(A) Pertussis toxin

(B) Diphtheria toxin

(C) Pseudomonas Exotoxin A

(D) S. aureus α-toxin

The correct answer to Q.37 is (D) S. aureus α-toxin, which forms membrane pores rather than transferring ADP-ribose to host proteins.​

Correct Answer

Option (D) S. aureus α-toxin lacks ADP-ribosyl transferase activity. This pore-forming β-barrel toxin lyses host cells by creating 1-2 nm transmembrane channels, disrupting membrane potential and causing necrosis, unlike the enzymatic ADP-ribosylation of AB toxins A, B, C.

Option Analysis

(A) Pertussis toxin: Has ADP-Ribosyl Activity

Pertussis toxin (PT) ADP-ribosylates Gαi proteins at Cys351 using NAD+, inhibiting Gi signaling and causing uncontrolled cAMP elevation, leading to whooping cough symptoms.​

(B) Diphtheria toxin: Has ADP-Ribosyl Activity

Diphtheria toxin ADP-ribosylates EF-2 at diphthamide residue, halting protein synthesis by blocking translocation; its A subunit catalyzes this in cytosol after receptor-mediated entry.​

(C) Pseudomonas Exotoxin A: Has ADP-Ribosyl Activity

ExoA ADP-ribosylates EF-2 like diphtheria toxin, inhibiting elongation factor function and causing cell death; common in Pseudomonas aeruginosa infections.​

(D) S. aureus α-toxin: No ADP-Ribosyl Activity

α-Toxin (also α-hemolysin) oligomerizes into heptameric pores on target membranes (erythrocytes, leukocytes), not using ADP-ribosylation; classified as a pore-forming toxin.​