The Urease-Catalyzed Hydrolysis of Thiourea and Thioacetamide

Jack bean urease catalyzes the hydrolysis of thiourea with a second-order rate constant (kcat/Km) of 1.6 (±0.2) × 10−3M−1s−1at pH7, 25°C. This value is lower than that for urea by a factor of 3 × 108. The corresponding substitution of S for O in acetamide reduces thekcat/Kmvalue by only a factor of 33. This greater reactivity of the oxo compounds than of the corresponding thiono compounds, and the tighter binding of urea (Ks= 2.9 mM) than of either the guanidinium ion (Ki= 30 mM) or thiourea (Ki= 70 mM), suggests that the substrate chalcogen (S or O) is more likely to be stabilized in the transition state by coordination to the enzyme via a neutral hydrogen-bond donor (i.e., Brønsted acid catalysis) than by coordination via one of the active-site nickel ions (i.e., Lewis acid catalysis).