This type of receptor, once bound by ligand, can change conformation to allow specific substances to flow down their concentration gradient. This type of receptor is referred to as . This type of receptor is not an enzyme itself, but is bound to a protein that can exchange GDP for GTP and then stimulate effector proteins, which are usually enzymes. This type of receptor is referred to as . This type of receptor is a transmembrane receptor. It has an extracellular domain that binds to ligand, and an intracellular domain that can directly phosphorylate other proteins. This is an example of . This type of receptor binds to hydrophobic ligands. This type of receptor is referred to as .