Photosynthetic carbon metabolism in Euglena gracilis. Part XXIII. Effects of photosynthetic intermediates on the activation state of ribulose 1,5-bisphosphate carboxylase/oxygenase from Euglena gracilis Z.

Abstract

The half-saturating concentration of ribulose 1, 5-bisphosphate carboxylase/oxygenase (RuBisCO) from Euglena gracilis Z for CO2 in its activation by CO2 in the presence of a saturating concentration of MgCl2 (Ka) was measured by analyzing the partial reversible inactivation of the fully activated enzyme in the medium with dilute CO2. The Ka of the Euglena enzyme was 12.5 μM. The Ka values were 6.3 μM for the enzyme from soybean, 10.8 μM from maize, 23.3 μM from Scenedesmus obliquus, and 20.8 μM from Anabaena 7120. The activated state of Euglena RuBisCO was stabilized by 6-phosphogluconate, fructose 1, 6-bisphosphate, and 3-phosphoglycerate in the medium containing low concentrations of CO2. Both fructose 6-phosphate and ATP stimulated inactivation in the medium. NADPH not only stabilized the activated state of the enzyme, but also enhanced the enzyme activity over the full activity measured in the absence of NADPH. NADP+ did not nullify the effects of NADPH on the activation at all. The physiological significance of the effects of these photosynthetic metabolites on the activated state of Euglena RuBisCO is discussed.