Waksman Institute of Microbiology, Rutgers University, Piscataway, New Jersey 08854, United States.
Department of Chemistry & Chemical Biology, Rutgers University, Piscataway, New Jersey 08854, United States.
J Phys Chem B. 2022 May 5;126(17):3257-3268. doi: 10.1021/acs.jpcb.2c00749. Epub 2022 Apr 21.
All contemporary oxygenic phototrophs─from primitive cyanobacteria to complex multicellular plants─split water using a single invariant cluster comprising MnCaO (the ) as the catalyst within photosystem II, the universal oxygenic reaction center of natural photosynthesis. This cluster is unstable outside of PSII and can be reconstituted, both in vivo and in vitro, using elemental aqueous ions and light, via photoassembly. Here, we demonstrate the first functional substitution of manganese in any oxygenic reaction center by in vitro photoassembly. Following complete removal of inorganic cofactors from cyanobacterial photosystem II microcrystal (PSIIX), photoassembly with free cobalt (Co), calcium (Ca), and water (OH) restores O evolution activity. Photoassembly occurs at least threefold faster using Co versus Mn due to a higher quantum yield for PSIIX-mediated charge separation (P*): Co → P* → CoQ. However, this kinetic preference for Co over native Mn during photoassembly is offset by significantly poorer catalytic activity (∼25% of the activity with Mn) and ∼3- to 30-fold faster photoinactivation rate. The resulting reconstituted Co-PSIIX oxidizes water by the standard four-flash photocycle, although they produce 4-fold less O per PSII, suggested to arise from faster charge recombination (CoQ ← CoQ) in the catalytic cycle. The faster photoinactivation of reconstituted Co-PSIIX occurs under anaerobic conditions during the catalytic cycle, suggesting direct photodamage without the involvement of O. Manganese offers two advantages for oxygenic phototrophs, which may explain its exclusive retention throughout Darwinian evolution: significantly slower charge recombination (MnQ ← MnQ) permits more water oxidation at low and fluctuating solar irradiation (greater net energy conversion) and much greater tolerance to photodamage at high light intensities (Mn is less oxidizing than Co). Future work to identify the chemical nature of the intermediates will be needed for further interpretation.
所有现代的需氧光合作用生物——从原始蓝藻到复杂的多细胞植物——都在光合作用的通用需氧反应中心 PSII 中使用包含 MnCaO（ ）的单一不变簇作为催化剂来分解水。该簇在 PSII 之外不稳定，可通过光组装，在体内和体外使用元素水离子和光进行重组。在这里，我们通过体外光组装证明了第一个含氧反应中心中锰的功能替代。在从蓝藻 PSII 微晶（PSIIX）中完全去除无机辅因子后，使用游离钴（Co）、钙（Ca）和水（OH）进行光组装可恢复 O 释放活性。由于 PSIIX 介导的电荷分离（P*）的量子产率更高（Co→P*→CoQ），因此 Co 比 Mn 的光组装速度至少快三倍。然而，在光组装过程中 Co 对天然 Mn 的这种动力学偏好被显著较差的催化活性（Mn 的活性约为 25%）和约 3 到 30 倍更快的光失活率所抵消。由此产生的重组 Co-PSIIX 通过标准的四闪光光循环氧化水，尽管它们每 PSII 产生的 O 少 4 倍，这被认为是由于催化循环中更快的电荷复合（CoQ←CoQ）所致。在催化循环中厌氧条件下，重组 Co-PSIIX 的更快光失活发生，这表明没有 O 的直接光损伤。锰为需氧光合作用生物提供了两个优势，这可能解释了它在达尔文进化中被完全保留的原因：MnQ←MnQ 更快的电荷复合允许在低和波动的太阳辐射下进行更多的水氧化（更高的净能量转换），并且在高光强下对光损伤的耐受性更高（Mn 的氧化性比 Co 小）。为了进一步解释，需要进行识别中间产物化学性质的进一步研究。
