Rubisco carboxylation template

In the Mg²⁺–carbamylated-Lys201 active site of rubisco the carbamate removes the C3 proton of ribulose 1,5-bisphosphate to give the 2,3-enediolate, which adds CO₂ at C2 to give 2′-carboxy-3-keto-arabinitol 1,5-bisphosphate; hydration at C3 and C2–C3 cleavage give one 3-phosphoglycerate and a C2 carbanion that is protonated to the second.

Mechanism step by step

The RuBP 2,3-enediolate adds CO₂ at C2; the β-keto acid is hydrated and cleaved to two 3-phosphoglycerates

1. Enolisation – The Lys201 carbamate (on Mg²⁺) removes the C3 proton: the 2,3-enediolate
2. Proton shift – The O3 proton moves to O2 (both oxygens are on Mg²⁺)
3. Carboxylation – C2 of the enediolate adds to CO₂: the β-keto acid
4. Hydration – Mg²⁺-bound OH⁻ adds to the C3 ketone: a gem-diolate
5. C2–C3 cleavage – C3 becomes a carboxylate (upper 3-PG); C2 keeps the electrons as a carbanion
6. Protonation – Lys175–H⁺ protonates C2 on one face: the second D-3-phosphoglycerate

Key points

• Rubisco is active only after CO₂ (not the substrate CO₂) carbamylates Lys201 and Mg²⁺ binds the carbamate; rubisco activase helps release inhibitory sugar phosphates.
• The upper 3-PG (C3–C5 of RuBP) leaves as the carboxylate from hydration; the lower one carries C1, C2 and the new CO₂ carbon.
• O₂ competes with CO₂ for the same enediolate: oxygenation gives 3-PG + 2-phosphoglycolate (photorespiration).
• Rubisco is slow (kcat ≈ 3 s⁻¹) and may be the most abundant protein on Earth; it fixes CO₂ in the Calvin cycle in the chloroplast stroma.

Drawn with 165 atoms, 21 arrows, 9 shapes – every one editable in MolDraw.

Sources

• Nelson & Cox, Lehninger Principles of Biochemistry, 8th ed., 2021, ch. 20 Textbook
• Berg, Tymoczko, Gatto & Stryer, Biochemistry, 9th ed., 2019, ch. 20 Textbook
• Cleland, Andrews, Gutteridge, Hartman & Lorimer, Chem. Rev. 1998, 98, 549 Review