Lipids in photosystem II: multifunctional cofactors.

To maintain its functionality, photosystem II (PSII) employs several types of auxiliary molecules (cofactors). As shown for PSII from Thermosynechococcus elongatus, lipids previously thought to play mostly the role of a hydrophobic matrix for embedding the membrane proteins, must be considered as a new, multifunctional type of cofactors, playing a vital role in the fine tuning of PSII and in its overall operation. The 2.9 Å resolution crystal structure of cyanobacterial homodimeric PSII showed the position of 25 lipid molecules per monomer, and allowed detailed analysis of individual binding sites as well as functional aspects related to lipids. The positions of the bound lipids suggest that they are essential for the assembly and disassembly of PSII, provide the proper environment for plastoquinone exchange, might tune electron transfer through contacts with chlorophylls and carotenoids, and might serve as an oxygen-outlet system from the lumen.