REGULATION OF RIBULOSE 1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE ACTIVATION BY INORGANIC-PHOSPHATE THROUGH STIMULATING THE BINDING OF THE ACTIVATOR CO2 TO THE ACTIVATION SITES

The mechanism of the regulation of the activation of ribulose 1,5-bisp hosphate carboxylase/oxygenase (RuBisCO) by inorganic phosphate (P-i) in the presence of limiting concentrations of CO2 was explored. The ac tivation state of RuBisCO increased sigmoidally following a biphasic k inetics against the concentration of P-i in the activation mixture wit h an intermediary plateau at 2 to 3 mM P-i when the enzyme was activat ed for 30 min. The intermediary plateau could not be seen when the pre incubation time was 10 min and the activation was completed at 10 mM P -i. RuBisCO from Euglena also showed a quite similar activation kineti cs. The activation was not due to the contaminating CO2 included in th e stock P-i solution or in the activation buffer containing the enzyme . The experiments with 2-carboxyarabinitol 1,5-bisphosphate showed tha t the P-i-stimulated activation was due to the promotion of binding of the activator CO2 to the activation sites. It was also found that P-i increased the affinity of RuBisCO for the activator CO2 5.4-fold acco mpanied by a decrease of the half-saturating concentration of CO2 to 1 .6 mu M at 20 mM MgCl2. Physiological significance of the effects of P -i on the activation of RuBisCO is discussed.