Diphtheria toxin, produced by a phage infecting Corynebacterium diphtheriae, is a proenzyme that, after activation, catalyzes the covalent attachment of the ADP ribose moiety of NAD to eukaryotic elongation factor 2 (eEF-2). Fragment A is the catalytic portion responsible for enzymatic ADP-ribosylation of elongation factor 2, while fragment B is responsible for binding of toxin to cell receptors and entry of fragment A. diphthamide-[translation elongation factor 2] + NAD(+) = H(+) + N-(ADP-D-ribosyl)diphthamide-[translation elongation factor 2] + nicotinamide Partially inhibited by 1,8-naphthalimide (NAP). Homodimer. Proteolytic activation by host furin cleaves the protein in two parts, Diphtheria toxin fragment A and Diphtheria toxin fragment B; which remain associated via a disulfide bond. Truncated N-terminus.