50. Oxygenase activity of RUBISCO generates
(1) Two molecules of PGA
(2) Two molecules of Phosphoglycolate
(3) One molecule each of PGA and phosphoglycolate
(4) Two molecules each of PGA and phosphoglycolate

Introduction

The oxygenase activity of RUBISCO is a key step in photorespiration and often asked in NEET and board exam questions. Understanding what products are formed and why helps students avoid confusion between carboxylation (normal Calvin cycle) and oxygenation (photorespiration).​

Concept: RUBISCO carboxylase vs oxygenase

RUBISCO (ribulose‑1,5‑bisphosphate carboxylase/oxygenase) acts on RuBP and can use either carbon dioxide (carboxylase activity) or oxygen (oxygenase activity) as a substrate.​

• In carboxylase activity, RuBP reacts with CO₂ to form two molecules of 3‑phosphoglycerate (3‑PGA), each a 3‑carbon compound.​

• In oxygenase activity (photorespiration), RuBP reacts with O₂ and forms one molecule of 3‑PGA (3‑carbon) and one molecule of phosphoglycolate (2‑carbon).​

Because RuBP has 5 carbons, splitting into a 3‑carbon (PGA) and a 2‑carbon (phosphoglycolate) product conserves carbon atoms in the oxygenase reaction.​

Why option (3) is correct

Option (3): “One molecule each of PGA and phosphoglycolate” is correct.​

• During photorespiration, RUBISCO’s oxygenase activity uses RuBP and O₂ and produces one 3‑phosphoglycerate (PGA, 3‑C) and one phosphoglycolate (2‑C).​

• The 3‑PGA can re‑enter the Calvin cycle, while phosphoglycolate enters the photorespiratory pathway, causing energy loss and carbon loss for the plant.​

Multiple physiology and biochemistry references state that RUBISCO’s oxygenation of RuBP yields exactly “one 3‑PGA and one 2‑phosphoglycolate,” matching option (3).​

Why option (1) is wrong

Option (1): “Two molecules of PGA” describes the carboxylase activity, not the oxygenase activity.​

• When CO₂ is the substrate, RuBP + CO₂ + H₂O → 2 molecules of 3‑PGA; this is the normal Calvin‑Benson cycle reaction.​

• The question specifically asks about “oxygenase activity,” which involves O₂ and photorespiration, so two PGA molecules are not produced in this case.​

Thus, option (1) confuses carboxylation with oxygenation and is incorrect for this question.​

Why option (2) is wrong

Option (2): “Two molecules of phosphoglycolate” does not match the known stoichiometry of the oxygenase reaction of RUBISCO.​

• Experimental and textbook descriptions consistently state one molecule of 3‑PGA and one molecule of phosphoglycolate are formed from one molecule of RuBP in the oxygenase reaction.​

• If two phosphoglycolate molecules were formed, total carbons would not balance correctly from the 5‑carbon RuBP substrate.​

Therefore, formation of two phosphoglycolate molecules is not supported and option (2) is incorrect.​

Why option (4) is wrong

Option (4): “Two molecules each of PGA and phosphoglycolate” suggests four products from one RuBP, which is not what happens biochemically.​

• One RuBP (5‑C) reacting with O₂ yields only two products: one 3‑PGA (3‑C) and one 2‑phosphoglycolate (2‑C), together accounting for the 5 carbons.​

• Producing two PGA and two phosphoglycolate molecules would require more carbon than is present in a single RuBP molecule, so this option is chemically impossible in this context.​

Hence, option (4) is not compatible with the known mechanism of RUBISCO oxygenase activity and is incorrect.​