REDOX REGULATION OF PHOTOSYNTHESIS BY THE CYTOCHROME bf COMPLEX: MECHANISMS AND CONSEQUENCES

Cytochrome b;Oxidation-reduction reaction;Cytochromes;Photosynthesis

Brantmier, Paul J.Kallas, Toivo ;

University of Wisconsin

关键词: Cytochrome b;    Oxidation-reduction reaction;    Cytochromes;    Photosynthesis;   

Others  :  https://minds.wisconsin.edu/bitstream/handle/1793/66241/Brantmier%2c%20Paul.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: University of Wisconsin

【 摘 要 】

Photosynthesis is a fundamental and central metabolic process of our planet. Honed bybillions of years of evolution, it forms the basis for the food chain that sustains ourecosphere. Photosynthesis begins with sunlight capture by light harvesting complexesassociated with photosynthetic membranes. The captured energy is funneled tophotosystems II and I (PSII & PSI) reaction centers. In the PSII reaction center, lightenergy pulls electrons from water, evolving oxygen. These electrons drive a series ofredox (oxidation-reduction) reactions passing through the cytochrome (Cyt) bf complexand PSI to generate chemical energy as adenosine triphosphate (ATP) andnicotine adenine dinucleotide phosphate (NADPH). The danger in shuffling electronsthrough sequential redox transfers is the risk of unintended electron transfer to moleculessuch as oxygen, forming reactive radical species. The Cyt bf complex has been proposedas a source of superoxide, one of many forms of reactive oxygen species (ROS) thatimpair growth, function, and survival. My thesis addresses the regulation of electrontransport and light harvesting processes by which photosynthetic organisms optimizelight energy distribution between the photosystems (so called ;;state transitions;;) andminimize ROS production. The Cyt bf complex has also long been implicated in sensingredox changes in electron transport and signaling the redistribution of light harvestingbetween PSII and PSI by mechanisms that are not well understood.I used specific inhibitors and a sensitive fluorescent probe (H2DCFDA) tocharacterize ROS production in the Cyt bf complex of the cyanobacteria (blue-greenmicro-algae), Synechocystis sp. PCC 6803 and Synechococcus sp. PCC 7002. Inhibitorswere also used in room-temperature and 77 degree K fluorescence studies to investigate the roleof the Cyt bf complex in redox sensing and signaling that mediates the redistribution oflight-harvesting phycobiliproteins (PBS) in these cyanobacteria. Findings from this workshow that the Cyt bf complex plays a central role in these processes. These andpreviously published data allowed me to formulate a detailed model of redox signaling bythe Cyt bf complex, and regulated redistribution of light energy by the formation ofdynamic light-harvesting (PBS) supercomplexes involving the Cyt bf complex, PSII, andPSI. The model proposes two distinct means by which these ;;state transitions;; occur. Ipropose Cyt bf-independent and - dependent sensing-signaling mechanisms. The Cyt bf- dependent mechanism depends on the presence of light and appears to require bindingevents or conformational changes in the Cyt bf low-potential electron transfer chain orquinone-reductase (Qn) site. I propose that the Cyt bf - independent mechanism liesdownstream of the bf complex. Together, these signal the formation of PSII-PBS-Cyt bfand PSI-trimer-PBS complexes during illumination, and the predominant formation ofPSI-monomer-PBS complexes during darkness. These in turn determine the relativelight-harvesting capacities of PSII and PSI. The model and supporting evidence arediscussed in the context of strategies that have evolved to maximize photosyntheticefficiency and minimize ROS production.