Theoretical analysis of the reaction mechanism of biotin carboxylase

A computational approach Is taken to clarify the reaction mechanism of biotin carboxylase (BC) by using the B3LYP density functional method. The overall reaction of BC is supposed to consist of two steps: in the first step, carboxyphosphate (CP) is generated from bicarbonate and ATP, and it is subject to nucleophilic attack on its carboxyl group by biotin to form carboxybiotin in the second step. The activation energies for the transition states of the first and second steps are computed to be 46.6 and 7.9 kcal/mol, respectively, demonstrating that the first step limits the overall reaction of BC. In the second step, the ureido moiety of biotin undergoes enolization with the aid of general acid-base catalysis by CP, followed by collapse of CP into CO ₂ and phosphate. The resulting bent CO₂ Is highly labile and condenses quickly with enolic biotin to give carboxybiotin. Implicit in this scheme as they are, ingenious proton movements between the two substrates, CP and biotin, dictate all of the succeeding chemical events.