What role does ATP play in reforming ribulose bisphosphate (RuBP)?

Sourav PanNovember 9, 2024

Answered

ATP plays a crucial role in the regeneration of ribulose bisphosphate (RuBP) during the Calvin cycle, which is essential for the continuation of carbon fixation in photosynthesis. Here’s how ATP is utilized in this process:

Role of ATP in Regenerating RuBP

1. Conversion of G3P to Ru5P:
   • After the reduction phase of the Calvin cycle, where glyceraldehyde 3-phosphate (G3P) is produced, five out of six G3P molecules are used to regenerate RuBP. These G3P molecules undergo a series of enzymatic reactions to form ribulose-5-phosphate (Ru5P). This conversion involves several enzymes, including transketolase and aldolase, which facilitate the rearrangement of carbon skeletons.
2. Phosphorylation of Ru5P:
   • The final step in regenerating RuBP involves the enzyme phosphoribulokinase (PRK), which catalyzes the phosphorylation of Ru5P to form RuBP. This reaction requires ATP, which donates a phosphate group to Ru5P. The reaction can be summarized as follows:
   Ru5P+ATP→RuBP+ADP+Pi

   This step is critical because RuBP is the substrate for the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO), which catalyzes the fixation of CO₂ into organic molecules.

3. Energy Requirement:
   • For every three CO₂ molecules fixed during the Calvin cycle, six G3P molecules are produced, but only one exits the cycle for carbohydrate synthesis. The remaining five G3P molecules are used to regenerate three molecules of RuBP, consuming three ATP molecules in this process. Overall, it takes a significant amount of ATP and NADPH to sustain the entire cycle: specifically, nine ATP and six NADPH are required for every three CO₂ molecules fixed .
4. Continuity of the Calvin Cycle:
   • The regeneration of RuBP is essential for maintaining the cycle’s continuity. Without sufficient RuBP, the Calvin cycle would halt, preventing further carbon fixation and thus limiting the plant’s ability to synthesize carbohydrates necessary for growth and energy storage.