Inhibitors of ADP-Ribosylating Bacterial Toxins Based on Oxacarbenium Ion Character at Their Transition States

The bacterial exotoxins, cholera toxin (CT), pertussis toxin (PT), and diphtheria toxin (DT), interfere with specific host proteins to cause tissue damage for their respective infections. The common toxic mechanism for these agents is mono-ADP-ribosylation of specific amino acids in G_sα , G_iα, and eEF-2 proteins, respectively, by the catalytic A chains of the toxins (CTA, PTA, and DTA). In the absence of acceptor proteins, these toxins also act as NAD⁺-N-ribosyl hydrolases. The transition-state structures for NAD⁺ hydrolysis and ADP-ribosylation reactions have oxacarbenium ion character in the ribose. We designed and synthesized analogues of NAD⁺ to resemble their oxacarbenium ion transition states. Inhibitors with oxacarbenium mimics replacing the NMN-ribosyl group of NAD+ show 200-620-fold increased affinity in the hydrolytic and N-ribosyl transferase reactions catalyzed by CTA. These analogues are also inhibitors for the hydrolysis of NAD⁺ by PTA with K_i values of 24-40 μM, but bind with similar affinity to the NAD⁺ substrates. Inhibition of the NAD⁺ hydrolysis and ADP-ribosyl transferase reactions of DTA gave K_i values from 19 to 48 μM. Catalytic rate enhancements by the bacterial exotoxins are small, and thus transition-state analogues cannot capture large energies of activation. In the cases of DTA and PTA, analogues known to resemble the transition states bind with approximately the same affinity as substrates. Transition-state analogue interrogation of the bacterial toxins indicates that CTA gains catalytic efficiency from modest transition-state stabilization, but DTA and PTA catalyze ADP-ribosyl transferase reactions more from ground-state destabilization. pH dependence of inhibitor action indicated that both neutral and cationic forms of transition-state analogues bind to DTA with similar affinity. The origin of this similarity is proposed to reside in the cationic nature of NAD⁺ both as substrate and at the transition state.