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CO2 fixation represents the first committed step of carbon assimilation in photosynthesis and other autotrophic pathways, where inorganic CO2 becomes part of organic molecules. This question tests fundamental biochemistry knowledge critical for plant biotech and microbial metabolism.

The correct answer is (B) Ribulose 1,5-bisphosphate carboxylase (RuBisCO), the most abundant enzyme on Earth that catalyzes CO2 + RuBP → 3-PGA in the Calvin cycle.

Why (B) RuBisCO Fixes CO2 into Organic Form

RuBisCO performs the carboxylation reaction: Ribulose-1,5-bisphosphate (5C) + CO₂ → 2 molecules of 3-phosphoglycerate (3C each). This six-carbon intermediate spontaneously splits, incorporating one CO₂ carbon into organic form. As the Calvin cycle entry point, RuBisCO fixes ~100 billion tons of CO₂ annually.

Explanation of All Options

Each enzyme has distinct metabolic roles:

• (A) Ribulose 5-phosphate kinase
  Wrong. Catalyzes Ru5P + ATP → RuBP + ADP in Calvin cycle regeneration phase. Produces RuBP substrate but doesn’t fix CO₂.

• (B) Ribulose 1,5-bisphosphate carboxylase (RuBisCO)
  Correct. Primary CO₂ fixation enzyme. RuBP + CO₂ → [unstable 6C] → 2 × 3-PGA. Rate-limiting step of photosynthesis.

• (C) Pyruvate dehydrogenase
  No. Converts pyruvate → acetyl-CoA + CO₂ (decarboxylation, not fixation). Links glycolysis to TCA cycle; releases CO₂.

• (D) Carbonic anhydrase
  Incorrect. CA catalyzes CO₂ + H₂O ⇌ HCO₃⁻ + H⁺. Converts between gas/ion forms for transport but doesn’t incorporate CO₂ into organics.

Quick Enzyme Function Table

Biotech relevance: Engineering RuBisCO kinetics is key to improving crop photosynthesis efficiency (RIPE project). Memory trick: “RuBisCO = Rubs In CO₂.” Carbonic anhydrase confusion common—CA moves CO₂, RuBisCO fixes it.