Circular dichroism of diphtheria toxin, Pseudomonas aeruginosa exotoxin A, and various derivatives

We have recorded the near- and far-ultraviolet circular dichroism spectra of diphtheria toxin, Pseudomonas aeruginosa exotoxin A, and derivatives of these toxins. The far-ultraviolet spectra of various forms of diphtheria toxin were virtually identical, implying that no major changes in secondary structure accompany proteolytic nicking or dimerization of toxin, or binding of the endogenous dinucleotide, adenylyl-(3'-5')-uridine 3'-monophosphate (AdoPUrdP). Alpha-helix content was estimated to be 29%, as compared with 8% for fragment A. Near-ultraviolet spectra were identical between nicked and intact diphtheria toxin. A broad negative transition with a minimum at 304 nm was assigned to the intrachain disulfide bridge within the B moiety. Dimeric diphtheria toxin showed perturbations of aromatic residues. Binding of AdoPUrdP to monomeric diphtheria toxin or of adenylyl-(3',5')-uridine (AdoPUrd) to fragment A perturbed one or more tryptophans. The latter results correlate with evidence for involvement of a tryptophan in NAD binding. Native exotoxin A was estimated to have 16% alpha-helix, and the activated form of exotoxin A, 11%. An enzymically active, 31 kDa proteolytic fragment of exotoxin A showed similar alpha-helix content (7%) to that of diphtheria toxin fragment A.