Biotin-bound CO2 and the mechanism of enzymatic carboxylation reactions.

Considerations are provided which show that the carboxylation site (oxygen or nitrogen) of biotin is most likely unknown. Kinetic studies of intramolecular reactions of the ureido function at acyl carbon demonstrate that ureido oxygen attack is the preferred mode when the acyl group is activated. Transfer from the O-acylisourea formed by this route (relative to an N-acylurea) is also favorable. These model studies are related to the carboxylation and CO(2) transfer steps of biotin-mediated reactions, and a mechanism consistent with these results is postulated.