Discuss the first two reactions (carboxylation and reduction phases) involved in the CBC. Clearly indicate the substrate enzymes and the products of these two reactions

Answer 1

The Calvin-Benson cycle (CBC) is the biochemical pathway that plants use to fix carbon dioxide (CO2) into organic molecules, primarily glucose. It consists of several reactions, but here we will focus on the first two reactions called carboxylation and reduction phases.

1. Carboxylation:
- Substrate: Ribulose-1,5-bisphosphate (RuBP)
- Enzyme: Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO)
- Product: 3-phosphoglycerate (PGA)

In the carboxylation phase, carbon dioxide (CO2) is added to a 5-carbon sugar molecule called RuBP, catalyzed by the enzyme RuBisCO. This reaction produces an unstable 6-carbon intermediate, which then immediately splits into two 3-carbon molecules called PGA.

2. Reduction:
- Substrate: 3-phosphoglycerate (PGA)
- Enzyme: Phosphoglycerate kinase and glyceraldehyde-3-phosphate dehydrogenase
- Product: Glyceraldehyde-3-phosphate (G3P)

In the reduction phase, the 3-phosphoglycerate (PGA) molecules are phosphorylated and reduced to form glyceraldehyde-3-phosphate (G3P). These conversions occur in multiple steps with the help of enzymes like phosphoglycerate kinase and glyceraldehyde-3-phosphate dehydrogenase. For every three molecules of CO2 entering the cycle, six molecules of G3P are produced; however, only one molecule of G3P is released from the cycle for the synthesis of glucose, while the other five molecules continue in the cycle as reactants for regeneration of RuBP.

These two reactions, carboxylation and reduction, are essential for converting CO2 into a usable form, G3P, which can then be used for the synthesis of glucose and other organic molecules within the plant.