Question

Mechanism of the ribulose-1,5 biphosphate carboxylase C-C bond formation and cleavage to generate two molecules of 3-phosphoglycerate.

Answer 1

Mechanism of the ribulose-1,5 biphosphate carboxylase C-C bond formation:

Carboxylation reactions are essentially the same as aldol condensations excepting that the carbonyl electrophile is CO₂ rather than a ketone or an aldehyde. The mechanism (Rubisco) involves the key carbon-fixing enzyme in plants and photosynthetic bacteria. Magnesium ion plays a key role throughout the reaction as a charge-stabilizing factor.

Step 1 is the deprotonation of an alpha-carbon to form an enolate. Step 2 is an intramolecular proton transfer, which has the effect of creating a different enolate intermediate and making C2 into the nucleophile for an aldol-like attack on CO₂ (step 3). Carbon dioxide has been 'fixed' into organic form and it has become a carboxylate group on a six-carbon sugar derivative.

Mechanism for the cleavage to generate two molecules of 3-phosphoglycerate:

Step 4 is a retro-Claisen mechanism, with water as a nucleophile and enolate as the leaving group. After protonation of this enolate, we are left with two molecules of 3-phosphoglycerate, which are incorporated into the 'gluconeogenesis' pathway of glucose synthesis.