tox diphtheria toxin precursor
Cartoon representation of the diphtheria toxin protein
Identifiers
Organism	Corynebacterium diphtheriae
Symbol	tox
Entrez	2650491
RefSeq (Prot)	NP_938615
UniProt	P00587
Other data
EC number	2.4.2.36
Chromosome	genome: 0.19 - 0.19 Mb
Search for Structures Swiss-model Domains InterPro

Diphtheria toxin is anexotoxin secreted mainly byCorynebacterium diphtheriae (but also byCorynebacterium ulcerans andCorynebacterium pseudotuberculosis), thepathogenicbacterium that causesdiphtheria. Thetoxingene is encoded by aprophage^{[annotation 1]} calledcorynephage β.^[1][2]The toxin causes the disease in humans by gaining entry into the cellcytoplasm and inhibitingprotein synthesis.^[3]

Diphtheria toxin is a singlepolypeptide chain of 535 amino acids consisting of twosubunits linked bydisulfide bridges, known as anA-B toxin. Binding to the cell surface of the B subunit (the less stable of the two subunits) allows the A subunit (the more stable part of the protein) to penetrate thehost cell.^[4]

Thecrystal structure of the diphtheria toxinhomodimer has been determined to 2.5Ångstrom resolution. Thestructure reveals a Y-shapedmolecule consisting of threedomains. Fragment A contains thecatalytic C domain, and fragment B consists of the T and R domains:^[5]

• Theamino-terminal catalytic domain, known as the C domain, has an unusual beta+alphafold.^[6] The C domain blocksprotein synthesis by transfer ofADP-ribose fromNAD to adiphthamide residue ofeukaryotic elongation factor 2 (eEF-2).^[7][3]
• A central translocation domain, known as the T domain or TM domain, has a multi-helicalglobin-likefold with two additionalhelices at the amino terminus but no counterpart to the firstglobinhelix. This domain is thought to unfold in themembrane.^[8] ApH-inducedconformational change in the T domain triggers insertion into theendosomal membrane and facilitates the transfer of the C domain into thecytoplasm.^[7][3]
• Acarboxy-terminal receptor-binding domain, known as the R domain, has abeta-sandwichfold consisting of nine strands in two sheets with Greek-key topology; it is a subclass ofimmunoglobulin-like fold.^[6] The R domain binds to acell surface receptor, permitting the toxin to enter thecell byreceptor-mediated endocytosis.^[7][3]

The diphtheria toxin has the same mechanism of action as the enzymeNAD(+)—diphthamide ADP-ribosyltransferase (EC2.4.2.36). It catalyzes theADP ribosylation of the unusualamino aciddiphthamide ineEF-2 by transferring the ADP-ribosyl group from NAD⁺. The ADP ribosylation of diphthamide inactivates the eEF-2 protein, thus, inhibiting thetranslation of mRNA. The catalysed reaction is as follows:

NAD⁺ + peptide diphthamide${\displaystyle \rightleftharpoons }$nicotinamide + peptideN-(ADP-D-ribosyl)diphthamide.

Theexotoxin A ofPseudomonas aeruginosa uses a similar mechanism of action.

The steps involved in generating toxicity are as follows:^{[citation needed]}

1. Processing
   1. The leader region is cleaved during secretion.
   2. Proteolytic nicking separates A and B subunits, which remain joined by disulfide bonds until they reach the cytosol.
2. The toxin binds to heparin-binding epidermal growth factor precursor (HB-EGF).^[9]: 116
3. The complex undergoes endocytosis by the host cell.
4. Acidification inside the endosome induces translocation of the A subunit into the cytosol.
   1. Disulfide bonds are broken.
   2. The B subunit remains in the endosome as a pore.
5. The A subunitADP-ribosylates host eEF-2, which is required for protein synthesis; when it is inactivated, the host cannot make protein and thus dies.

Diphtheria toxin is extraordinarily potent.^[4] Thelethal dose for humans is about 0.1 μg of toxin per kg of body weight. Death occurs throughnecrosis of theheart andliver.^[10] Diphtheria toxin has also been associated with the development ofmyocarditis. Myocarditis secondary to diphtheria toxin is considered one of the biggest risks to unimmunized children.

Diphtheria toxin was discovered in 1888 byÉmile Roux andAlexandre Yersin. In 1890,Emil Adolf von Behring developed an anti-toxin based on the blood of horses immunized with attenuated bacteria.^[11] In 1951, Freeman found that the toxin gene was not encoded on the bacterial chromosome, but by alysogenicphage (corynephage β)^[2] infecting all toxigenic strains.^[12][13][14]

The drugdenileukin diftitox uses diphtheria toxin as anantineoplastic agent.

Resimmune is animmunotoxin that is in clinical trials incutaneous T cell lymphoma patients. It uses diphtheria toxin (truncated by the cell binding domain) coupled to an antibody toCD3ε (UCHT1).^[15]

Similar to other A-B toxins, diphtheria toxin is adept at transporting exogenous proteins across mammalian cell membranes, which are usually impermeable to large proteins. This unique ability can be repurposed to deliver therapeutic proteins, instead of the catalytic domain of the toxin.^[16][17]

This toxin has also been used in neuroscientific and cancer research to ablate specific populations of cells which express the diphtheria toxin receptor (heparin-binding EGF-like growth factor). Administration of the toxin into the organism which does not naturally express this receptor (e.g. mice) will result in the selective ablation of the cell population which do express it.^[18][19]

• Diphtheria+Toxin at the U.S. National Library of MedicineMedical Subject Headings (MeSH)

• Hepatotoxins
• Protein domains
• Peripheral membrane proteins
• Bacterial toxins
• Diphtheria
• Protein synthesis inhibitors

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