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 elongation factor 2. Fragment A is 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 Homodimer.