Photosystem II is a membrane-bound complex found in plants, eukaryotic algae and cyanobacteria which converts photo-excitation energy into chemical energy in the form of both oxidising and reducing power, catalysing the oxidation of water and reduction of quinone. The structure of this enzyme is tuned to balancing thermodynamic and quantum efficiency while minimising photodamage. This thesis tests a number of hypotheses regarding structural and functional aspects of this enzyme, addressing (1)

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... he importance of structural differences between normal- and high-light-induced protein isoforms, (2) the binding mode of the inhibitor DCMU, (3) electron transfer from the primary quinone acceptor QA to the terminal quinone acceptor QB and (4) the structural origins of functional differences between redox-active tyrosines YZ and YD, using the thermophilic cyanobacterium Thermosynechococcus elongatus as a model organism. A crystal structure of PSII containing the PsbA3, the high-light isoform of the D1 protein which binds cofactors involved in the active electron transfer chain, is presented, demonstrating structural similarities with PsbA1. Crystallographic evidence is also presented which supports the binding of DCMU to D2-Ser264 and D2-Phe265 in this isoform, similar to predictions from PsbA1. Kinetic studies show that the half-times of electron transfer from QA- to QB and QA- to QB- are around 400 μs and 1 ms respectively, and the implications of these rates for structural studies are discussed. Finally, electron paramagnetic resonance experiments provide evidence that a hydrogen-bonding network linking YD and CP47-Glu364 is not a major determinant of functional differences between YZ and YD.