Organisation of the photosystem two light harvesting system

摘要

Like other types of photosystems, Photosystem II (PSII) consists of a reaction center served by a light harvesting system. Higher plants, algae and cyanobacteria all contain an inner PSII antenna system composed of CP43 and CP47 which are chlorophyll a (chla) binding proteins encoded by the psbB and psbC genes, respectively. They bind about 15 chls each and have 6 transmembrane helices. Six chla molecules are bound to the PSII reaction center complex composed of the D1 and D2 proteins thus giving a total of about 36 chla molecules associated with the PSII core complex. In higher plants and green algae, the outer PSII antenna is made up of chla/chlb binding proteins encoded by the nuclear located cab genes. The dominant forms (Lhcb1, Lhcb2 and Lhcb3) make up the light harvesting complex, LHCII, which exists as a trimer, binding about 42 chl each (24 chla and 18 chlb). In addition, three minor forms of Cab proteins are found in PSII; Lhcb4, also known as CP29, Lhcb5 (CP26) and Lhcb6 (CP24). Some Cab proteins are more tightly held to the PSII core while others are more easily dissociated, leading to the concept of an inner and outer pool of LHCII. On average the total antenna size corresponds to about 250 chl per reaction center. In red algae and cyanobacteria the secondary antenna is contained within the phycobilisomes.