Lopreore C, Byers L D
Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, USA.
Arch Biochem Biophys. 1998 Jan 15;349(2):299-303. doi: 10.1006/abbi.1997.0477.
Jack bean urease catalyzes the hydrolysis of thiourea with a second-order rate constant (kcat/Km) of 1.6 (+/- 0.2) x 10(-3) M-1 S-1 at pH7, 25 degrees C. This value is lower than that for urea by a factor of 3 x 10(8). The corresponding substitution of S for O in acetamide reduces the kcat/Km value 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).
刀豆脲酶在pH7、25℃条件下催化硫脲水解，二级速率常数（kcat/Km）为1.6（±0.2）×10⁻³ M⁻¹ s⁻¹。该值比尿素的相应值低3×10⁸倍。在乙酰胺中用S取代O，kcat/Km值仅降低33倍。这种含氧代化合物比相应的硫代羰基化合物具有更高的反应活性，以及尿素（Ks = 2.9 mM）比胍离子（Ki = 30 mM）或硫脲（Ki = 70 mM）结合更紧密，这表明底物的硫属元素（S或O）在过渡态时更有可能通过与酶通过中性氢键供体配位（即布朗斯特酸催化）而不是通过活性位点镍离子之一配位（即路易斯酸催化）来实现稳定。
