Diphtheria Toxin

**Semantic type:** Amino Acid, Peptide, or Protein|Pharmacologic Substance

**Definition:** An exotoxin protein encoded by a bacteriophage that infects Corynebacterium diphtheriae (C. diphtheriae) and is the causative agent for the pathology associated with diphtheria. C. diphtheriae secretes a single chain protein toxin that is quickly cleaved into two fragments (the A and B subunits) that are linked by disulfide bridges. The B subunit binds to the plasma membrane of a host cell and forms a pore that allows the catalytic A subunit to be internalized. Once in the cytoplasm, the catalytic domain of the toxin moiety dissociates from the B subunit and catalyzes the transfer of the ADP-ribose moiety of NAD to a diphthamide residue of elongation factor 2 (EF-2). This covalent modification inactivates EF-2 and disrupts polypeptide chain elongation, resulting in an inhibition of protein synthesis and, subsequently, cell death.

**Synonyms:** - DIPHTHERIA TOXIN (CORYNEPHAGE BETA)