Structure-Function of Membrane Protein Complexes Involved in Oxygenic Photosynthesis
Abstract
The electron transport chain of oxygenic photosynthesis involves four major heterooligomeric membrane protein complexes: the photosystem II and photosystem I reaction center complexes, the cytochrome b6f complex, and the ATP synthase. Three aspects of the electron transport chain have been investigated in the present studies: (i) structure-function studies of the central proton-electron conducting cytochrome b6f complex, focusing on the effect of lipids in structural stabilization and electron transfer function; (ii) transmembrane electron transfer pathways in the cytochrome b6f and mitochondrial cytochrome bc1 complexes, determined by heterogeneity in the internal polarity of the membrane protein complexes; and (iii) purification and characterization of a novel ~1 MDa supercomplex, dominated by the presence of photosystem I (PSI), ATP-synthase and ferredoxin-NADP+ reductase (FNR) from higher plant system Spinacea. Conclusions drawn from these studies are: (a) Addition of individual lipids, as well as incorporation into nanodiscs and bicelles, impart significant structural stabilization to a “delipidated” cytochrome b6f complex. Functions of individual lipids present in native thylakoid membrane have been determined. While the anionic sulfolipid, SQDG is critical for the stability of the dimeric complex, an anionic phospholipid, phosphatidyl-glycerol, restores the activity of an inactivated, delipidated dimeric cytochrome complex, hence emphasizing the role of lipids in the electron transfer function of the cytochrome complex; (b) The presence of internally bound lipids, along with water molecules, make the internal environment of membrane protein complexes, like the cytochrome complex, heterogeneous with respect to polarity or dielectric constant. The dielectric heterogeneity in turn influences the directionality of electron transfer through the cytochrome complexes, which was found to be different in the b6f complex, and its homologous counterpart in photosynthetic bacteria, the cytochrome bc1 complex; (c) The novel super complex formed by PSI, ATP synthase and FNR, acts as a localized domain to facilitate and expedite the shuttling of the two cofactors formed as byproducts of this complex, ATP and NADPH, to the CO2 fixation cycle. These studies provide an insight on the dynamic properties of the hetero-oligomeric membrane protein complexes in the chloroplast thylakoid membrane, and also show that both the internal and external membrane lipid environments, defined by the membrane lipid bilayer, affect the structure and function of the cytochrome b6f complex.
Degree
Ph.D.
Advisors
Cramer, Purdue University.
Subject Area
Energy|Microbiology
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