<prompt> Receptor for diphtheria toxin lies at ?Cell membraneMucous membraneNucleusNone  <response>: Ans. is a i.e., Cell membrane Diphtheria toxin The diphtheria toxin acts by inhibiting protein synthesis. It inhibits polypeptide chain elongation in the presence of nicotinamide adinine dinucleotide NAD by inactivating elongation factor, EF - 2. The diphtheria toxin is a protein which consists of two fragments, A and B. Both fragments are necessary for the toxic effect : Fragment A - has enzymatic activity and inhibits protein synthesis by inhibiting the chain elongation by inactivating the elongation factor - 2 Ef - 2. Fragment B - responsible for binding the toxin to the cells. The toxin binds to specific receptors HB-EGF receptors present on susceptible cells and enters by receptor-mediated endocytosis. Toxin has special affinity for ceain tissues such as myocardium, adrenals and nerve endings. 