Numerous receptors have intrinsic or tightly related enzyme activity.  On the required of ligand to their extracellular face like receptors undergo a conformational change and activate an enzyme activity.  In  case  of  the  insulin receptor  that  is  a complex  of  two  α- and  two  β-subunits  held  together  through disulfide bonds the polypeptide hormone insulin the ligand binds to the extracellular face of the α-subunits. The receptor then undergoes a conformational modify leading to the autophosphorylation of the cytosolic domain of the β-subunit.  By the specifically hydroxyl groups in the side chains of certain tyrosine residues are phosphorylated with ATP being the phosphate donor.  Phosphorylated   receptor  is  then  identify  by  other proteins   in  the  cytosol   which  in  turn  modulate   various   intracellular   events permitting  the  cell  to  respond  to  the  hormone  appropriately.  Additionally, β-subunit can straight phosphorylate other goal proteins within the cell.

        figure:  Signal transduction through an enzyme-linked receptor such as the insulin receptor.