Photosynthesis is responsible for creating NADPH and ATP and the Calvin-Benson-Bassham cycle (CBB) uses those high energy molecules to drive the production of glyceraldehyde-3-phosphate (G-3-P). G-3-P can then be used to synthesize hexose sugars which are the primary source of nutrients for heterotrophs.
Because ATP and NADPH are required for the CBB to proceed it is necessary for photosynthesis to occur prior. Photosynthesis (a light dependent reaction) uses light energy to produce ATP and NADPH which can then be used to drive synthesis of of carbohydrate molecules in the CBB, namely glyceraldehyde-3-phosphate. Although the CBB cycle has been given the nick name the "dark reaction" the enzymes involved are activated by light. Light stimulates changes in pH in the different regions of the plant cell which then in turn create a better environment for the CBB enzymes. The enzymes in the CBB cycle are very similar to other enzymes found in other metabolic path ways with the exception that they are found in the stoma instead of in the cytoplasm like in glycolysis.
Steps of the Calvin Cycle
The diagram directly below is an extremely abbreviated version of the CBB cycle.
This stage is very similar to the isomerization phase of PPP. Enzymes for these reactions are in red
- The enzyme Rubisco (ribulose bisphosphate carboxylase) catalyses the carboxilation of ribulose-1,5-bisphosphate in a two step reaction. Ribulose-1,5-bisphosphate must first be phosphorolated by the enzyme Phosphoribulose kinase. The outcome of this carboxylation are two molecules of 3-Phosphoglycerate.
This phase of CBB very closely resembles part of gluconeogenesis. Enzymes for these reactions are in red
- 3-Phosphoglycerate is then phosphorolated with the aid of Phosphoglycerate kinase to yield 1,3-Bisphosphoglycerate.
- Next 1,3-Bisphosphoglycerate is reduced by NADPH to yield NADP+ and Glyceraldehyde-3-phosphate with the aid of Glyceraldehyde-3-phosphate dehydrogenase. One of every six Glyceraldehyde-3-phosphate molecules is exported into the cytoplasm to be use in the synthesis of Glucose and other metabolic processes.
This phase of CBB very closely resembles the rearrangement phase of PPP. Enzymes for these reactions are in red
- Glyceraldehyde-3-phosphate is then reversibly converted to Dihydroxyacetone phosphate by Triose phosphate isomerase.
- Next Dihydroxyacetone is converted into fructose-6-phosphate (F-6-P) by Aldolase and Fructose bisphosphatase. Aldolase condenses the two DHAP molecules to form Fructose-1,6-bisphosphate. Because of its high (-)delta G the transformation of Fructose-1,6-bisphosphate to Fructose-6-phosphate is thought to be the rate limiting step of the CBB cycle. F-6-P can then be converted into glucose via two enzymatic steps with the help of Phosphoglucoisomerase and glucose-6-Phosphatase.
- Dihydroxyacetone can also go on to condense with Erythrose-4-phosphate to form Sedoheptulose-1,7-bisphosphate(SBP). This reaction is also catalyzed by Aldolase.
- SBP is then de-phosphorolated by Sedoheptulose bisphosphatase to yield Sedoheptulose-7-phosphate (S7P).
- After several rearrangement reactions utilizing Transketolase and Transaldolase enzymes, Xylulose-5-Phosphate (X5P) and Ribose-5-phosphate (R5P) are synthesized.
- Lastly X5P and R5P are isomerised by Phosphopentose epimerase and Phosphopentose isomerase to yield Ribulose-5-phosphate which can then be put back into the cycle.
- Garrett, H., Reginald and Charles Grisham. Biochemistry. Boston: Twayne Publishers, 2008.
- Raven, Peter. Biology. Boston: Twayne Publishers, 2005.
- What are the three phases of the CBB cycle?
- What is considered the rate determining step of the CBB cycle?
- Where does the ATP and NADPH come from that is used in the CBB cycle?
- Examine the CBB and identify the steps that are similar to the PPP and also the steps similar to gluconeogenesis.
- After watching the animation write up balanced reactions for each phase of CBB cycle.
- Darik Benson (University of California Davis)