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Are plastocyanin and ferredoxin specific electron carriers or generic redox capacitors? Classical and murburn perspectives on two chloroplast proteins

Within the context of light reaction of photosynthesis, the structure-function correlations of the chloroplast proteins of plastocyanin and ferredoxins (Fd) are analyzed via two perspectives: 1) The Z-scheme, which considers PC/Fd as specific affinity binding-based electron-relay agents, thereby deterministically linking the functions of Cytochrome b6f (Cyt. b6f) and Photosystem I (PS I) to NADP+ reduction by Fd:NADPH oxidoreductase (FNR) via protein-protein contacts and 2) The murburn explanation for oxygenic photophosphorylation, which deems PC/Fd as generic ‘redox capacitors’, temporally accepting and releasing one-electron equivalents in reaction milieu. 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 on wildtype and mutant proteins interactions do not align well with model 1. Furthermore, micromolar physiological concentrations of PC (when Kd values 100 μM) and the non-conducive architecture of chloroplasts render the classical model untenable. In the 2nd model, PC is optional and higher concentrations of PC (sought by model 1) could inhibit ET, quite like the role of cytochrome c of mitochondria and cytochrome b5 of cytoplasmic microsomes. Also, PC is found in both lumen and stroma, and plants lacking PC survive and grow. Thus, evidence from structure, interactive dynamics with redox partners and physiological implications of PC/Fd supports the murburn perspective that these proteins serve as generic redox-capacitors in chloroplasts.