Absence of a Formate-Induced Release of Bicarbonate from Photosystem II.

Formate has been proposed to inhibit electron flow in photosystem II by replacing endogenous bound bicarbonate on the reaction center complex. A mass spectrometer was used to measure directly the CO(2)/HCO(3) (-) released when maize thylakoids, showing normal rates of electron flow, were treated with formate. Although the formate inhibited electron flow by 95%, no release (displacement) of CO(2)/HCO(3) (-) was detected. This is consistent with the concept that membrane-bound HCO(3) (-) is not a requirement for normal rates of electron flow through photosystem II. Moreover, formate and other monovalent anions do not inhibit electron flow by removing bound HCO(3) (-) but by binding to empty sites. The "bicarbonate effect" is a reversal, by high concentrations of exogenous bicarbonate, of anion inhibition of photosystem II.