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
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.
The Calvin-Benson cycle (CBC) is a series of chemical reactions that take place during photosynthesis in order to convert carbon dioxide (CO2) into glucose and other organic compounds. The cycle consists of three main phases: carboxylation, reduction, and regeneration.
1. Carboxylation Phase:
In this phase, an enzyme called Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) catalyzes the carboxylation reaction. The substrate for this reaction is ribulose-1,5-bisphosphate (RuBP), which is a 5-carbon compound.
The reaction involves the addition of a carbon dioxide (CO2) molecule to RuBP, resulting in an unstable 6-carbon compound. This compound quickly breaks down into two molecules of 3-phosphoglycerate (PGA), which are 3-carbon compounds. This reaction is often referred to as fixation of CO2.
Overall reaction:
3 molecules of RuBP + 3 molecules of CO2 → 6 molecules of PGA
2. Reduction Phase:
In this phase, ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) produced in the light-dependent reactions are used to convert the PGA molecules into glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar.
The reduction is a multi-step process that involves the conversion of PGA into G3P. First, ATP provides energy to phosphorylate PGA, forming 1,3-bisphosphoglycerate (1,3-BPG). Then, NADPH acts as a reducing agent, donating high-energy electrons and hydrogens to convert 1,3-BPG into G3P. One G3P molecule is produced for every three molecules of CO2 fixed.
Overall reaction:
6 molecules of PGA + 6 molecules of ATP + 6 molecules of NADPH → 6 molecules of G3P
It is important to note that only one out of every six G3P molecules synthesized is used to produce glucose or other organic compounds. The remaining five G3P molecules are regenerated into RuBP to continue the cycle.
These first two reactions, carboxylation and reduction, convert CO2 into usable organic compounds, laying the foundation for the subsequent reactions in the regeneration phase of the Calvin-Benson cycle.