Abstract

In the light reaction of oxygenic photosynthesis, plastocyanin (PC) and ferredoxins (Fd) are small/diffusible redox-active proteins playing key roles in electron transfer/transport phenomena. In the Z-scheme mechanistic purview, they are considered as specific affinity binding-based electron-relay agents, linking the functions of Cytochrome b₆f (Cyt. b₆f), Photosystem I (PS I) and Fd:NADPH oxidoreductase (FNR). The murburn explanation for photolytic photophosphorylation deems PC/Fd as generic ‘redox capacitors’, temporally accepting and releasing one-electron equivalents in reaction milieu. Herein, we explore the two theories with respect to structural, distributional and functional aspects of PC/Fd. Amino acid residues located on the surface loci of key patches of PC/Fd vary in electrostatic/contour (topography) signatures. Crystal structures of four different complexes each of Cyt.f-PC and Fd-FNR show little conservation in the contact-surfaces, thereby discrediting ‘affinity binding-based electron transfers (ET)’ as an evolutionary logic. Further, thermodynamic and kinetic data of wildtype and mutant proteins interactions do not align with Z-scheme. Furthermore, micromolar physiological concentrations of PC and the non-conducive architecture of chloroplasts render the classical model untenable. In the murburn model, as PC is optional, the observation that plants lacking PC survive and grow is justified. Further, the low physiological concentration/distribution of PC in chloroplast lumen/stroma is supported by murburn equilibriums, as higher concentrations would limit electron transfers. Thus, structural evidence, interactive dynamics with redox partners and physiological distribution/role of PC/Fd support the murburn perspective that these proteins serve as generic redox-capacitors in chloroplasts.

Communicated by Ramaswamy H. Sarma

摘要

在有氧光合作用的光反应中，质体蓝素 (PC) 和铁氧还蛋白 (Fd) 是小的/可扩散的氧化还原活性蛋白，在电子转移/传输现象中起关键作用。在 Z 方案机制范围内，它们被认为是基于特异性亲和结合的电子中继剂，将细胞色素b ₆ f (Cyt. b ₆ f)、光系统 I (PS I) 和 Fd:NADPH 氧化还原酶 (FNR)。对光解光磷酸化的 murburn 解释将 PC/Fd 视为通用的“氧化还原电容器”，在反应环境中暂时接受和释放单电子当量。在这里，我们探讨了关于 PC/Fd 的结构、分布和功能方面的两种理论。位于 PC/Fd 关键斑块表面基因座上的氨基酸残基在静电/轮廓（形貌）特征上有所不同。Cyt 的四种不同复合物的晶体结构。F-PC 和 Fd-FNR 在接触面中几乎没有守恒，因此不相信“基于亲和结合的电子转移 (ET)”是一种进化逻辑。此外，野生型和突变蛋白相互作用的热力学和动力学数据与 Z 方案不一致。此外，PC 的微摩尔生理浓度和叶绿体的非导电结构使经典模型站不住脚。在 murburn 模型中，由于 PC 是可选的，因此缺乏 PC 的植物存活和生长的观察是合理的。此外，Murburn 平衡支持 PC 在叶绿体腔/基质中的低生理浓度/分布，因为较高的浓度会限制电子转移。因此，结构性证据，

由 Ramaswamy H. Sarma 传达