A functional model for the role of cytochrome b559 in the protection against donor and acceptor side photoinhibition.

Abstract

A quinone-independent photoreduction of the low potential form of cytochrome b559 has been studied using isolated reaction centers of photosystem II. Under anaerobic conditions, the cytochrome can be fully reduced by exposure to strong illumination without the addition of any redox mediators. Under high light conditions, the extent and rate of the reduction is unaffected by addition of the exogenous electron donor Mn2+ and, during this process, no irreversible damage occurs to the reaction center. However, prolonged illumination in strong light brings about irreversible bleaching of chlorophyll, indicative of photoinhibitory damage. When the cytochrome is fully reduced and excess Mn2+ is present, the effect of moderate light is to facilitate the photoaccumulation of reduced pheophytin. The dark reoxidation of the reduced cytochrome is very slow under anaerobic conditions but significantly speeded up on addition of oxidized 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone. From these results it is suggested that the low potential form of cytochrome b559 can accept electrons directly from reduced pheophytin and in so doing help to protect the reaction center against acceptor side photoinhibition as suggested by Nedbal et al. [Nedbal, J., Samson, G. & Whitmarsh, J. (1992) Proc. Natl. Acad. Sci. USA 89, 7929-7933]. This conclusion has been incorporated into a model that further suggests that in its high potential form the cytochrome primarily acts to protect against donor side photoinhibition due to increased lifetime of highly oxidized species as previously proposed by Thompson and Brudvig [Thompson, L. & Brudvig, G. W. (1988) Biochemistry 27, 6653-6658]. The particular feature of our scheme is that it incorporates reversible interconversion between the two redox forms so as to protect against either type of photoinhibition.

Mechanism of photosystem II photoinactivation and D1 protein degradation at low light: The role of back electron flow

“…Partial protection against this possibility may be ascribed to the presence of the RCII carotens. Oxidation of the reduced semiquinone acceptors in the dark may occur also by alternative routes (14,39) that may be harmless to PSII. The relative contribution of the alternative decay paths and their significance is not yet known.…”

Aspects of nitrogen metabolism in coffee plants

“…These highly reactive chemical entities may cause damage, namely lipid peroxidation, bleaching of pigments (e.g., in P 680 ), protein degradation (e.g., D1), enzyme inactivation and even DNA damage (Asada and Takahashi, 1987;Winston, 1990;Barber and De Las Rivas, 1993;Foyer et al, 1994;Niyogi, 1999), resulting in impaired membrane functionality and lower ability of photon-use and enhanced photoproduction of reactive molecules (Asada, 1994). To prevent oxidative stress at the chloroplast level, due to the production of active forms of oxygen and chl, plants have evolved complex systems to remove such highly reactive chemical entities at their generation site.…”

Activation of Photosynthesis and Resistance to Photoinhibition in Cyanobacteria within Biological Desert Crust

“…• 2 , and possibly involvement of back electron flow within PSII via cytochrome b 559 (Barber and Rivas, 1993;Poulson et al, 1995). It is also possible that the manganese cluster of part of the PSII population is partially disassembled during the desiccation, and reassembly might require photoactivation via electron flow-dependent oxidation of the cluster ).…”