Photosynthetic Carbohydrate Synthesis:- Four Enzymes of the Calvin Cycle Are Indirectly Activated by Light
The reductive assimilation of CO2 requires a lot of ATP and NADPH, and their stromal concentrations increase when chloroplasts are illuminated (Fig. 1). The light-induced transport of protons across the thylakoid membrane also increases the stromal pH from about 7 to about 8, and it is accompanied by a flow of Mg2 from the thylakoid compartment into the stroma, raising the [Mg2+] from 1 to 3 mM to 3 to 6 mM. Several stromal enzymes have evolved to take advantage of these light-induced conditions, which signal the availability of ATP and NADPH: the enzymes are more active in an alkaline environment and at high [Mg2+]. For example, activation of rubisco by formation of the carbamoyl lysine is faster at alkaline pH, and high stromal [Mg2 ] favors formation of the enzyme’s active Mg2 complex. Fructose 1,6-bisphosphatase requires Mg2 and is very dependent on pH (Fig. 2); its activity increases more than 100-fold when pH and [Mg+2] rise during chloroplast illumination. Four Calvin cycle enzymes are subject to a special type of regulation by light. Ribulose 5-phosphate kinase, fructose 1,6-bisphosphatase, sedoheptulose 1,7-bisphosphatase, and glyceraldehyde 3-phosphate dehydroge nase are activated by light-driven reduction of disulfide bonds between two Cys residues critical to their catalytic activities. When these Cys residues are disulfide bonded (oxidized), the enzymes are inactive; this is the normal situation in the dark. With illumination, electrons flow from photosystem I to ferredoxin , which passes electrons to a small, soluble, disulfide containing protein called thioredoxin (Fig. 3), in a reaction catalyzed by ferredoxin-thioredoxin reductase. Reduced thioredoxin donates electrons for the reduction of the disulfide bonds of the light-activated enzymes, and these reductive cleavage reactions are accompanied by conformational changes that increase enzyme activities. At nightfall, the Cys residues in the four enzymes are reoxidized to their disulfide forms, the enzymes are inactivated, and ATP is not expended in CO2 assimilation. Instead, starch synthesized and stored during the daytime is degraded to fuel glycolysis at night. Glucose 6-phosphate dehydrogenase, the first en zyme in the oxidative pentose phosphate pathway, is also regulated by this light-driven reduction mechanism, but in the opposite sense. During the day, when photo synthesis produces plenty of NADPH, this enzyme is not needed for NADPH production. Reduction of a critical disulfide bond by electrons from ferredoxin inactivates the enzyme.
FIGURE 1 Source of ATP and NADPH. ATP and NADPH produced by the light reactions are essential substrates for the reduction of CO2. The photosynthetic reactions that produce ATP and NADPH are accompanied by movement of protons (red) from the stroma into the thylakoid, creating alkaline conditions in the stroma. Magnesium ions pass from the thylakoid into the stroma, increasing the stromal [Mg2+].
FIGURE 2 Activation of chloroplast fructose 1,6-bisphosphatase. Reduced fructose 1,6-bisphosphatase (FBPase-1) is activated by light and by the combination of high pH and high [Mg+2] in the stroma, both of which are produced by illumination.
FIGURE 3 Light activation of several enzymes of the Calvin cycle. The light activation is mediated by thioredoxin, a small, disulfide-containing protein. In the light, thioredoxin is reduced by electrons moving from photosystem I through ferredoxin (Fd) (blue arrows), then thioredoxin reduces critical disulfide bonds in each of the enzymes sedoheptulose 1,7-bisphosphatase, fructose 1,6 bisphosphatase, ribulose 5-phosphate kinase, and glyceraldehye 3-phosphate dehydrogenase, activating these enzymes. In the dark, the –SH groups undergo reoxidation to disulfides, inactivating the enzymes.
