The complex architecture of organice photosynthesis (original) (raw)

2005, Nat Rev Mol Cell Biol

Oxygenic photosynthesis-the conversion of sunlight into chemical energy by plants, green algae and cyanobacteria-underpins the survival of virtually all higher life forms. The production of oxygen and the assimilation of carbon dioxide into organic matter determines, to a large extent, the composition of our atmosphere and provides all life forms with essential food and fuel. The study of the photosynthetic apparatus is a prime example of research that requires a combined effort between numerous disciplines, which include quantum mechanics, biophysics, biochemistry, molecular and structural biology, as well as physiology and ecology. The time courses that are measured in photosynthetic reactions range from femtoseconds to days, which again highlights the complexity of this process. Plant photosynthesis is accomplished by a series of reactions that occur mainly, but not exclusively, in the chloroplast (BOX 1). Initial biochemical studies showed that the chloroplast thylakoid membrane is capable of light-dependent water oxidation, NADP reduction and ATP formation 1. Biochemical and biophysical studies 2-6 revealed that these reactions are catalysed by two separate photosystems (PSI and PSII) and an ATP synthase (F-ATPase): the latter produces ATP at the expense of the PROTONMOTIVE FORCE (pmf) that is formed by the light reaction. The cytochrome-b 6 f complex mediates electron transport between PSII and PSI and converts the redox energy into a high-energy intermediate (pmf) for ATP formation 7. After the invention of SDS-PAGE 8,9 , the biochemical composition of the four multisubunit protein complexes began to be elucidated 10-14. According to the partial reactions that they catalyse, PSII is defined as a water-plastoquinone oxidoreductase, the cytochrome-b 6 f complex as a plastoquinone-plastocyanin oxidoreductase, PSI as a plastocyanin-ferredoxin oxidoreductase and the F-ATPase as a pmf-driven ATP synthase 15 (FIG. 1). PSI and PSII contain chlorophylls and other pigments that harvest light and funnel its energy to a reaction centre. Energy that has been captured by the reaction centre induces the excitation of specialized reactioncentre chlorophylls (PRIMARY ELECTRON DONORS; a special chlorophyll pair in PSI), which initiates the translocation of an electron across the membrane through a chain of cofactors. Water, the electron donor for this process, is oxidized to O 2 and 4 protons by PSII. The electrons that have been extracted from water are shuttled through a quinone pool and the cytochrome-b 6 f complex to plastocyanin, a small, soluble, copper-containing protein 16. Solar energy that has been absorbed by PSI induces the translocation of an electron from plastocyanin at the inner face of the membrane (thylakoid lumen) to ferredoxin on the opposite side (stroma; FIG. 1). The reduced ferredoxin is subsequently used in numerous regulatory cycles and reactions, which include nitrate assimilation, fatty-acid desaturation and NADPH production. The CHARGE SEPARATION