The Burden Borne by Urease

At the active site of urease, urea undergoes nucleophilic attack by water, whereas urea decomposes in solution by elimination of ammonia so that its rate of spontaneous hydrolysis is unknown. Quantum mechanical simulations have been interpreted as indicating that urea hydrolysis is extremely slow, compared with other biological reactions proceeding spontaneously, and that urease surpasses all other enzymes in its power to enhance the rate of a reaction. We tested that possibility experimentally by examining the hydrolysis of 1,1,3,3-tetramethylurea, from which elimination cannot occur. In neutral solution at 25 degrees C, the rate constant for the uncatalyzed hydrolysis of tetramethylurea is 4.2 x 10-12 s-1, which does not differ greatly from the rate constants observed for the uncatalyzed hydrolysis of acetamide (5.1 x 10-11 s-1) or N,N-dimethylacetamide (1.8 x 10-11 s-1) under the same conditions. We estimate that the proficiency of urease as a catalyst, (kcat/Km)/knon, is 8 x 1017 M-1, slightly higher than the values for other metalloenzymes (carboxypeptidase b and cytidine deaminase) that catalyze the hydrolysis of similar bonds.