diphtheria toxin

Diphtheria toxin is the main virulence factor of Corynebacterium diphtheriae which causes the disease diphtheria. It was the first exotoxin to be discovered. It is produced by strains of C. diphtheriae that are lysogenized by a bacteriophage called phage ß.

Diphtheria toxin is synthesized as a single polypeptide (535 amino acids) with a molecular weight of 62000. It consists of two fragments, Fragment A (active) at the aminoterminal end, which is the toxic fragment and Fragment B (binding) which attaches the toxin to the host cells (or to any eukaryotic cell). Fragment A is only produced after uptake of the toxin, by protease cleavage and reduction of the disulphide bonds. It inactivates elongation factor 2 (EF-2) by ADP ribosylation making the transfer of an amino acid from tRNA impossible, and thereby inhibiting protein synthesis.

1. (A)-(B) ->(B) links to cell, toxin enters cell
2. (A) is cleaved by protease and sulphydryl donor
3. (A) + (NAD) -> (A)-(NAD)
4. toxin catalyses binding of ADP moiety to EF-2 and transfer of ADP-ribose ->(NAD)-(EF-2) + (A)
5. EF-2 is ADP-ribosylated at a single amino acid terminal.
6. Protein synthesis is stopped

Prokaryotic and mitochondrial protein synthesis is unaffected because a different elongation factor is involved.

A single bacterium can produce 5000 toxin molecules an hour and the toxic fragment is so stable within the cell that a single molecule can kill a cell.

Myocardial and peripheral nerve cells are especially susceptible, although the reason for this is not known.

Iron concentration in the environment in which the bacteria are growing has an effect on toxin production. In media containing sufficient iron for optimal growth no toxin is produced, whereas when iron concentration is reduced to growth-limiting levels, toxin production occurs. There is some evidence that iron binds to a regulatory protein and that this iron-binding protein combines with a control region of the DNS of phage ß and prevents expression of the toxin gene. Mutants in the regulatory region known to synthesize toxin even in the presence of high iron concentrations.