The electron donor side of photosystem II: the oxygen evolving complex.

The 02-evolving complex (OEC) of photosynthesis resides near the inner side of the thylakoid membrane. Currently, it is believed to be composed of at least four polypeptides: (1) an intrinsic 27-34 kilodalton (kD) polypeptide that may be associated with manganese (Mn) ; (2) a peripheral 33 k D polypeptide, perhaps also associated with Mn; (3) a peripheral 24 kD polypeptide; and (4) a peripheral 18 kD polypeptide. At least three other polypeptides are associated with this complex to form Photosystem I1 (PSII): (1) a reaction center (RC) polypeptide, CP-47, of 47-51 kD molecular weight (M,) containing a chlorophyll a (P680), which is the primary electron donor, and a pheophytin (Pheo), which is the primary electron acceptor; also, a bound plastoquinol electron donor (Z), and a bound plastoquinone electron acceptor (aA) are suggested to be incorporated in this polypeptide; (2) a 32 kD polypeptide containing a second bound plastoquinone electron acceptor (QB); and (3) a 10 kD cytochrome (Cyt) b559, which exists in two forms: a high potential and a low potential. The primary reaction of PSI? the charge separation: P680.Pheo + hv .+ P680*Pheo-, in which the energy of a photon or an exciton is transformed to achieve the energetically uphill electron transfer. This electron is then transferred to the quinones on the reducing side of PSII. The oxidized reaction center (RC) chlorophyll (P680+) is reduced by removing an electron from Z, an event that can be observed