Reevaluation of the role of bicarbonate and formate in the regulation of photosynthetic electron flow in broken chloroplasts

The stimulation of the Hill reaction in CO₂-depleted broken chloroplasts (Pisum sativum L. cv Rondo) by the total amount of dissolved CO₂ and HCO₃⁻ (bicarbonate^*) was measured at several formate concentrations. Formate appears to be a competitive inhibitor of the bicarbonate^* stimulation of electron flow. From these experiments we have obtained a reactivation constant (K_r) of 78 ± 31 micromolar NaHCO₃ and an inhibition constant (K_i) of 2.0 ± 0.7 millimolar HCOONa at pH 6.5. In the absence of formate, significant electron flow was measured at a bicarbonate^* concentration well below K_r, suggesting that electron flow from Q, the primary electron acceptor of photosystem II, to plastoquinone can proceed when no bicarbonate^* is bound to the regulatory site at the Q_B-protein. If so, bicarbonate^* stimulation of electron flow is mainly a diminution of the inhibition of electron flow by formate. In view of the results, it is proposed that regulation of linear electron flow by bicarbonate^* and formate is a mechanism that could link cell metabolism to photosynthetic electron flow.