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

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Abstract
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 CO2/HCO3− 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 CO2/HCO3− was detected. This is consistent with the concept that membrane-bound HCO3− 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 HCO3− but by binding to empty sites. The “bicarbonate effect” is a reversal, by high concentrations of exogenous bicarbonate, of anion inhibition of photosystem II.