1. Which parts of a cell express receptors?

Receptors may be located at the cell membrane, or within the cytosol of the cell.

2. Can you name the four main types of receptor involved in cellular signalling? Give some examples.

• Ion channel linked receptor: e.g. nicotinic cholinoceptors at the neuromuscular junction

• G-protein coupled receptor: e.g. muscarinic cholinoceptors and adrenoceptors

• Tyrosine kinase linked receptor: e.g. various growth factors, insulin receptor

• Intracellular receptor: steroid hormone receptors

3. What basically happens when a ligand binds to a G-protein coupled receptor?

Receptoro stimulation by the ligand causes binding of the receptor to its G-protein. This causes the G-protein to release (inactive) guanosine diphosphate (GDP) and uptake (active) guanosine triphosphate (GTP). Depending on the type of G-protein that the receptor is coupled to, the G-protein may then activate the enzyme adenylyl cyclase, or inhibit it, or it may stimulate the enzyme phospholipase C.

4. What are the components of the G-protein?

This is composed of α, β and γ subunits:

• α subunit: variation in this determines the type of G-protein. This component binds to GDP and GTP

• β and γ components bind reversibly to the α subunit

5. What is the functional significance of the α subunit?

This determines the type of G-protein and therefore its function. There are several types of α subunit, each linked to a particular type of G-protein. Three examples are:

• Gs: receptor binding to this system leads to activation of adenylyl cyclase, e.g. occurs with β1- and β2-adrenoceptor stimulation and glucagon signals through this pathway

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