What is the difference between hormone binding to its receptor and allosteric regulator binding to an enzyme?

Ligand binding to proteins may occur with varying strengths; some ligands bind tightly to proteins while others bind less tightly. The strengths of reversible binding are determined experimentally by varying concentrations of ligands, and measuring the saturation of the protein in the various ligand concentrations. One such laboratory study investigated the binding of a hormone to three different receptor proteins in the cell membrane. The data collected are shown in the table below: 1) Provide a brief explanation as to why ligand binding to proteins must be a reversible process.  2)  Calculate the dissociation constant (Kd) for the hormone binding to each of the three proteins.

Epinephrine stimulates glycogen breakdown by activating glycogen phosphorylase. However, in the research lab, no glycogen breakdown occurred when epinephrine was added to a test tube containing the enzyme and its substrate, glycogen. Explain why glycogen phosphorylase could be activated by epinephrine only when the hormone was added to intact cells in a solution.

Describe the process and results of receptor-ligand binding.