Evidence against a temperature-dependent conformational change in urocanase from Pseudomonas putida

Enzymatic activity of urocanase (4-imidazolone-5-propionate hydro-lyase, EC 4.2.1.49) has an unusual resistance to temperature changes, and a temperature-dependent conformational change has been suggested (Hug, D.H. and Hunter, J.K. (1974) Biochemistry 13, 1427-1431). A conformational change or dissociation has been proposed in the range of 29-31 degrees C (Cohn, M.S., Lynch, M.C. and Phillips, A.T. (1975) Biochim. Biophys. Acta 377, 444-453). In this work, no evidence was found for a temperature-dependent conformational change or dissociation. Arrhenius plots of Km and Vmax were linear; the sedimentation coefficient was independent of temperature; tryptophanyl fluorescence was a linear function of temperature; and heat capacity calorimetry showed no transitions below 60 degrees C.