Photosystem I

Plants harvest light energy by oxygenic photosynthesis, which is undoubtedly one of the most important biological processes on the earth. By liberating oxygen, consuming carbon dioxide, and generating biological sources of energy and reducing power, oxygenic photosynthesis has made our planet hospitable for the survival and evolution of complex life forms. The light reactions of oxygenic photosynthesis, in which the energy of photons is converted into chemical energy, take place in the thylakoid membranes of chloroplasts and cyanobacteria. During the photosynthetic electron transport, three membrane-protein complexes, PSII, Cyt b,f, and PSI, function in a coordinated fashion to yield a proton gradient across membrane and strong reductants on the stromal side. ATP synthase of thylakoids uses the proton motive force to synthesize ATP. PSI is the light-driven generator of the reducing power in chloroplasts. It is a heteromultimeric pigment-protein complex that functions as a light-driven plastocyanin-Fd oxidoreductase (see Chitnis et al., 1995, for details and additional refs.). Reduced Fd is used by Fd-NADP+ oxidoreductase to generate NADPH. Thus, PSI plays a vital role in chloroplast metabolism; it utilizes light to generate reducing power, NADPH, and reduced Fd. These reductants are used in diverse biochemical processes, including Calvin cycle, nitrite reduction to ammonia, fatty acid desaturation, pyruvate decarboxylation, glutamate synthesis, sulfite reduction, and enzyme regulation through reduced thioredoxin. PSI also participates in cyclic electron flow and contributes to the proton gradient that is used to generate ATP. In chloroplasts, PSI is localized in the nonappressed regions of thylakoids. The PSI complex contains polypeptides and cofactors that are required for absorption and use of photons. PSI consists of at least 11 different polypeptides in cyanobacteria and 13 in chloroplasts (Table I). A11 PSI proteins are believed to be present as one copy per P700 reaction center. They vary considerably in their molecular weights, hydrophobicities, and locations with respect to the lipid bilayer.