Drug and Ligand Receptor Classifications

Pharmacodynamics is the study of how a drug causes a molecular, cellular, or physiological response. For a drug (ligand) to cause an effect, in many cases it requires a specific receptor. Drug receptors are cellular macromolecules which bind to the drug. When bound it can change and adjust cellular functions, such as protein synthesis and gene expression.

Below are the different types of drug receptors with a brief overview of each one.

Ligand-gated Ion channel
This transmembrane protein forms a tunnel in the cell membrane with a hydrophilic core and a lipophilic exterior facing the membrane. While bound to the drug, this gated channel opens, allowing passage of certain molecules in or out of the cell.

Intracellular ligand receptors
These receptors bind to lipophilic ligands such as steroids or lipid soluble vitamins A, D, and E. Because of the ligands lipophilic nature, they can slip through the cell's membrane and reach the receptor which is located within the cell. Often these receptors, when bound, trigger a change with the cell's gene activity (i.e. transcription)

Receptors with enzymatic activity
These receptors span across the cell membrane. When the drug binds to the receptor on the external portion of the receptor, in promotes enzymatic activity within the cell. The enzymes bind substrates and convert it to a product. These receptors can either contain the intracellular enzymes themselves, having it be one complete protein. Or the receptor can be associated with an intracellular enzyme, but are two separate proteins.

G Protein-Coupled Receptors
These proteins, also known as 7 membrane spanning, serpentine or heptahelical receptors have an intracellular C-terminus and an extracellular N-terminus. These activate signal transduction pathways using a G protein and its three subunits. (alpha, beta, and gamma) The alpha portion binds with GDP (guanosine diphosphate) which phosphorylates it to GTP (guanosine triphosphate) which activates the alpha portion. The alpha subunit can further affect signaling proteins.