Medium effects in enzyme-catalyzed decarboxylations.

Carbon isotope effects and steady-state kinetic parameters have been measured for the decarboxylation of arginine and homoarginine by the pyridoxal 5'-phosphate dependent arginine decarboxylase from Escherichia coli. In water at pH 5.25, 5 degrees C, homoarginine shows an isotope effect k12/k13 = 1.601, indicating that the decarboxylation step is entirely rate determining. In the presence of 16 mol % ethylene glycol under otherwise identical conditions, the decarboxylation rate is increased 3-fold, and the carbon isotope effect is 1.044, indicating that the rate of the decarboxylation step is increased by the presence of the less polar solvent. The decarboxylation or arginine under the same conditions shows a similar trend: in water, the isotope effect is 1.027, decreasing to 1.003 in 16% ethylene glycol, with little change in the steady-state rate. Again, the rate of the decarboxylation step is substantially increased by the presence of the nonpolar solvent. Thus, pyridoxal phosphate dependent enzymatic decarboxylations show a medium effect similar to that observed in a number of nonenzymatic decarboxylations. This suggests that these enzymes may accelerate the decarboxylation step by providing a nonpolar environment. Evidence is also presented that desolvation of the substrate carboxyl group may contribute to catalysis.