Role of Photosynthetic Electron Transfer in Light Activation of Calvin Cycle Enzymes

The effect of light in activating fructose-1,6 biphosphate phosphatase (E.C. 3.1.3.11), sedoheptulose-1,7, biphosphate phosphatase (E.C. 3.1.3.11), ribulose-5 phosphate kinase (E.C. 2.7.1.19), ribulose-1,5 biphosphate carboxylase (E.C. 4.1.1.39) and (NADPH) glyceraldehyde-3 phosphate dehydrogenase (E.C. 1.2.1.13) in intact spinach chloroplasts in the presence of antimycin A, tetramethylethylenediamine (TMEDA) or chlorophenyl-1,1-dimethylurea (CMU) was examined. Antimycin A and TMEDA were added as stimulating agents for photosynthetic electron transfer in intact chloroplasts while CMU was added for its inhibitory characteristics. Light exerted its control through the mediation of the photosynthetic electron transfer. Antimycin A and TMEDA promoted the light activation. CMU nullified the light activation as well as the stimulatory effect of antimycin A and TMEDA. Thus the control by light of the activities of the Calvin cycle enzymes involves a reduced agent formed by the photosynthetic electron transport chain. From the presently available evidence, it seems appropriate to hypothesize that the light activation of the enzymes is not a single mechanism. In fact three types of enzymes can be distinguished: Ru-5 P kinase and (NADPH) G-3 P dehydrogenase, maximal activation of which appears within the first minute of illumination and is promoted by antimycin A and by TMEDA; F-1,6 P2 phosphatase and S-1,7 P2 phosphatase, ferredoxin-dependent enzymes, activation of which is slightly slower but is also promoted by antimycin A and by TMEDA; finally Ru-1,5 P2 carboxylase, activation of which is still slower and characterized by the absence of any response to antimycin A as well as to TMEDA.