分子：这被称为质子湾。后半部分，从质子湾到S3B，由8个氢键水分子组成的单链。这一部分是严格保守的，并与高柠檬酸的氢键密切相关，高柠檬酸是螯合钼的重要成分。这就是质子线，并提出了一个详细的格洛图斯机制，通过这个质子线将质子串联转移到S3B。这种受控制的从蛋白质表面到S3B的连续质子传递是固氮酶完整化学机制的分子内氢化范例的重要组成部分。每个到达S3B的质子在电子转移到FeMo-CO的激发下成为氢原子，该氢原子迁移到FeMo-CO活性表面的其他成分并结合底物和中间体，从而允许随后的多次质子沿着质子线转移。建议对所提出的质子供应机制进行实验验证。固氮酶中的水链与细胞色素C氧化酶的质子泵途径类似。
The enzyme nitrogenase, when reducing natural and unnatural substrates, requires large numbers of protons per chemical catalytic cycle. The active face of the catalytic site (the FeMo-cofactor, FeMo-co) is situated in a protein domain which is largely hydrophobic and anhydrous, and incapable of serial provision of multiple protons. Through detailed analysis of the high quality protein crystal structures available the characteristics of a chain of water molecules leading from the protein surface to a key sulfur atom (S3B) of FeMo-co are described. The first half of the water chain from the surface inwards is branched, slightly variable, and able to accommodate exogenous small molecules: this is dubbed the proton bay. The second half, from the proton bay to S3B, is comprised of a single chain of eight hydrogen bonded water molecules. This section is strictly conserved, and is intimately involved in hydrogen bonds with homocitrate, an essential component that chelates Mo. This is the proton wire, and a detailed Grotthuss mechanism for serial translocation of protons through this proton wire to S3B is proposed. This controlled serial proton relay from the protein surface to S3B is an essential component of the intramolecular hydrogenation paradigm for the complete chemical mechanisms of nitrogenase. Each proton reaching S3B, instigated by electron transfer to FeMo-co, becomes a hydrogen atom that migrates to other components of the active face of FeMo-co and to bound substrates and intermediates, allowing subsequent multiple proton transfers along the proton wire. Experiments to test the proposed mechanism of proton supply are suggested. The water chain in nitrogenase is comparable with the purported proton pumping pathway of cytochrome c oxidase.