Reevaluation of the Role of Bicarbonate and Formate in the Regulation of Photosynthetic Electron Flow in Broken Chloroplasts

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Abstract
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 (Kr) of 78 ± 31 micromolar NaHCO3 and an inhibition constant (Ki) 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 Kr, 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 QB-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.

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