Key Concepts of Section 15.2

• The concentration of ligand at which half the ligand’s receptors are occupied, the dissociation constant (K_d), can be determined experimentally and is a measure of the affinity of the receptor for the ligand (see Figure 15-8).

• Because of receptors’ high affinity for their target ligand, the extracellular domains of receptors often can be used as parts of drugs to reduce the level of free hormone.

• The near-maximal response of a cell to a particular ligand generally occurs at ligand concentrations at which less than 100 percent of its receptors are bound to the ligand (see Figure 15-8).

• Affinity chromatography techniques can be used to purify receptors even when they are present in low abundance.

• Immunoprecipitation using antibodies specific for protein kinases can be used to measure the activity of these kinases. Western blotting assays using antibodies specific for phosphorylated peptides can measure phosphorylation of a specific amino acid on any desired protein within a cell (see Figure 15-10).

• Pull-down assays using the protein-binding domain of a target protein can be used to quantify activation of a GTP-binding protein within a cell (see Figure 15-11).