Question

5. List the most important enzyme and the inputs and outputs of the Calvin cycle. 6. Describe the three main stages of the Calvin Cycle.

Answer 1

The CO₂ incorporation into carbohydrates by photosynthetic eukaryotic organisms, occurs within stroma of chloroplast and is known as Calvin cycle. The reactions of Calvin cycle can occur without light when sufficient ATP and NADPH are supplied. Such reactions are called dark reactions or life independent reactions.

The Calvin cycle reactions are divided in to 3 phases known as carbon fixation, reduction and regeneration.

Carbon fixation: The mechanism of carbon fixation occurs by the incorporation of inorganic CO₂ in to organic molecules completed in a single reaction. Ribulose – 1, 5 - bisphosphate carboxylase (Rubisco) is an enzyme that catalyzes the carboxylation of Ribulose – 1, 5 - bisphosphate to produce 2 molecules of glycerate – 3 – phosphate. This enzyme requires Mg²⁺.

Reduction: In the reduction phase of Calvin cycle the glycerate – 3 – phosphate (G3P) produced earlier is converted to glyceraldehyde – 3 – phosphate. Around 6 molecules of glycerate – 3 – phosphate are phosphorylated by 6 ATP molecules to produce 6 glycerate – 1, 3 – bisphosphate in the first two reactions. The glycerate – 1, 3 – bisphosphate molecules are then reduced by NADP⁺ and glyceraldehyde – 3 – phosphate dehydrogenase producing 6 glyceraldehyde – 3 – phosphate molecules.

Regeneration: In Calvin cycle one molecule of glyceraldehyde – 3 – phosphate is the net production of fixed carbon. The other 5 glyceraldehyde – 3 – phosphate molecules are processed to regenerate 3 molecules of Ribulose – 1, 5 – bisphosphate involving the glyceraldehyde – 3 – phosphate. About 4 molecules of   glyceraldehyde – 3 – phosphate are isomerized forming two dihydroxyacetonephosphate (DHAP). One molecule of DHAP is condensed with the third molecule of glyceraldehyde – 3 – phosphate to form fructose – 1, 6 – bisphosphate. This reaction catalyzed by aldolase enzyme. The fructose – 1, 6 – bisphosphate is hydrolyzed to fructose – 6 – phosphate by fructose – 1, 6 – biphosphatase. The fructose – 6 – phosphate combines with the 4^th molecule of glyceraldehyde – 3 – phosphate subsequently by transketolase enzyme to form xylulose – 5 – phosphate and erythrose – 4 – phosphate.

The erythrose – 4 – phosphate condenses with the 2^nd molecule of DHAP to form sedoheptulose – 1, 7 – bisphosphate catalyzed by aldolase enzyme. The sedoheptulose – 1, 7 – bisphosphate is then hydrolyzed to sedoheptulose – 7 – phosphate. The 5^th molecule of glyceraldehyde – 3 – phosphate condenses with sedoheptulose – 7 – phosphate to form ribose – 5 – phosphate and xylulose – 5 – phosphate. This reaction is catalyzed by transketolase enzyme. The ribose – 5 – phosphate and xylulose – 5 – phosphate isomerized separately to ribulose – 5 – phosphate. In the last reaction 3 molecules of ribulose – 5 – phosphate are phosphorylated by 3 ATP to form 3 molecules of ribulose – 1, 5 – phosphate catalyzed by ribulose – 5 – phosphatekinase.

The remaining molecule of glyceraldehyde – 3 – phosphate is used within the chloroplast for starch synthesis or transported to cytoplasm for synthesis of sucrose or other metabolites.