title: Ligand interactions of diphtheria toxin. II. Relationships between the NAD site and the P site.
abstract: Prior studies have described two functionally distinct ligand-binding sites on whole diphtheria toxin, the NAD site, which catalyzes the intracellular ADP-ribosylation reaction, and the P site, which affects toxin binding to sensitive cells. Occupancy of the P site by ATP or other phosphorylated compounds inhibits toxin attachment to cells. Here we show that binding of NAD site and P site ligands is competitive; and we characterize ligand-binding properties of two mutant forms of the toxin, CRM 45 and CRM 197. The data suggest that the NAD site, on the A moiety, lies immediately adjacent to the P site, formed by a strongly cationic region on the COOH-terminal half of B. The cationic character of the P site slightly alters the substrate specificity of the NAD site, and occupancy of either of the sites blocks ligand binding to the other. Possible roles of the P site in toxin attachment are discussed.
mesh_terms: D000246:Adenosine Diphosphate Ribose / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D001665:Binding Sites; D004167:Diphtheria Toxin / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D009243:NAD; D011485:Protein Binding
chemical_list: D004167:Diphtheria Toxin; D008024:Ligands; D009243:NAD; D000246:Adenosine Diphosphate Ribose; D000255:Adenosine Triphosphate