Ion channel receptors and G protein-coupled receptors

Ion channel receptors and G protein-coupled receptorsSpec C2.1.8, C2.1.9, C2.1.10

Receptors for neurotransmitters: the acetylcholine receptor

1. Acetylcholine binds to binding sites on the acetylcholine receptor in the postsynaptic membrane.
2. The receptor changes shape and opens an ion channel within the receptor itself.
3. Positively charged ions, mainly Na⁺, diffuse into the cell down their concentration gradient.
4. The inside becomes less negative: the voltage across the plasma membrane (the membrane potential) changes. This depolarization may cause other changes, such as an action potential in a neuron or contraction of a muscle fibre.

G protein-coupled receptors

A G protein-coupled receptor (GPCR) is a transmembrane receptor linked on the inside of the membrane to a G protein. Humans have many different GPCRs (hundreds of genes), detecting ligands such as hormones, neurotransmitters, odours and light.

1. The ligand binds to the receptor on the outside of the plasma membrane.
2. The receptor changes conformation and activates a G protein on the cytoplasmic side.
3. The G protein releases GDP and binds GTP. Part of it (the α subunit) separates and activates or inhibits a target protein, such as an enzyme or an ion channel.
4. The G protein switches itself off by hydrolysing GTP to GDP, so the signal stops when ligand binding stops.

Epinephrine (adrenaline) receptors

1. Epinephrine is carried in the blood and binds to its receptor (a GPCR) on a target cell such as a liver cell. It does not enter the cell.
2. The receptor activates a G protein, which binds GTP.
3. The activated G protein activates the enzyme adenylyl cyclase in the membrane.
4. Adenylyl cyclase converts ATP into cyclic AMP (cAMP), the second messenger.
5. cAMP activates protein kinase A, which phosphorylates other enzymes in a cascade. In liver cells this activates glycogen breakdown, releasing glucose into the blood.

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