The hormones that are enzymes also amplify their signal by phosphorylating many proteins before their action is extinguished by downregulation. Some hormones cross the target cell membrane and bind to an intracellular receptor. The vast majority of intracellular receptors are transcription factors that alter the expression of genes because these responsive genes have response elements that bind receptor. In this case hormone action is indirect. E. G-protein coupled receptors work with the signaling ligand, e.g., adrenaline, glucagon, etc., binding to a receptor on the cell surface .
Different tissues have a different constellations of hormone receptors.
In addition, the hormone receptor for a particular hormone may be an isoform of the receptor found on another tissue. The classics in this case are the adrenergic receptors, many of which bind epinephrine, but are tissue specific. G-protein coupled receptors are intramembrane proteins with seven transmembrane alpha helices. The bound receptor activates a heterotrimeric G-protein. Heterotrimeric G-proteins are a class of proteins.