Evidence that formate destabilizes the S-1 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-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 (S-0), rather than in the singly oxidized state (S-1) as is normally the case. An electron donation to the oxidized states S-2 or S-3 during the flash sequence by reduced tyrosine 'Y-D' cannot account for the data unless the rate of this reaction is dramatically accelerated by formate. Rather, it appears that the normally dark-stable S-1 state of the oxygen-evolving mechanism is modified by formate so that it becomes reducible to S-0. The reduced form of DCBQ is an efficient mediator to promote the shift to So. As in other instances where a chemical reduction of S-1 is achieved, the 'formal S-0' state caused by the formate/DCBQ treatment is not spectroscopically identical with the S-0 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.