Inhibition of photophosphorylation by ribulose-1,5-bisphosphate carboxylase

Abstract

Purified ribulose-1,5-bisphosphate carboxylase inhibited coupled glycerate 3-phosphate, NADP and ferricyanide reduction in saturating light. Complete inhibition of coupled electron transport was achieved at an added carboxylase protein concentration of 5 mg · ml −1. The rate of uncoupled electron transport was not inhibited by the carboxylase protein. Thus inhibition of photophosphorylation was indicated. Inhibition was promoted by high Mg 2+ concentration and was not reversed in the light by subsequent additions of inorganic phosphate (P i) or ADP. However, if P i was added prior to ribulose-1,5-bisphosphate carboxylase then partial protection against the inhibition of coupled electron transport was observed. The apparent K m P i of photophosphorylation was considerably increased by increasing carboxylase concentration. This may contribute to a higher apparent K m in vivo. When coupled electron flow associated with glycerate 3-phosphate-reduction was totally inhibited in the presence of carboxylase, the addition of ATP or an ATP-generating system facilitated electron flow but did not reverse inhibition of coupled electron transport. At limiting light intensities similar effects were produced by the addition of stromal protein (instead of purified carboxylase protein), but inhibition was not observed in saturating light. The inhibition of coupled electron transport was found with both the active and inactive forms of carboxylase but not with bovine serum albumin.