A.
Enzyme-coupled receptors are transmembrane proteins that display their ligand-binding domains on the outer suce of the plasma membrane. The cytoplasmic domain of the receptor either acts as an enzyme itself or forms a complex with another protein that acts as an enzyme.
B.
Enzyme-coupled receptors were discovered through their role in responses to extracellular signal proteins that regulate the growth, proliferation, differentiation, and survival of cells in animal tissues.
C.
Most of these signal proteins function as local mediators and can act at very low concentrations (about 10ˉ 9 to 10ˉ 11 M). Responses to them are typically slow (on the order of hours), and their effects may require many intracellular transduction steps that usually lead to a change in gene expression.
D.
The largest class of enzyme-coupled receptors consists of receptors with a cytoplasmic domain that functions as a tyrosine kinase, which phosphorylates particular tyrosines on specific intracellular signaling proteins. These receptors, called receptor tyrosine kinases (RTKs).