Molecular Asymmetry in Urease of Water Melon (Citrullus vulgaris)

Kinetics of urease-catalysed urea hydrolysis follows Arrhenius equation in the temperature range 10–50°C and shows an energy of activation equal to 7.14 kcal/mol. The kinetics of thermal inactivation of the enzyme is biphasic, in that half of the initial activity is destroyed more rapidly than the remaining half. The data are consistent with the rate equation: A_t= A_faste^−k +_fast-t+ A_slow e^−k_slow^−t where A, is the residual activity at time t, A_fast and As_slow k_fast and k_slow are the amplitudes and the first-order rate constants of the fast and the slow phases, respectively. A similar activity decay (namely biphasic) is also observed on storing the enzyme at −4 and -4°C. The data suggest the existence of half-and-half distribution of sites which is a manifestation of a pre-existent site heterogeneity in the oligomeric protein molecule.