title: Probing the function of conserved residues in the serine/threonine phosphatase PP2Calpha.
abstract: Human PP2Calpha is a metal-dependent phosphoserine/phosphothreonine protein phosphatase and is the representative member of the large PPM family. The X-ray structure of human PP2Calpha has revealed an active site containing a dinuclear metal ion center that is coordinated by several invariant carboxylate residues. However, direct evidence for the catalytic function of these and other active-site residues has not been established. Using site-directed mutagenesis and enzyme kinetic analyses, we probed the roles of conserved active-site amino acids within PP2Calpha. Asp-60 bridges metals M1 and M2, and Asp-239 coordinates metal M2, both of which were replaced individually to asparagine residues. These point mutations resulted in >or=1000-fold decrease in k(cat) and >or=30-fold increase in K(m) value for Mn(2+). Mutation of Asp-282 to asparagine caused a 100-fold decrease in k(cat), but no significant effect on K(m) values for metal and substrate, consistent with Asp-282 activating a metal-bound water nucleophile. Mutants T128A, E37Q, D38N, and H40A displayed little or no alterations on k(cat) and K(m) values for substrate or metal ion (Mn(2+)). Analysis of H62Q and R33A yielded k(cat) values that were 20- and 2-fold lower than wild-type, respectively. The mutant R33A showed a 8-fold higher K(m) for substrate, while the K(m) observed with H62Q was unaffected. A pH-rate profile of the H62Q mutant showed loss of the ionization that must be protonated for activity. Brönsted analysis of substrate leaving group pK(a) values for H62Q indicated a greater dependency (slope -0.84) on leaving group pK(a) in comparison to wild-type (slope -0.33). These data provide strong evidence that His-62 acts as a general acid during the cleavage of the P-O bond.
authors.affiliation: Oregon Health & Science University, Department of Biochemistry and Molecular Biology, Portland, Oregon 97201-3098, USA.
authors.forename: Michael D
authors.initials: MD
authors.lastname: Jackson
authors.forename: Clark C
authors.initials: CC
authors.lastname: Fjeld
authors.forename: John M
authors.initials: JM
authors.lastname: Denu
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D017124:Conserved Sequence; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008024:Ligands; D016297:Mutagenesis, Site-Directed; D010749:Phosphoprotein Phosphatases / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D011487:Protein Conformation; D000071636:Protein Phosphatase 2C; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism
chemical_list: D008024:Ligands; D011994:Recombinant Proteins; D010749:Phosphoprotein Phosphatases; D000071636:Protein Phosphatase 2C