The reductive pentose phosphate cycle for photosynthetic CO2 assimilation: enzyme modulation.

The reductive pentose phosphate cycle (Benson-Calvin cycle) is the main biochemical pathway for the conversion of atmospheric CO2 to organic compounds. Two unique systems that link light-triggered events in thylakoid membranes with enzyme regulation are located in the soluble portion of chloroplasts (stroma): the ferredoxin-thioredoxin system and ribulose 1,5-bisphosphate carboxylase/oxygenase-Activase (Rubisco-Activase). The ferredoxin-thioredoxin system (ferredoxin, ferredoxin-thioredoxin reductase, and thioredoxin) transforms native (inactive) glyceraldehyde-3-P dehydrogenase, fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase, and phosphoribulokinase to catalytically competent forms. However, the comparison of enzymes reveals the absence of common amino acid sequences for the action of reduced thioredoxin. Thiol/disulfide exchanges appear as the underlying mechanism, but chloroplast metabolites and target domains make the activation process peculiar for each enzyme. On the other hand, Rubisco-Activase facilitates the combination of CO2 with a specific epsilon-amino group of ribulose 1,5-bisphosphate carboxylase/oxygenase and the subsequent stabilization of the carbamylated enzyme by Mg2+, in a reaction that depends on ATP and ribulose 1,5-bisphosphate. Most of these studies were carried out in homogeneous solutions; nevertheless, a growing body of evidence indicates that several enzymes of the cycle associate either with thylakoid membranes or with other proteins yielding supra-molecular complexes in the chloroplast.