3 Types of Cell Surface Receptors
1. Ion Channel Linked Receptors
• Ionotropic receptors
• Ion channel gated receptor
• pore-forming membrane proteins that allow ions to pass through a channel pore
a) Ligand gated ion channels
Allow ions to flow into or out of the cell in response to the binding of a ligand to their respective receptors. Eg: Acetylcholine Receptors.
At the nerve muscle junction, the neurotransmitter acetylcholine binds to specific receptors that allows Na+ ions to flow into and K+ ions out of the target cell.
GABAA receptors: that allow chloride ions to flow into a cell upon agonist (GABA) binding.
b) Voltage-Gated Ion Channels
Ion channels that open and close in response to changes in membrane potential
2. Enzyme Linked Receptors:
• These receptors have a catalytic activity that is activated by binding of the ligand.
• have intrinsic enzymatic activity or are associated with an enzyme (usually a kinase)
Kinases (Protein Kinases [PKs]) • enzymes that catalyze the phosphorylation of target molecules to cause their activation. In other words, they add a phosphate group to a molecule/protein/another kinase.
Phosphatases • enzymes that catalyze the dephosphorylation of target molecules. This dephosphorylation usually inactivates the target molecule (effector). Phosphatases act in opposition to kinases.
It is the balanced activity of kinases and phosphatases that results in effective, signal transduction.
An example are tyrosine-kinase receptors (RTK).
Binding of an often-dimeric ligand induces dimerization of the receptors that leads to cross phosphorylation of the cytosolic domains and phosphorylation of other proteins.
RTKs bind growth factors like EGFR, NGF to signal processes that result in the regulation of cell growth, differentiation and survival through transcription of associated genes.
3. G protein Coupled Receptors (GPCR) or 7 transmembrane receptors
• largest family of transmembrane signaling proteins
• known as seven-(pass)-transmembrane domain receptors (7TM receptor), due to the seven alpha helices that transverse the cellular membrane.
• The receptor recognizes the appropriate ligand to activate the G proteins.
- there are approximately 950 different human genes encoding GPCRs
These receptors are coupled to intracellular GTP-binding proteins (G-proteins). Once activated, G-proteins trigger the production of a variety of second messengers (e.g. cyclic AMP [cAMP], inositol triphosphate [IP3], diacylglycerol [DAG], etc.) helping to regulate a number of body functions ranging from sensation to growth to hormone release.
*A second messenger is a substance that is released in the cytoplasm following activation of a receptor. It is non-specific, short lived and can generate a variety of responses in the cell. Eg: cAMP, DAG, Ca2+