Dynamic metabolism of photosystem II reaction center proteins and pigments

Abstract

Photosystem II (PSII) reaction center is an intrinsic membrane‐protein complex in the chloroplast that catalyzes primary charge separation between P680, a chlorophyll a dimer, and the primary quinone acceptor QA. This supramolecular protein complex consists of D1, D2, α and β subunits of cytochrome b559, the psbI gene product, and a few low molecular mass proteins. Ligated to this complex are pigments: chlorophyll a, pheophytin a, β‐carotenes, and non‐heme iron. One of the major outcomes of light‐mediated photochemistry is the fact that in the light, D1 protein is rapidly turned over compared to the other proteins of the reaction center; the relative lability of proteins being: D1≫D2>Cyt b559. D1 degradation in visible light exhibits complex, multiphasic kinetics. D1 degradation can be uncoupled from photosynthetic electron transport, which suggests that degradation may perform some separate function(s) beyond maintaining photosynthetic activity. The presence of a physiologically relevant level of ultraviolet‐B (UV‐B) radiation in a background of photosynthetically active radiation stimulates D1/D2 heterodimer degradation in a synergistic manner. D1 undergoes several post‐translational modifications including N‐acetylation, phosphorylation, and palmitoylation. Light‐dependent phosphorylation of D1 occurs in all flowering plants but not in the green alga Chlamydomonas or in cyanobacteria, and the same may be true for D2. The roles of these modifications in D1/D2 assembly, turnover, or function are still a matter of conjecture. Nor do we yet know about the fate of the liganded pigments, such as the chlorophyll and carotenoids bound to the reaction center proteins. Environmental extremes that negatively impact photosynthesis seem to involve D1 metabolism. Thus, D1 protein is a major factor of PSII instability, and its replacement after its degradation is a primary component of the PSII repair cycle.