Electron transfers amongst cytochrome f, plastocyanin and photosystem I: kinetics and mechanisms

Abstract

The review covers the theory and practice of the determination of kinetic constants for the electron transfer reactions in chloroplast thylakoid membranes between plastocyanin and cytochrome f in cytochrome bf complexes, and between plastocyanin and the reaction centre of photosystem I. Effects of ionic strength and pH are featured. The contribution of mutant studies is included. It is concluded that nearly all data from in vitro experiments can be interpreted with a reaction scheme in which an encounter complex between donor and acceptor is formed by long-range electrostatic attraction, followed by rearrangement during which metal centres become close enough for rapid intra-complex electron transfer. In vivo experiments so far cast doubt on this particular sequence, but their interpretation is not straightforward. Means of modelling the bimolecular complex between cytochrome f and plastocyanin are outlined, and two likely structures are illustrated. The complex formed by plastocyanin and photosystem I in higher plants involves the PsaF subunit, but its structure has not been fully determined.

Backbone Dynamics of Plastocyanin in Both Oxidation States

“…where S f 2 and S s 2 are the squares of the order parameters characterizing the amplitude of the internal motions for the fast (Ͻ20 ps) and slow (Ͼ20 ps) time scales, respectively; 4 , and is the angle between the N-H bond vector and the unique axis of the principal frame of the diffusion tensor, D. S 2 ϭ S f 2 S s 2 is the square of the generalized order parameter for internal motion. In order to take into account the presence of a contribution to the experimental 15 N R 2 relaxation rate from conformational exchange processes, an additional adjustable parameter (R ex ), which sums up to the value of R 2 calculated taking into account only the dipolar and chemical shift anisotropy contributions, may be introduced.…”

“…Plastocyanins are copper-containing electron transfer proteins that shuttle electrons between the reduced cytochrome f of the membrane-bound b 6 f complex and the photo-oxidized chlorophyll special pair P700 ϩ of photosystem I during oxygenic photosynthesis in green algae, higher plants, and cyanobacteria (1)(2)(3)(4). In plastocyanins, the Cu(II)/Cu(I) ion resides in the so-called type I copper center, characterized by the presence of one cysteine and two histidine residues strongly bound to copper in a trigonal plane (5)(6)(7).…”

“…Plastocyanin is the only soluble electron carrier between cytochrome b 6 f and photosystem I in higher plants (4). Depending on the organism and the copper levels in the growth medium, cytochrome c 6 can replace plastocyanin in some cyanobacteria and most eukaryotic algae (9,10).…”

The Unique Proline of the Prochlorothrix hollandica Plastocyanin Hydrophobic Patch Impairs Electron Transfer to Photosystem I

“…Whereas higher plants produce just Pc, there is a number of intermediate species, both cyanobacteria and eukaryotic algae, that are able to synthesize cytochrome c 6 as an alternative redox carrier under copper deficiency (4). The interaction between these two metalloproteins and PSI has been studied by laser-flash absorption spectroscopy in a wide variety of evolutionarily differentiated organisms, including prokaryotic and eukaryotic systems (5,6). All these comparative kinetic analyses have allowed us to propose different reaction mechanisms for PSI reduction (7,8).…”

Computer Simulation of Protein-Protein Association in Photosynthesis

“…In photosynthesis of higher plants the electron flow passes through mobile electron carrier proteins plas-tocyanin (Pc) and ferredoxin (Fd). Pc, a small copper protein, makes transient complexes with cytochrome f of cytochrome bf complex (Cyt f ) and with the luminal part of photosystem I (PSI) shuttling electrons from cytochrome bf complex to P700, the reaction center of PSI [10] in the luminal space of chloroplasts. Fd diffuses in the chloroplasts stromal space and transfers electrons from PSI to ferredoxin-NADP + reductase (FNR), forming transient complexes with the stromal part of PSI and FNR.…”