Regulation of Photosynthesis

The dark reactions of photosynthesis are actually stimulated by the light reactions. The central enzyme in the dark reactions, ribulose-1,5-bisphosphate carboxylase, is stimulated by high pH and CO 2 and Mg2+. Remember that pumping of protons from the stroma increases the pH. As protons are lost from the stroma due to pumping, Mg2+ enters to replace the lost charges.

Three other enzymes of the Calvin cycle are activated by reduction of disulfides to sulfhydryls. These enzymes are sedoheptulose-1,7-bisphosphatase (Figure 17.22), glyceradehyde-3-phosphate dehydrogenase, and ribulose-5-phosphate kinase. This disulfide reduction reaction is dependent upon a disulfide exchange reaction promoted by the protein thioredoxin (Figure 17.23). In this reaction, thioredoxin becomes oxidized and must be reduced to function again. Reduction is catalyzed by the ferredoxin-thioredoxin reductase, and uses electrons from ferredoxin (Figure 17.23). High levels of ferredoxin accumulate when NADP+ stores are low, and this occurs during light exposure. Thus, when light is present, the Calvin cycle is stimulated. It should be noted that thioredoxin has another effect - it stimulates the CF 0-CF 1 complexes as well.

In the dark, plants must metabolize some of the stored energy. In general, the pathways of catabolism - glycolysis, the citric acid cycle, and the pentose phosphate pathway - are inhibited in the presence of sunlight and become more active in the dark. The key light-inhibited enzymes are phosphofructokinase (from glycolysis) and glucose-6-phosphate dehydrogenase (pentose phosphate pathway). The latter enzyme also can be reduced by thioredoxin, but in this case the reduction causes inhibition of the enzyme.

See also: Photosystem II, Photosystem I