Point mutations in the guanine phosphoribosyltransferase from Giardia lamblia modulate pyrophosphate binding and enzyme catalysis

Abstract

Guanine phosphoribosyltransferase (GPRTase) from Giardia lamblia, an enzyme required for guanine salvage and necessary for the survival of this parasitic protozoan, has been kinetically characterized. Phosphoribosyltransfer proceeds through an ordered sequential mechanism common to many related purine phosphoribosyltransferases (PRTases) with α‐D‐5‐phosphoribosyl‐1‐pyrophosphate (PRPP) binding to the enzyme first and guanosine monophosphate (GMP) dissociating last. The enzyme is a highly unique purine PRTase, recognizing only guanine as its purine substrate (Km = 16.4 µm) but not hypoxanthine (Km > 200 µm) nor xanthine (no reaction). It also catalyzes both the forward (kcat = 76.7 s−1) and reverse (kcat = 5.8·s−1) reactions at significantly higher rates than all the other purine PRTases described to date. However, the relative catalytic efficiencies favor the forward reaction, which can be attributed to an unusually high Km for pyrophosphate (PPi) (323.9 µm) in the reverse reaction, comparable only with the high Km for PPi (165.5 µm) in Tritrichomonasfoetus HGXPRTase‐catalyzed reverse reaction. As the latter case was due to the substitution of threonine for a highly conserved lysine residue in the PPi‐binding loop [Munagala et al. (1998) Biochemistry37, 4045–4051], we identified a corresponding threonine residue in G. lamblia GPRTase at position 70 by sequence alignment, and then generated a T70K mutant of the enzyme. The mutant displays a 6.7‐fold lower Km for PPi with a twofold increase in the Km for PRPP. Further attempts to improve PPi binding led to the construction of a T70K/A72G double mutant, which displays an even lower Km of 7.9 µm for PPi. However, mutations of the nearby Gly71 to Glu, Arg, or Ala completely inactivate the GPRTase, suggesting the requirement of flexibility in the putative PPi‐binding loop for enzyme catalysis, which is apparently maintained by the glycine residue. We have thus tentatively identified the PPi‐binding loop in G. lamblia GPRTase, and attributed the relatively higher catalytic efficiency in the forward reaction to the unusual loop structure for poor PPi binding in the reverse reaction.