REDOX REGULATION OF PHOTOSYNTHESIS BY THE CYTOCHROME bf COMPLEX: MECHANISMS AND CONSEQUENCES
File(s)
Date
2013-05Author
Brantmier, Paul J.
Advisor(s)
Kallas, Toivo
Metadata
Show full item recordAbstract
Photosynthesis is a fundamental and central metabolic process of our planet. Honed by
billions of years of evolution, it forms the basis for the food chain that sustains our
ecosphere. Photosynthesis begins with sunlight capture by light harvesting complexes
associated with photosynthetic membranes. The captured energy is funneled to
photosystems II and I (PSII & PSI) reaction centers. In the PSII reaction center, light
energy pulls electrons from water, evolving oxygen. These electrons drive a series of
redox (oxidation-reduction) reactions passing through the cytochrome (Cyt) bf complex
and PSI to generate chemical energy as adenosine triphosphate (ATP) and
nicotine adenine dinucleotide phosphate (NADPH). The danger in shuffling electrons
through sequential redox transfers is the risk of unintended electron transfer to molecules
such as oxygen, forming reactive radical species. The Cyt bf complex has been proposed
as a source of superoxide, one of many forms of reactive oxygen species (ROS) that
impair growth, function, and survival. My thesis addresses the regulation of electron
transport and light harvesting processes by which photosynthetic organisms optimize
light energy distribution between the photosystems (so called 'state transitions') and
minimize ROS production. The Cyt bf complex has also long been implicated in sensing
redox changes in electron transport and signaling the redistribution of light harvesting
between PSII and PSI by mechanisms that are not well understood.
I used specific inhibitors and a sensitive fluorescent probe (H2DCFDA) to
characterize ROS production in the Cyt bf complex of the cyanobacteria (blue-green
micro-algae), Synechocystis sp. PCC 6803 and Synechococcus sp. PCC 7002. Inhibitors
were also used in room-temperature and 77 degree K fluorescence studies to investigate the role
of the Cyt bf complex in redox sensing and signaling that mediates the redistribution of
light-harvesting phycobiliproteins (PBS) in these cyanobacteria. Findings from this work
show that the Cyt bf complex plays a central role in these processes. These and
previously published data allowed me to formulate a detailed model of redox signaling by
the Cyt bf complex, and regulated redistribution of light energy by the formation of
dynamic light-harvesting (PBS) supercomplexes involving the Cyt bf complex, PSII, and
PSI. The model proposes two distinct means by which these 'state transitions' occur. I
propose Cyt bf-independent and - dependent sensing-signaling mechanisms. The Cyt bf
- dependent mechanism depends on the presence of light and appears to require binding
events or conformational changes in the Cyt bf low-potential electron transfer chain or
quinone-reductase (Qn) site. I propose that the Cyt bf - independent mechanism lies
downstream of the bf complex. Together, these signal the formation of PSII-PBS-Cyt bf
and PSI-trimer-PBS complexes during illumination, and the predominant formation of
PSI-monomer-PBS complexes during darkness. These in turn determine the relative
light-harvesting capacities of PSII and PSI. The model and supporting evidence are
discussed in the context of strategies that have evolved to maximize photosynthetic
efficiency and minimize ROS production.
Subject
Cytochrome b
Oxidation-reduction reaction
Cytochromes
Photosynthesis
Permanent Link
http://digital.library.wisc.edu/1793/66241Description
A Thesis Submitted In Partial Fulfillment of the Requirements for the Degree of Master of Science - Biology-Molecular Biology