Bicarbonate Inhibits Ribulose-1,5-Bisphosphate Carboxylase

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) rapidly extracted from leaves of wheat ( Triticum aestivum ) and purified activated RuBPCO were incubated in the presence and absence of 20 millimolar HCO 3 − and changes in activation state were followed. Rapid inactivation occurred in the presence, but not in the absence, of HCO 3 − . Effects of CO 2 concentration and pH during preincubation before assay on activation state of RuBPCO were investigated in equilibrium studies. Twenty percent inactivation occurred at high CO 2 concentration if pH was high, but not if it was low, suggesting that RuBPCO was inactivated by HCO 3 − . The inactivation by HCO 3 − was more rapid than the dissociation of activating CO 2 in CO 2 -free buffer (both in the presence of 20 millimolar MgCl 2 ), suggesting that HCO 3 − was bound to the active enzyme complex. The dissociation of inactivating HCO 3 − from the enzyme was slow enough that inhibition could be demonstrated in experiments with HCO 3 − treatments during preincubation and constant conditions during assay. Inorganic phosphate did not seem to interfere with the binding of HCO 3 − .