title: A catalytic loop within Pseudomonas aeruginosa exotoxin A modulates its transferase activity.
abstract: Mutagenesis techniques were used to replace two loop regions within the catalytic domain of Pseudomonas aeruginosa exotoxin A (ETA) with functionally silent polyglycine loops. The loop mutant proteins, designated polyglycine Loops N and C, were both less active than the wild-type enzyme. However, the polyglycine Loop C mutant protein, replaced with the Gly(483)-Gly(490) loop, showed a much greater loss of enzymatic activity than the polyglycine Loop N protein. The former mutant enzyme exhibited an 18,000-fold decrease in catalytic turnover number (k(cat)), with only a marginal effect on the K(m) value for NAD(+) and the eukaryotic elongation factor-2 binding constant. Furthermore, alanine-scanning mutagenesis of this active-site loop region revealed the specific pattern of a critical region for enzymatic activity. Binding and kinetic data suggest that this loop modulates the transferase activity between ETA and eukaryotic elongation factor-2 and may be responsible for stabilization of the transition state for the reaction. Sequence alignment and molecular modeling also identified a similar loop within diphtheria toxin, a functionally and structurally related class A-B toxin. Based on these results and the similarities between ETA and diphtheria toxin, we propose that this catalytic subregion represents the first report of a diphthamide-specific ribosyltransferase structural motif. We expect these findings to further the development of pharmaceuticals designed to prevent ETA toxicity by disrupting the stabilization of the transition state during the ADP-ribose transfer event.
authors.affiliation: Guelph-Waterloo Centre for Graduate Work in Chemistry and Biochemistry, Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
authors.forename: S P
authors.initials: SP
authors.lastname: Yates
authors.forename: A R
authors.initials: AR
authors.lastname: Merrill
mesh_terms: D036002:ADP Ribose Transferases; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D001427:Bacterial Toxins; D001483:Base Sequence; D020134:Catalytic Domain; D005098:Exotoxins / Q000737:chemistry* / Q000378:metabolism; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011065:Poly(ADP-ribose) Polymerases / Q000378:metabolism*; D009626:Terminology as Topic; D037521:Virulence Factors; D000097668:Pseudomonas aeruginosa Exotoxin A
chemical_list: D036002:ADP Ribose Transferases; D001427:Bacterial Toxins; D005098:Exotoxins; D011065:Poly(ADP-ribose) Polymerases; D037521:Virulence Factors; D000097668:Pseudomonas aeruginosa Exotoxin A