Define receptor proteins. List and briefly describe three major classes of receptor proteins. - G-protein-coupled receptors - ion channel receptors - enzyme-linked receptors / Receptor tyrosine kinases (RTKs)

Receptor proteins are specialized integral membrane proteins that take part in communication between the cell and the outside world. These proteins are embedded in the plasma membranes of cells and they receive chemical signals from outside the cell. When such chemical signals bind to a receptor, they cause some form of cellular/tissue response, such as a change in the electrical activity of a cell.

1. G-protein-coupled receptors (GPCRs): These are a large family of receptor proteins that interact with G proteins to activate intracellular signaling pathways. When a ligand binds to the GPCR, it causes a conformational change in the GPCR, which allows it to act as a guanine nucleotide exchange factor (GEF). The GPCR can then activate an associated G protein by exchanging the GDP bound to the G protein for a GTP.

2. Ion channel receptors: These are a type of membrane receptor that can open or close in response to the binding of a specific ligand, allowing ions to pass through the membrane and cause a range of cellular responses. They are crucial for processes such as neurotransmission and the regulation of heartbeat.

3. Enzyme-linked receptors / Receptor tyrosine kinases (RTKs): These are transmembrane proteins with an intracellular kinase domain and an extracellular domain that binds to a ligand. The binding of a ligand to the extracellular domain causes the receptor to dimerize and autophosphorylate, leading to the activation of various intracellular signaling pathways.