Effet de l'ATP et de Mg2+ sur les activités ribulose-diphosphate carboxylase et ribose-5-phosphate isomérase et kinase d'une préparation enzymatique de chloroplastes

Abstract

CO₂ fixation in the presence of Ru-DP or R-5-P, catalyzed by proteins precipitated from isolated chloroplasts, is affected by ATP. The enzymes of the chloroplastic preparation are mainly Ru-DP carboxylase, R-5-P isomerase and Ru-5-P kinase. ATP inhibits the activity of the Ru-DP carboxylase. This is true for the enzyme of the crude protein preparation as well as for the purified enzyme. The inhibition by ATP is overcome by Mg²⁺. This may be related to the formation of a chelate with ATP and Mg²⁺, which lowers the level of the Ru-DP carboxylase – Mg²⁺ – CO₂ complex.However, R-5-P kinase activity is enhanced by ATP. As a result of the different responses of the two enzymes to ATP, CO₂ fixation coupled to Ru-DP formation from R-5-P, is enhanced by low concentrations of ATP and inhibited by concentrations above 2 mM.