How the Electron-Transfer Cascade is Maintained in Chlorophyll‑d Containing Photosystem I

Photosystem I (PSI) from Acaryochloris marina utilizes chlorophyll d (Chld) with a formyl group as its primary pigment, which is more red-shifted than chlorophyll a (Chla) in PSI from Thermosynechococcus elongatus. Using the cryo-electron microscopy structure and solving the linear Poisson–Boltzmann equation, here we report the redox potential (Em) values in A. marina PSI. The Em(Chld) values at the paired chlorophyll site, [PAPB], are nearly identical to the corresponding Em(Chla) values in T. elongatus PSI, despite Chld having a 200 mV lower reduction power. The accessory chlorophyll site, A–1, in the B branch exhibits an extensive H-bond network with its ligand water molecule, contributing to Em(A–1B) being lower than Em(A–1A). The substitution of pheophytin a (Pheoa) with Chla at the electron acceptor site, A0, decreases Em(A0), resulting in an uphill electron transfer from A–1. The impact of the A–1 formyl group on Em(A0) is offset by the reorientation of the A0 ester group. It seems likely that Pheoa is necessary for A. marina PSI to maintain the overall electron-transfer cascade characteristic of PSI in its unique light environment.海洋色球藻（Acaryochloris marina）来源的光系统I（Photosystem I，PSI）以带有甲酰基的叶绿素d（Chlorophyll d，Chld）作为核心色素，其光谱红移程度高于长形热蓝藻（Thermosynechococcus elongatus）PSI中的叶绿素a（Chlorophyll a，Chla）。本研究利用冷冻电子显微镜（cryo-electron microscopy）结构，并通过求解线性泊松-玻尔兹曼方程（linear Poisson–Boltzmann equation），报道了海洋色球藻PSI的氧化还原电位（redox potential，Em）数值。成对叶绿素位点[PAPB]处的Em(Chld)数值，与长形热蓝藻PSI中对应的Em(Chla)数值近乎一致，尽管Chld的还原能力比Chla低200毫伏。B分支中的辅助叶绿素位点A-1，与其配位水分子形成了广泛的氢键网络，这使得Em(A-1_B)低于Em(A-1_A)。在电子受体位点A0处，以Chla替换脱镁叶绿素a（Pheophytin a，Pheoa）会降低Em(A0)，进而引发从A-1位点起始的逆电势电子传递。A-1位点的甲酰基对Em(A0)的影响，被A0位点酯基的重新取向所抵消。海洋色球藻PSI若要在其独特的光照环境中维持PSI完整的电子传递级联特性，脱镁叶绿素a似乎是必不可少的。