ABSTRACT
In plants, algae, and photosynthetic bacteria, the photosynthetic apparatus converts light energy into chemical energy. This machinery is an assembly of membrane proteins, in which the action of light starts in an “antenna,” an ensemble of pigments (chlorines such as chlorophylls, polyenes such as carotenoids, and phycobilins) embedded in proteins. The antenna absorbs light and funnels excitation energy toward large specific proteins named reaction centers, where photoinduced charge separation takes place. Reaction centers (RC) can thus be considered as microscopic photocells that realize the first steps of energy conversion. In this system, all elementary processes take place rapidly (in less than 1 ps for pigment-topigment energy transfer, in less than 3 ps for primary electron transfer), so that the overall quantum yield is higher than 90%, with only a small fraction of excitation being lost by internal conversion, by fluorescence (less than 2%), or by conversion to the triplet state (less than 4%). For many years, RCs were simply hypothesized as specific sites in the photosynthetic membrane, but progress in membrane biochemistry has led to their effective isolation as large proteins made of several polypeptides (from two in the simplest case of nonsulfur green bacteria to at least 12 in Photosystem I and Photosystem II of oxygenic organisms) and of several pigment molecules (all RCs contain at least six chlorin-type molecules and one or more carotenoids). They contain several chemical groups implicated in electron transfer (these groups are called “redox centers”: quinones, hemes, iron-sulfur centers, Mn atoms, amino acid residues of polypeptides, etc.).
