Egg-White Avidin in its functional complex with Biotin                                  Avidin is a glycoprotein made of four identical subunits.  Biotin is a vitamin that is involved in carboxylation-decarboxylation reactions and forms a highly stable complex with Avidin.  Two Avidin chains are contained in an asymmetric unit.  Avidin's binding with biotin is one of the tightest binding intereactions known.