Diphtheria toxin

Diphtheria toxin is an exotoxin secreted by Corynebacterium diphtheriae, the pathogen bacterium that causes diphtheria. Unusually, the toxin gene is encoded by a bacteriophage (a virus that infects bacteria). The toxin causes the disease diphtheria in humans by gaining entry into the cell cytoplasm and inhibiting protein synthesis.

Structure
Diphtheria toxin is a single polypeptide chain of 535 amino acids consisting of two subunits linked by disulfide bridges. Binding to the cell surface of the less stable of these two subunits allows the more stable part of the protein to penetrate the host cell.

The crystal structure of the diphtheria toxin homodimer has been determined to 2.5A resolution. The structure reveals a Y-shaped molecule consisting of 3 domains. Fragment A contains the catalytic C domain, and fragment B consists of the T and R domains


 * The N-terminal catalytic domain, known as the C domain, has an unusual beta+alpha fold. The C domain blocks protein synthesis by transfer of ADP-ribose from NAD to a diphthamide residue of EF-2.


 * A central translocation domain, known as the T domain or TM domain. The T domain has a multi-helical globin-like fold with two additional helices at N-termini, but which has no counterpart to the first globin helix. This domain is thought to unfold in the membrane. pH-induced conformational change in the T domain triggers insertion into the endosomal membrane and facilitates the transfer of the C domain into the cytoplasm.


 * A C-terminal receptor-binding domain, known as the R domain. This domain has a beta-sandwich fold consisiting of nine strands in two sheets with greek-key topology; it is a subclass of immunoglobin-like fold. The R domain binds to cell surface receptor, permitting the toxin to enter the cell by receptor mediated endocytosis.

Mechanism
This is NAD+-diphthamide ADP-ribosyltransferase. It catalyzes the ADP-ribosylation of eukaryotic elongation factor-2 (eEF2), inactivating this protein. It does so by ADP-ribosylating the unusual amino acid diphthamide. In this way, it acts as a RNA translational inhibitor. The catalysed reaction is as follows:


 * NAD+ + peptide diphthamide $$\rightleftharpoons$$ nicotinamide + peptide N-(ADP-D-ribosyl)diphthamide

The exotoxin A of Pseudomonas aeruginosa uses a similar mechanism of action.

Lethal dose
Diphtheria toxin is extraordinarily potent. The lethal dose for humans is about 0.1 μg of toxin per kg of bodyweight. A massive release of toxin into the body will likely cause lethal necrosis of the heart and liver.

History
Diphtheria toxin was discovered in 1890 by Emil Adolf von Behring. In 1951, Freeman found that the toxin gene was not encoded on the bacterial chromosome, but by a lysogenic phage infecting all toxigenic strains.

Clinical use
The drug denileukin diftitox uses diphtheria toxin as an antineoplastic agent.