Inhibition of photosystem II by formate. Possible evidence for a direct role of bicarbonate in photosynthetic oxygen evolution.

In broken chloroplasts the presence of 100 mM sodium formate at pH 8.2 will specifically lengthen the Photosystem II relaxation times of the reactions S'2 leads to S3 and S'3 leads to S0. Rates of reactions S'0 leads to S1 and S'1 leads to S2 remain unaffected. Evidence is presented which indicates the discrimination among S-states by formate cannot be attributed to a block imposed on the reducing side of Photosystem II. The results are interpreted in context of the known interaction of formate and CO2 which is bound to the Photosystem II reaction center complex. It is proposed that those S-state transitions which show extended relaxation times in the presence of formate must result in the momentary release and rebinding of CO2. Furthermore since formate is acting on the oxygen-evolving side of Photosystem II, it would seem that CO2 is released in reactions that occur there. A chemical model of oxygen evolution is presented. It is based on the hypothesis that hydrated CO2 is the immediate source of photosynthetically evolved oxygen and explains why, under certain conditions formate slows only the S-state transitions S'2 leads to S3 and S'3 leads to S0.