Stimulation of Ribulose Bisphosphate Carboxylase Activity by Inorganic Orthophosphate without an Increase in Bound Activating CO2: Co-operativity between the Subunits of the Enzyme

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Abstract
Parry, M. A. J., Schmidt, C. N. G., Cornelius, M. J., Keys, A. J., Millard, B. N. and Gutteridge, S. 1985. Stimulation of ribulose bisphosphate carboxylase activity by inorganic orthophosphate without an increase in bound activating CO2: co-operativity between the subunits of the enzyme.—J. exp. Bot. 36: 1396–1404 Ribulose bisphosphate carboxylase/oxygenase (E.C. 4.1.1.39) from wheat (Triticum aestivum L.), already activated by reaction with CO2 and Mg2+, was increased in activity on addition of inorganic orthophosphate. This further activation took place without a significant increase in the amount of bound activating CO2 and the effect was relatively greater with smaller amounts of bound CO2. With less than 2·0 mol of CO2 bound per mol holoenzyme, phosphate increased activity about five-fold whilst with 7·0 mol of bound activating CO2 per mol holoenzyme, phosphate increased activity by a factor of only 1 ·8. This decrease in the effect of orthophosphate with increase in bound activating CO2 suggests negative co-operativity between activated sites. The stimulation of activity by inorganic orthophosphate must be a process distinct from activation by CO2; it was observed with both the slow and the rapidly activating forms of ribulose bisphosphate carboxylase/oxygenase from wheat.