RuBisCO Enzyme: Carboxylation and Oxygenation Activity
Biology · Photosynthesis in Higher Plants · NEET
RuBisCO (Ribulose bisphosphate Carboxylase-Oxygenase) is a single enzyme with two jobs. When it binds CO2 (carboxylation), it fixes CO2 onto RuBP to make two molecules of 3-PGA in the Calvin cycle. When it binds O2 (oxygenation), it starts photorespiration, making one 3-carbon PGA and one 2-carbon phosphoglycolate. Memory hook: the name itself tells you both jobs - "Carboxylase-Oxygenase" = it can grab CO2 OR O2 at the same active site.
RuBisCO has one active site that binds either CO2 or O2. CO2 (green, carboxylation) gives two 3-PGA and feeds the Calvin cycle; O2 (red, oxygenation) gives one 3-PGA plus 2-carbon phosphoglycolate and starts wasteful photorespiration.
Your doubts, answered
Why is RuBisCO called both a carboxylase and an oxygenase?
Because its active site can bind BOTH CO2 and O2. When it binds CO2 it acts as a carboxylase and fixes carbon (Calvin cycle). When it binds O2 it acts as an oxygenase and starts photorespiration. One enzyme, two activities - that is exactly why NCERT writes the full name as Ribulose bisphosphate Carboxylase-Oxygenase (RuBisCO).
Does RuBisCO prefer CO2 or O2?
RuBisCO has a much greater affinity for CO2 than for O2. The binding is competitive, so it is the relative amount of CO2 and O2 that decides which one binds. In C3 plants, when O2 is high compared to CO2, some O2 binds RuBisCO and CO2 fixation drops.
What is formed when RuBisCO uses O2 instead of CO2?
In oxygenation, RuBP + O2 gives ONE molecule of 3-carbon phosphoglycerate (3-PGA) and ONE molecule of 2-carbon phosphoglycolate. Compare this with carboxylation, which gives TWO molecules of 3-PGA. So the O2 pathway wastes carbon - this is photorespiration.
Why does photorespiration happen only in C3 plants and not C4 plants?
C4 plants concentrate CO2 around RuBisCO inside bundle sheath cells using Kranz anatomy. High CO2 keeps RuBisCO acting as a carboxylase, so O2 rarely binds. C3 plants have no such CO2-pump, so on hot dry days O2 competes and photorespiration occurs. RuBisCO is the same enzyme in both - only the CO2 concentration at its site differs.
Is RuBisCO the most abundant enzyme in the world?
Yes. NCERT states RuBisCO is the most abundant enzyme (protein) in the whole biosphere. This is a very common one-line NEET fact, often paired with 'collagen is the most abundant protein in the animal world.'
⚠️ The NEET trap ✗ RuBisCO is only a carboxylating enzyme that fixes CO2, and PEP carboxylase does the oxygenation. ✓ RuBisCO itself does BOTH carboxylation and oxygenation at the same active site. PEP carboxylase (PEPcase) is a different C4 enzyme that only fixes CO2 and has NO oxygenase activity. 🧠 The exam loves mixing RuBisCO with PEPcase. Remember: only RuBisCO has the double name carboxylase-OXYGENASE, so only RuBisCO can start photorespiration.
Real NEET questions
2025
Which of the following statements about RuBisCO is true?
A · It is an enzyme involved in the photolysis of water.
B · It catalyzes the carboxylation of RuBP. ✓
C · It is active only in the dark.
D · It has higher affinity for oxygen than carbon dioxide.
Solution: RuBisCO catalyses carboxylation - CO2 is added to the 5-carbon RuBP to form two molecules of 3-PGA. Water photolysis is done by the water-splitting complex of PS II, not RuBisCO. RuBisCO has GREATER affinity for CO2, not O2, so option D is wrong. Answer: it catalyzes the carboxylation of RuBP.
2026
The photorespiration reaction catalysed by RuBisCO is: RuBP + O2 -> 3-Phosphoglycerate + X. Identify X.
A · Phosphoenolpyruvate
B · 2-Phosphoglycolate ✓
C · Oxaloacetate
D · Malate
Solution: In photorespiration RuBisCO acts as an oxygenase: RuBP binds O2 and, instead of two molecules of PGA, forms one molecule of 3-phosphoglycerate (3C) and one molecule of 2-phosphoglycolate (2C). So X = 2-phosphoglycolate.
2023
Statement I: RuBisCO is the most abundant enzyme in the world. Statement II: Photorespiration does not occur in C4 plants. Choose the correct answer.
A · Both Statement I and Statement II are correct ✓
B · Both Statement I and Statement II are incorrect
C · Statement I is correct but Statement II is incorrect
D · Statement I is incorrect but Statement II is correct
Solution: Statement I is correct - RuBisCO is the most abundant enzyme in the biosphere. Statement II is correct - in C4 plants Kranz anatomy concentrates CO2 at the enzyme site in bundle sheath cells, so RuBisCO acts as a carboxylase and photorespiration does not occur. Both statements are correct.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Ribulose bisphosphate Carboxylase-Oxygenase. The name captures both its activities: carboxylase (binds CO2) and oxygenase (binds O2).
What is the substrate of RuBisCO?
Its substrate is RuBP (Ribulose 1,5-bisphosphate), a 5-carbon molecule that is also the primary CO2 acceptor of the Calvin cycle. RuBisCO adds either CO2 or O2 to this same RuBP.
How many 3-PGA molecules form during carboxylation?
Carboxylation of one RuBP by CO2 forms TWO molecules of 3-carbon 3-PGA, the first stable product of the C3 pathway. Oxygenation instead forms only ONE 3-PGA plus one 2-carbon phosphoglycolate.
Which enzyme catalyses the first step of the Calvin cycle?
RuBisCO catalyses carboxylation, the first and most crucial fixation step of the Calvin cycle, joining CO2 to RuBP.
Do C4 plant mesophyll cells have RuBisCO?
No. In C4 plants RuBisCO is present only in bundle sheath cells, not mesophyll cells. Mesophyll cells use PEP carboxylase instead, which has no oxygenase activity.
