Mechanisms of quinol oxidation in photosynthesis

Abstract

The mechanisms by which para-benzoquinols can be oxidized is reviewed. Emphasis is placed on the information available from chemical and electrochemical studies which may provide insight into the biochemical mechanisms of plastoquinol oxidation in the chloroplast. Three mechanisms of quinol oxidation are possible: (1) The removal of an electron from the quinol, QH inf2 (sup·t) , directly to produce the radical cation, QH 2 (·+) . This may be achieved electrochemically only at very high potential in acidic media. The reaction may be of relevance to D1, the donor to P-680. (2) The removal of an electron from the anionic quinol. QH(-), formed by quinol deprotonation. It is likely that the catalytic mechanism of the cytochrome bf complex involves this mechanism. (3) The removal of an electron from the dianionic quinol, Q(2-). This route will be dominant only under basic or aprotic conditions and at very low potentials.

Structure of the Cytochrome b6f Complex: Quinone Analogue Inhibitors as Ligands of Heme cn

“…Heme c n has no amino acid side-chain serving as an axial ligand, but only an axial H 2 O that bridges the 4 Å distance between the Fe atom of heme c n and a propionate oxygen of heme b n . 8,9 In retrospect, it was realized that heme c n was previously defined spectrophotometrically and designated as component G.. 30,31 The position of heme c n in close proximity to the previously welldefined heme b n that has been proposed to function in a "Q cycle" [32][33][34][35][36][37][38][39][40] and alternatively, or as well, in a ferredoxin-linked cyclic pathway, 7-9 begs the question of the location of the plastoquinone that should provide the electron acceptor in these pathways.…”

Kinetics of the plastoquinone pool oxidation following illumination

“…The produced PQ ▪− could either dismutate, or be oxidized by dioxygen The difference of redox potentials of pairs PQ/PQ ▪− , −170 mV, and , −160 mV, is not high. The dismutation is possibly more significant due to high‐equilibrium constant, 10 10 [17], however, it can be partially hampered by an electrostatic repulsion (p K of PQ ▪− is 7.0) as well as the distance between PQ ▪− molecules formed.…”

“…The difference of redox potentials of pairs PQ/PQ ÅÀ , À170 mV, and O 2 =O ÅÀ 2 , À160 mV, is not high. The dismutation is possibly more significant due to high-equilibrium constant, 10 10 [17], however, it can be partially hampered by an electrostatic repulsion (pK of PQ ÅÀ is 7.0) as well as the distance between PQ ÅÀ molecules formed.…”

Antioxidant and signaling functions of the plastoquinone pool in higher plants

“…Plastosemiquinone and plastohydroquinone have the different pK values and the ratio of the protonated and deprotonated forms depends on pH, which is different on the stromal vs luminal surface of the thylakoid membrane. The redox couples with the involvement of the plastoquinone forms with different degrees of ionization have different redox potentials at different pH values (Hauska et al , Rich ). Thus the possible pH changes at both surfaces of thylakoid membrane under stress conditions should be also taken into consideration.…”