Question

explain in details why is it important, and logical, that some enzymes of the Calvin cycle are regulated by the presence or absence of light?

Answer 1

Solution:-

The dark reaction of photosynthesis involves reduction and fixation of carbon. In all oxygenic photosynthetic organisms, CO2 reduction and fixation occurs through carbon reduction cycle called Calvin Cycle. Calvin cycle is a series of biochemical reactions which reduce CO2 to three-carbon sugar.

Some enzymes in this cycle are light regulated. Five enzymes are regulated by light including ribulose 1,5 bisphosphate (RuBP) carboxylase/oxygenase Rubisco, NADP glyceraldehyde-3-phosphate dehydrogenase, fructose-1,6-bisphosphatase, Sedoheptulose-1,7- bisphosphatase and ribulose-5-phosphate kinase. Activity of last four enzymes is controlled by light via ferredoxin-thioredoxin system. These enzymes contain one or more disulfide groups that exist in oxidized state in dark thus the enzyme is inactive or subactive. In the light the -S-S- group is subjected to a redox change and reduced to the sulfhydryl state (-SH HS-) resulting in activation of enzyme. Regulation of Rubisco by this system is indirect via thioredoxin accessory enzyme, Rubisco activase. Thioredoxin is coupled to ferredoxin by ferredoxin thioredoxin reductase. In light, thioredoxin is reduced by electrons moving from PSI through ferredoxin (Fd), then thioredoxin reduces critical disulfide bonds in enzymes such as sedoheptulose 1,7-bisphosphatase and fructose 1,6-bisphosphatase, activating these enzymes. In the dark, the -SH groups undergo reoxidation to disulfides by inactivating the enzymes.