Calvin Cycle

BioFiles 2007, 2.6, 27.

BioFiles 2007, 2.6, 27.

The Calvin cycle is a more complex process used by algae, plants and photosynthetic bacteria to convert carbon dioxide and water into carbohydrate and molecular oxygen with the aid of sunlight. The light reactions split water into molecular oxygen and reducing power in the form of NADPH, while the dark reactions reduce carbon dioxide to carbohydrates (see Figure 6).

Figure 6. Calvin Cycle.

The fixation of carbon dioxide with D-ribulose-1,5-bisphosphate (1) is catalyzed by ribulose-1,5-bisphosphate carboxylase and yields D-3-phosphoglycerate (2).

This compound yields D-1,3-bisphosphoglycerate (3) upon phosphorylation with ATP and D-glyceraldehyde-3-phosphate (4) upon reduction with NADPH.

The next steps are isomerization to dihydroxyacetonephosphate (5), aldol condensation to D-fructose-1,6-bisphosphate (6) and further hydrolysis to D-fructose-6-phosphate.

A transketolase-catalyzed two-carbon unit transfer from Dfructose- 6-phosphate to D-glyceraldehyde-3-phosphate yields D-xylulose-5-phosphate (7) and D-erythrose-4-phosphate (8).

Aldol condensation of erythrose-4-phosphate (8) with dihydroxyacetone-phosphate (5) produces D-sedoheptulose- 1,7-bisphosphate (9), which after enzymatic hydrolysis, yields D-sedoheptulose-7-phosphate (10).

The next transketolase-catalyzed reaction of D-sedoheptulose- 7-phosphate (10) and D-glyceraldehyde-3-phosphate gives again D-xylulose-5-phosphate (7) and D-ribose-5-phosphate (11), both of which are enzymatically isomerized to ribulose-5-phosphate (12).

This is then converted back to ribulose-1,5-bisphosphate (1) by phosphorylation with ATP, completing the Calvin cycle.

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