title: Mutation of an active site residue in Escherichia coli uracil-DNA glycosylase: effect on DNA binding, uracil inhibition and catalysis.
abstract: The role of the conserved histidine-187 located in the leucine intercalation loop of Escherichia coli uracil-DNA glycosylase (Ung) was investigated. Using site-directed mutagenesis, an Ung H187D mutant protein was created, overproduced, purified to apparent homogeneity, and characterized in comparison to wild-type Ung. The properties of Ung H187D differed from Ung with respect to specific activity, substrate specificity, DNA binding, pH optimum, and inhibition by uracil analogues. Ung H187D exhibited a 55000-fold lower specific activity and a shift in pH optimum from pH 8.0 to 7.0. Under reaction conditions optimal for wild-type Ung (pH 8.0), the substrate preference of Ung H187D on defined single- and double-stranded oligonucleotides (25-mers) containing a site-specific uracil target was U/G-25-mer > U-25-mer > U/A-25-mer. However, Ung H187D processed these same DNA substrates at comparable rates at pH 7.0 and the activity was stimulated approximately 3-fold relative to the U-25-mer substrate. Ung H187D was less susceptible than Ung to inhibition by uracil, 6-amino uracil, and 5-fluorouracil. Using UV-catalyzed protein/DNA cross-linking to measure DNA binding affinity, the efficiency of Ung H187D binding to thymine-, uracil-, and apyrimidinic-site-containing DNA was (dT20) = (dT19-U) >/= (dT19-AP). Comparative analysis of the biochemical properties and the X-ray crystallographic structures of Ung and Ung H187D [Putnam, C. D., Shroyer, M. J. N., Lundquist, A. J., Mol, C. D., Arvai, A. S., Mosbaugh, D. W., and Tainer, J. A. (1999) J. Mol. Biol. 287, 331-346] provided insight regarding the role of His-187 in the catalytic mechanism of glycosylic bond cleavage. A novel mechanism is proposed wherein the developing negative charge on the uracil ring and concomitant polarization of the N1-C1' bond is sustained by resonance effects and hydrogen bonding involving the imidazole side chain of His-187.
authors: Departments of Microbiology, Environmental and Molecular Toxicology, Biochemistry and Biophysics, and the Environmental Health Science Center, Oregon State University, Corvallis, Oregon 97331, USA.
authors: M J
authors: MJ
authors: Shroyer
authors: S E
authors: SE
authors: Bennett
authors: C D
authors: CD
authors: Putnam
authors: J A
authors: JA
authors: Tainer
authors: D W
authors: DW
authors: Mosbaugh
mesh_terms: D001483:Base Sequence; D001665:Binding Sites; D002384:Catalysis; D002853:Chromatography, Liquid; D004247:DNA / Q000378:metabolism*; D045647:DNA Glycosylases; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000201:enzymology*; D006863:Hydrogen-Ion Concentration; D016297:Mutagenesis, Site-Directed; D009699:N-Glycosyl Hydrolases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D013379:Substrate Specificity; D014498:Uracil / Q000494:pharmacology*; D051981:Uracil-DNA Glycosidase
chemical_list: D014498:Uracil; D004247:DNA; D045647:DNA Glycosylases; D009699:N-Glycosyl Hydrolases; D051981:Uracil-DNA Glycosidase