An alternative explanation for the catalytic proficiency of orotidine 5′-phosphate decarboxylase

Orotidine 5′-phosphate decarboxylase (ODCase) is the most proficient enzyme known, enhancing the rate of decarboxylation of orotidine 5′-phosphate (OMP) by a factor of 10¹⁷, which corresponds to a ΔΔG‡ of ∼24 kcal/mol. Ground-state destabilization through local electrostatic stress has been recently proposed as the basis of catalytic rate enhancement for a mechanism that is the same as in solution. We have carried out gas-phase ab initio quantum mechanical calculations combined with a free energy method, a continuum solvent model, and molecular dynamics simulations to assess an alternative mechanism. Although we are not able to reproduce the experimentally observed ΔΔG‡ quantitatively, we present evidence that this ΔΔG‡ is very large, in the range found experimentally. We thus conclude that the preferred mechanism may well be different from that in solution, involving an equilibrium pre-protonation of OMP C5 by a catalytic lysine residue that greatly reduces the barrier to subsequent decarboxylation.