Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8654, Japan.
Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan.
J Phys Chem B. 2022 Apr 28;126(16):3059-3066. doi: 10.1021/acs.jpcb.2c01320. Epub 2022 Apr 18.
Most low-potential FeS clusters exist in the conserved binding sequence CxxCxxC (CCC). Fe(II) and Fe(III) at the first (C) and third (C) cysteine ligand sites form a mixed-valence Fe···Fe pair in the reduced Fe(II)Fe(III) cluster. Here, we investigate the mechanism of how the conserved protein environment induces mixed-valence pair formation in the FeS clusters, F, F, and F in photosystem I, using a quantum mechanical/molecular mechanical approach. Exchange coupling between Fe sites is predominantly determined by the shape of the FeS cluster, which is stabilized by the preorganized protein electrostatic environment. The backbone NH and CO groups in the conserved CxxCxxC and adjacent helix regions orient along the Fe···Fe axis, generating an electric field and stabilizing the Fe(II)Fe(III) state in F and F. The overlap of the d orbitals via -S- (superexchange) is observed for the single Fe(II)···Fe(III) pair, leading to the formation of the mixed-valence Fe···Fe pair. In contrast, several superexchange Fe(II)···Fe(III) pairs are observed in F due to the highly symmetric pair of the CDGPGRGGTC sequences. This is likely the origin of F serving as an electron acceptor in the two electron transfer branches.
大多数低势能 FeS 簇存在于保守的结合序列 CxxCxxC (CCC) 中。位于第一个 (C) 和第三个 (C) 半胱氨酸配体位置的 Fe(II) 和 Fe(III) 形成还原态 Fe(II)Fe(III) 簇中的混合价 Fe···Fe 对。在这里，我们使用量子力学/分子力学方法研究了保守的蛋白质环境如何诱导光合作用系统 I 中的 FeS 簇 F、F 和 F 中形成混合价对的机制。Fe 位点之间的交换耦合主要由 FeS 簇的形状决定，该形状由预组织的蛋白质静电环境稳定。保守的 CxxCxxC 和相邻螺旋区域中的骨架 NH 和 CO 基团沿 Fe···Fe 轴排列，产生电场并稳定 F 和 F 中的 Fe(II)Fe(III) 状态。通过 -S-（超交换）观察到单个 Fe(II)···Fe(III) 对的 d 轨道重叠，导致混合价 Fe···Fe 对的形成。相比之下，由于 CDGPGRGGTC 序列的高度对称对，在 F 中观察到几个超交换 Fe(II)···Fe(III) 对。这可能是 F 在两个电子转移分支中充当电子受体的起源。
