Structural characterisation of photosystem II.
Photosystem II (PSII) is the multi-subunit pigment-protein complex which catalyses water oxidation in higher plants, algae and cyanobacteria. Its atomic structure and protein cofactor organisation has not yet been determined. Elucidation of the structure would aid our understanding of PSII function and regulation, and may provide us with the knowledge to design new solar driven catalysts for the production of renewable energy. This thesis details the biochemical and structural studies on PSII complexes differing in subunit composition that have been isolated in order to enhance our current understanding of PSII structure.
The largest of several novel PSII complexes isolated was a 725 kDa PSII-LHCII complex from the higher plant, spinach (Spinacea oleracea), consisting of the light harvesting proteins Lhcb1, 2, 4 and 5 bound to a dimeric oxygen evolving PSII core. The Lhcb proteins, 33 kDa and 23 kDa extrinsic proteins were sequentially removed, allowing their positions to be determined at a resolution of 25 Å by producing enhanced images from electron micrographs of negatively stained particles. A three-dimensional reconstruction was also achieved with these images at a resolution of 30 Å. The results suggest that the dimeric 725 kDa particle bound one LHCII trimer per reaction centre, connected to a centrally located PSII core dimer by one copy of Lhcb4 and 5. It is further concluded that CP43 is located to the outer side of the central dimer core and that there is a single copy of the 33 kDa and 23 kDa proteins per reaction centre. The significance of the dimeric organisation is discussed in terms of the in vivo structure. A similar complex of the PSII-LHCII type was also isolated from the green alga, Chlamydomonas reinhardtii and enhanced images obtained to enable comparison with the higher plant data. A smaller spinach CP47-RC dimeric PSII complex (390 kDa), was subjected to single particle image averaging with the view to more closely identifying the position of the reaction centre proteins within the PSII-LHCII complex.