At the cells of the target organ, the hormone acts as a “first or extracellular messenger”, binding to a specific receptor site for that hormone on the plasma membrane. The hormone – receptor complex activates the enzyme adenylate cyclase in the membrane. The activated enzyme converts ATP into a nucleotide called cyclic AMP, which becomes the “second (or intracellular) messenger.” Cyclic AMP diffuses throughout the cytoplasm and activates an enzyme called protein kinase, which causes the cell to respond with its distinctive physiological activity. After inducing the target cell to perform its specific function, the enzyme phosphodiesterase inactivates cyclic AMP. In the meantime, the receptor on the plasma membrane loses the first messenger and now becomes available for a new reaction. Fig. 3.7.
What Is the function of a receptor site on a plasma membrane?
Many hormones, such as most amino acid derivatives, and the peptide hormones that are too large, or too polar, to pass through cell membranes, bind to receptor sites present on the surface of target cell membranes. The hormone and receptor form a complex that triggers a cascade of molecular events within a cell.