Enzymic Carboxyl Transfer from N-Carboxybiotin. A Molecular Orbital Evaluation of Conformational Effects in Promoting Reactivity

On the basis of analysis of conformational energetics and enzymatic reactivity patterns, it is shown that the activation of N-carboxybiotin toward carboxyl transfer can be initiated by rotation of the carboxyl group out of the plane of the urea ring of N-carboxybiotin. This rotation provides proper polarization and bond weakening as expected from resonance models. It also provides a correct alignment for bond formation between the carbanion derived from the substrate and the carboxyl group of N-carboxybiotin. The proposal is supported by molecular orbital calculations on ground-state and transition-state models. These calculations also indicate that puckering of the imidazolidone ring does not lower activation barriers. The activation energy calculated for bond rotation is consistent with measured barriers in related systems. The mechanism accounts for the low nonenzymic activity of N-carboxylated ureas, the source of enzymic activation, and the recently reported “triggering” of N-carboxybiotin by substrate analogues.