Bicarbonate-reversible formate inhibition at the donor side of photosystem II

Flash-induced absorbance changes at 295 nm show that in the spinach Photosystem II preparation called DT-20 the four-step redox cycle of the oxygen evolving complex is blocked by very low formate concentrations, 50% at 10 μM, and fully restored by a 2-fold higher bicarbonate concentration. These concentrations are four orders of magnitude lower than those involved in the well-known acceptor-side inhibition by formate and did not affect 295 nm absorbance changes in thylakoids, 'BBY' PS II membrane fragments, or PS II core particles. The effect is not related to the chloride dependence of oxygen evolution and appears to be an all-or-none inhibition already present on the first flash.