Steady-State Kinetics of the Hypoxanthine-Guanine-Xanthine Phosphoribosyltransferase from Tritrichomonas foetus:  The Role of Threonine-47

Tritrichomonas foetus, an anaerobic flagellated protozoan, causes urogenital trichomoniasis in cattle. Hypoxanthine-guanine-xanthine phosphoribosyl transferase (HGXPRTase), an essential enzyme in T. foetus required for salvaging exogenous purine bases, has been regarded as a promising target for anti-tritrichomonial chemotherapy. The steady-state kinetic analyses of synthesis and pyrophosphorolysis of IMP, GMP, and XMP and product inhibition studies have been used to elucidate the reaction mechanisms. Double-reciprocal plots of initial velocities versus the varying concentrations of one substrate at a fixed concentration of the other show intersecting lines indicating a sequential mechanism for both the forward and the reverse reactions. In terms of the k cat/K m ratios, hypoxanthine is the most effective substrate whereas guanine and xanthine are converted equally well into their corresponding nucleotides. The minimum kinetic model from the data in product inhibition studies is an ordered bi−bi mechanism, where the substrates bind to the enzyme (first PRPP followed by the purine bases), and the products released (first PPi followed by purine nucleotide) in a defined order. The K ms for PPi in the T. foetus HGXPRTase-catalyzed reactions are unusually high, close to the millimolar range. Since the crystal structure of this enzyme [Somoza et al. (1996) Biochemistry 35, 7032−7040] suggests potential binding between the threonine-47 in a conserved cis-peptide loop and PPi whereas human HGPRTase has lysine-68 [Eads et al. (1994) Cell 78, 325−334] at the corresponding position, we prepared a T47K enzyme mutant and found in the T47K-catalyzed reaction a 4−10-fold decrease of K m for PPi. The lack of ionic interactions between Thr-47 and PPi and an increased distance between the loop and the active site as compared to the human HGPRTase are thus proposed to be responsible for the high K m for PPi in the T. foetus HGXPRTase-catalyzed reaction.