Figure 52.
LIGANDS There are many different types of ligands that can bind to receptors. There can be gaseous ligands, like nitric oxide, and hydrophobic molecules, like steroids. Peptides and proteins can be ligands as well. Ligands that enter the cells include steroid hormones, such as estradiol and testosterone. Vitamin D is another steroid hormone that binds first to carrier proteins, enter the cell, and bind to intracellular receptors. Nitric oxide is a ligand that binds inside the cell because it can pass through the plasma membrane. Ligands that bind outside of the cell are water-soluble, polar, or charged so they cannot readily pass through the plasma membrane. The most common ligands of this type are
169
