Proteins, glycerolipids and carotenoids in the functional photosystem II architecture.

Photosystem II (PSII), the first supercomplex of the electron transport chain, governs the energy transfer using harvested light energy, which is transformed into biochemical energy. Phosphatidylglycerol and sulfoquinovosyl diacylglycerol, the anionic lipids of photosynthetic organisms, together with a neutral lipid, digalactosyldiacylglycerol, assist in the assembly of photosynthetic complexes. These lipids and carotenoids serve as mortar for the proteins which act as bricks in the construction of the active photosynthetic machinery, and they have determinative roles in the oligomerization of protein subunits. X-ray crystallographic localization of glycerolipids and carotenoids revealed that they are present at functionally and structurally important sites of both the PSI and PSII reaction centers. Phosphatidylglycerol is involved in the formation of the reaction-center oligomers and controls electron transport at the acceptor site of PSII. Digalactosyldiacylglycerol, together with phosphatidylglycerol, is involved in the electron transport at the donor site. Phosphatidylglycerol and carotenoids are needed to glue CP43 to the reaction center core. Carotenoids are protective agents, which prevent photosynthetic complexes from degradation caused by reactive oxygen species.