Department of Microbiology and Molecular Genetics, Oklahoma State University, 307 Life Sciences East, Stillwater, OK, 74078, USA.
Photosynth Res. 2013 Oct;116(2-3):215-29. doi: 10.1007/s11120-013-9907-1. Epub 2013 Aug 24.
The ability of PSII to extract electrons from water, with molecular oxygen as a by-product, is a remarkable biochemical and evolutionary innovation. From an evolutionary perspective, the invention of PSII approximately 2.7 Ga led to the accelerated accumulation of biomass in the biosphere and the accumulation of oxygen in the atmosphere, a combination that allowed for the evolution of a much more complex and extensive biosphere than would otherwise have been possible. From the biochemical and enzymatic perspective, PSII is remarkable because of the thermodynamic and kinetic obstacles that needed to have been overcome to oxidize water as the ultimate photosynthetic electron donor. This article focuses on how proton release is an integral part of how these kinetic and thermodynamic obstacles have been overcome: the sequential removal of protons from the active site of H2O-oxidation facilitates the multistep oxidation of the substrate water at the Mn4CaOx, the catalytic heart of the H2O-oxidation reaction. As noted previously, the facilitated deprotonation of the Mn4CaOx cluster exerts a redox-leveling function preventing the accumulation of excess positive charge on the cluster, which might otherwise hinder the already energetically difficult oxidation of water. Using recent results, including the characteristics of site-directed mutants, the role of the second sphere of amino acid ligands and the associated network of water molecules surrounding the Mn4CaOx is discussed in relation to proton transport in other systems. In addition to the redox-leveling function, a trapping function is assigned to the proton release step occurring immediately prior to the dioxygen chemistry. This trapping appears to involve a yet-to-be clarified gating mechanism that facilitates to coordinated release of a proton from the neighborhood of the active site thereby insuring that the backward charge-recombination reaction does not out-compete the forward reaction of dioxygen chemistry during this final step of H2O-oxidation.
PSII 从水中提取电子，同时将分子氧作为副产物，这是一种非凡的生化和进化创新。从进化的角度来看，大约 27 亿年前 PSII 的发明导致了生物圈内生物量的加速积累和大气中氧气的积累，这种组合使得生物界的进化变得更加复杂和广泛，否则是不可能的。从生化和酶学的角度来看，PSII 之所以引人注目，是因为需要克服热力学和动力学障碍才能将水氧化为最终的光合作用电子供体。本文重点介绍质子释放如何成为克服这些动力学和热力学障碍的重要组成部分：从 H2O 氧化的活性位点连续去除质子有助于在 Mn4CaOx 上逐步氧化底物水，Mn4CaOx 是 H2O 氧化反应的催化核心。如前所述，Mn4CaOx 簇的促进去质子化发挥了氧化还原水平化功能，防止簇上积累过多的正电荷，否则可能会阻碍已经具有挑战性的水氧化。利用最近的结果，包括定点突变的特征、第二氨基酸配体层的作用以及围绕 Mn4CaOx 的相关水分子网络，讨论了质子在其他系统中的传输作用。除了氧化还原水平化功能外，还将质子释放步骤分配给氧化学发生之前的捕获功能。这种捕获似乎涉及到一个尚未阐明的门控机制，该机制促进了从活性位点附近协调释放质子，从而确保在 H2O 氧化的最后一步中，反向电荷重组反应不会与氧化学的正向反应竞争。
