Evidence that formate destabilizes the S1 state of the oxygen-evolving mechanism in Photosystem II

Flash-induced absorption changes at 295 nm and oxygen flash yields were measured in Photosystem II-enriched membranes and thylakoids from spinach. In the presence of formate and reduced 2,6-dichloro-t p-benzoquinone (DCBQ), the normal period-4 oscillation in the oxygen-evolving mechanism is shifted one flash, a maximum appearing on the 4th rather than the 3rd flash. Analysis of the flash-induced sequence shows that in 90–100% of the centers, the oxygen evolving complex is present in the most reduced state (S0), rather than in the singly oxidized state (S1) as is normally the case. An electron donation to the oxidized states S2 or S3 during the flash sequence by reduced tyrosine ‘YD’ cannot account for the data unless the rate of this reaction is dramatically accelerated by formate. Rather, it appears that the normally dark-stable S1 state of the oxygen-evolving mechanism is modified by formate so that it becomes reducible to S0. The reduced form of DCBQ is an efficient mediator to promote the shift to S0. As in other instances where a chemical reduction of S1 is achieved, the ‘formal S0’ state caused by the formate/DCBQ treatment is not spectroscopically identical with the S0 state occurring under illumination. Incubation at high redox potential (550 mV) did not reverse the S-state shift caused by the treatment with formate and DCBQ at lower potential, but induced a distortion of the absorption change sequence that could indicate an enhanced yield of oxygen on the second flash.