Search for intermediates of photosynthetic water oxidation.

Photosystem II of cyanobacteria and plants incorporates the catalytic centre of water oxidation. Powered and clocked by quanta of light the centre accumulates four oxidising equivalents before oxygen is released. The first three oxidising equivalents are stored on the Mn(4)Ca-cluster, raising its formal oxidation state from S0 to S3 and the third on YZ, producing S3 YZ ox. From there on water oxidation proceeds in what appears as a single reaction step (S3 YZ ox(H2O)2 <==>O2 + 4H+ + S0. Intermediate oxidation products of bound water had not been detected, until our recent report on the stabilisation of such an intermediate by high oxygen pressure (NATURE 430, 2004, 480-483). Based on the oxygen titration (half-point 2.3 bar) the standard free-energy profile of a reaction sequence with a single intermediate was calculated. It revealed a rather small difference (-3 kJ mol-1) between the starting state [S3YZ OX(H2O2) and the product state S0YZ + O2 + 4H+ . Here we describe the tests for side effects of exposing core particles to high oxygen pressure. We found the reduction of P680 +* in ns and the reduction/dismutation of quinones at the acceptor side of PSII both unaffected, and the inhibition of the oxygen evolving reaction by exposure to high O2-pressure was fully reversible by decompression to atmospheric conditions.