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Diphtheria toxin is a single polypeptide chain of 535 amino acids consisting of two subunits linked by disulfide bridges, known as an A-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 the host cell.3
Diphtheria toxin is extraordinarily potent.3 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.9
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 lysogenicphage infecting all toxigenic strains.101112
^Freeman VJ (June 1951). "Studies on the virulence of bacteriophage-infected strains of Corynebacterium diphtheriae". J. Bacteriol.61 (6): 675–88. PMC386063. PMID14850426.
^Freeman VJ, Morse IU (March 1952). "Further observations on the change to virulence of bacteriophage-infected a virulent strains of Corynebacterium diphtheria". J. Bacteriol.63 (3): 407–14. PMC169283. PMID14927573.