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 CO(2)-depleted broken chloroplasts (Pisum sativum L. cv Rondo) by the total amount of dissolved CO(2) and HCO(3) (-) (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 NaHCO(3) 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.