Biochemistry - Chemical Reactions in Living Cells, Volume 3 - D. Metzler 1980

Light in Biology
Photosynthesis
Regulation of Photosynthesis

The key reaction of CO₂ reduction in The Calvin Cycle is the carboxylation of ribulose diphosphate [Eq. (7-81)]. Carboxylase is allosterically activated by fructose 6-phosphate and inhibited by fructose 1,6-diphosphate (Fig. 13-24) [119]. Thus, the accumulation of fructose 1,6-diphosphate serves as a signal that shuts down the carboxylation process, whereas the appearance of high concentrations of fructose 6-phosphate initiates the Reactions of the Calvin cycle. It follows that the functioning of the Calvin cycle, much like Gluconeogenesis reactions (Chap. 11, Sec. E,5), fundamentally depends on fructose 1,6-diphosphatase, which is subject to potent regulatory influences. In METABOLISM/14.html">Chloroplasts, this enzyme is activated by light, The Effect of which is mediated by reducedferredoxin. The latter, together with a certain "protein factor," converts the inactive fructose diphosphatase (presumably by reducing a specific group on the enzyme) into its active form (Fig. 13-24) [119]. Light also activates several other Calvin cycle Enzymes [120], while the crucial enzyme glyceraldehyde-3-phosphate dehydrogenase appears to alter its Specificity (from NADH to NADPH) upon exposure to light [121]. Another aspect of the regulatory process is the 3-phosphoglycerate-mediated activation of ADP-glucose synthesis from glucose 1-phosphate—an example of feed-forward regulation (Fig. 13-24). Additional aspects of photosynthetic regulation not covered here are discussed in [122].

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FIG. 13-24. Selected regulatory MECHANISMS OF CARBON dioxide assimilation during Photosynthesis [119].