Orotic acid decarboxylation in water and nonpolar solvents: a potential role for desolvation in the action of OMP decarboxylase.

OMP decarboxylase (ODCase) generates a very large rate enhancement without the assistance of metals or other cofactors. The uncatalyzed decarboxylation of 1-methylorotate in water is shown to involve the monoanion, although uncharged 1-methylorotic acid is decarboxylated at a similar rate. To measure the extent to which the rate of the nonenzymatic decarboxylation of orotate derivatives might be enhanced by their removal from solvent water, the 1-phosphoribosyl moiety of OMP was replaced with 1-substituents that would allow it to enter less polar solvents. When the tetrabutylammonium salt of 1-cyclohexylorotate was transferred from water to a series of dipolar aprotic solvents, its rate of decarboxylation increased markedly, varying with the relative ability of each solvent to release the substrate in the ground state from stabilization by solvent water acting as a proton donor. These findings are consistent with the view that separation of the substrate from solvent water may contribute, at least to a limited extent, to the rate enhancement produced by ODCase. This enzyme's active site, like that of another cofactorless enzyme recently shown to produce a rate enhancement similar in magnitude (uroporphyrinogen decarboxylase), is equipped with an ammonium group positioned in such a way as to balance the electrostatic charge of the carboxylate group of the substrate and later supply a proton to the incipient carbanion in a relatively waterless environment.