Abstract
Photoactivation of phosphoenolpyruvate carboxylase in C(4) plants is detected more efficiently when activity is assayed at suboptimum pH (e.g. 7.2); the magnitude of the light effect is often larger at low phosphoenolpyruvate concentration.Darkness and low assay pH induce an allosteric behavior (positive cooperativity with phosphoenolpyruvate) which is relieved in light or by higher pH; thus, normal Michaelis-Menten kinetics are exhibited only when the enzyme is extracted during the day and assayed at pH 8.2.Light activation, pH, and substrate level appear to be components of a regulatory device suppressing the activity in darkness and enhancing it under light.