What is the mechanism of action of diphtheria toxin?

Diphtheria toxin exerts its effects primarily by blocking protein synthesis within affected cells. This toxin is produced by the bacterium Corynebacterium diphtheriae and functions as an ADP-ribosylating toxin. The active part of the toxin enters cells and modifies elongation factor-2 (EF-2) through ADP-ribosylation, which inhibits the function of EF-2. This factor is crucial for the translocation step during protein synthesis, and its inactivation prevents the ribosome from moving along mRNA, effectively halting the synthesis of proteins.

This mechanism leads to cellular toxicity and ultimately cell death due to the inability to produce essential proteins necessary for various cellular functions. The severity of diphtheria and associated symptoms arise from this profound effect on protein synthesis, making it a central pathological mechanism in the disease process.

In comparison, the other mechanisms listed, such as disrupting the cell membrane, blocking nucleic acid synthesis, or interfering with neurotransmission, do not accurately describe how diphtheria toxin operates. Diphtheria's pathology is specifically tied to its action on protein synthesis rather than these other unrelated processes, which are mechanisms seen with different types of toxins or antimicrobial agents.