Spare quinones in the QB cavity of crystallized photosystem II from Thermosynechococcus elongatus

The recent crystallographic structure at 3.0 Å resolution of PSII from Thermosynechococcus elongatus has revealed a cavity in the protein which connects the membrane phase to the binding pocket of the secondary plastoquinone QB. The cavity may serve as a quinone diffusion pathway. By fluorescence methods, electron transfer at the donor and acceptor sides was investigated in the same membrane-free PSII core particle preparation from T. elongatus prior to and after crystallization; PSII membrane fragments from spinach were studied as a reference. The data suggest selective enrichment of those PSII centers in the crystal that are intact with respect to O2 evolution at the manganese–calcium complex of water oxidation and with respect to the integrity of the quinone binding site. One and more functional quinone molecules (per PSII monomer) besides of QA and QB were found in the crystallized PSII. We propose that the extra quinones are located in the QB cavity and serve as a PSII intrinsic pool of electron acceptors.