Carboxylation, biotin and

STEPS 3-4 Carboxylation and acyl transfer. The third priming reaction again starts with acetyl CoA, which is carboxylated by reaction with HCO3" and ATP to yield malonyl CoA plus ADP. This step involves the coenzyme biotin, which is bonded to the lysine residue of acetyl CoA carboxylase enzyme and acts as a carrier of CO2. The enolate ion of acetyl CoA reacts with carboxylated biotin and transfers the CO2 group in a nucleophilic acyl substitution reaction. 

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