Q.56 Rubisco catalyzes conversion of ribulose 1,5–bisphosphate to
P. two molecules of stable 3–phosphoglycerate
Q. one molecule of 3–phosphoglycerate and one molecule of phosphoglycolate
R. one molecule of ribulose 5–phosphophate
S. one molecule of ribose 5–phosphate
Choose only the correct combination
(A) P and S (B) Q and S (C) P and Q (D) R and S

Rubisco, or ribulose-1,5-bisphosphate carboxylase/oxygenase, primarily catalyzes the carboxylation of ribulose 1,5-bisphosphate (RuBP) in the Calvin cycle to produce two molecules of 3-phosphoglycerate under high CO₂ conditions. However, its oxygenase activity during photorespiration yields one molecule of 3-phosphoglycerate and one of phosphoglycolate. The correct option is (C) P and Q, as both represent products of Rubisco’s dual catalytic functions.

Option Analysis

• P (two molecules of stable 3-phosphoglycerate): Correct for carboxylation reaction (RuBP + CO₂ → 2 × 3-PGA), the main CO₂ fixation step in photosynthesis.

• Q (one molecule of 3-phosphoglycerate and one molecule of phosphoglycolate): Correct for oxygenation reaction (RuBP + O₂ → 3-PGA + phosphoglycolate), initiating photorespiration.

• R (one molecule of ribulose 5-phosphate): Incorrect; RuBP is regenerated from 3-PGA via multiple Calvin cycle steps, not directly produced by Rubisco.

• S (one molecule of ribose 5-phosphate): Incorrect; ribose 5-phosphate forms in the non-oxidative pentose phosphate pathway, unrelated to direct Rubisco catalysis.

Rubisco catalyzes conversion of ribulose 1,5-bisphosphate (RuBP), the key CO₂ acceptor in photosynthesis, playing a central role in carbon fixation for CSIR NET Life Sciences aspirants. This enzyme’s dual activity—carboxylase and oxygenase—determines photosynthetic efficiency versus photorespiratory losses.

Carboxylation Mechanism

Rubisco catalyzes conversion of ribulose 1,5-bisphosphate with CO₂, forming an unstable 6-carbon intermediate that splits into two stable 3-phosphoglycerate (3-PGA) molecules. This step initiates the Calvin-Benson cycle, enabling sugar synthesis in chloroplasts.

• Enolization of RuBP exposes C2 for CO₂ attack.

• Hydration and cleavage yield 2 × 3-PGA.

Oxygenase Activity and Photorespiration

When O₂ competes with CO₂, Rubisco catalyzes conversion of ribulose 1,5-bisphosphate to one 3-PGA and one phosphoglycolate (2-carbon), triggering photorespiration. Phosphoglycolate salvage recycles ~75% carbon but releases CO₂, reducing efficiency by 25-30% in C3 plants.

• Occurs at high O₂/CO₂ ratios (e.g., closed stomata).

• Involves peroxisomes and mitochondria for glycine/serine conversion.

Exam Relevance for CSIR NET

Options P and Q reflect both reactions, making (C) correct—vital for questions on enzyme bifunctionality. R and S confuse regeneration/isomerization steps.

• P: Calvin cycle fixation.

• Q: Photorespiratory oxygenation.
  Avoid pitfalls: Stable 3-PGA emphasizes carboxylation’s productivity.