Receptor superfamilies and speed of response
Receptors that share a basic design and signalling speed form a superfamily, and each superfamily has a characteristic timescale of response, from milliseconds to hours to days.
Remember
The more steps between binding and the final effect, the slower the response, so each receptor superfamily has its own timescale, from milliseconds to hours to days.
Memory aid
opening a channel is like flicking a switch, whereas changing gene expression is like building a new machine
The more steps between binding and the final effect, the slower the response.
A receptor is a protein that recognises a chemical messenger, such as a hormone or a neurotransmitter, and turns that recognition into a cellular response. A ligand is the molecule that binds a receptor, for example a drug or a neurotransmitter. Receptors fall into four broad superfamilies, and each has a characteristic timescale of response.
Ligand-gated ion channels are receptors that are also ion channels, so the ligand binds the channel itself and the channel opens or closes. Ions (charged particles such as sodium) then cross the cell membrane. Binding and ion movement are directly linked, with no chain of intermediate steps, so the response typically comes within milliseconds.
G-protein coupled receptors switch on a G-protein inside the cell, which then triggers second messengers (small molecules that relay the signal inside the cell). These extra steps mean the response takes seconds. Kinase-linked receptors switch on an enzyme (a kinase) inside the cell that adds phosphate groups to other proteins, and they respond in minutes to hours.
Nuclear (intracellular) receptors sit inside the cell, in the cytoplasm or the nucleus. A ligand that is hydrophobic (fat-soluble) enters the cell and binds the receptor, which regulates gene transcription (the copying of a gene into messenger RNA). New messenger RNA and then new protein have to be made before the cell changes, so these responses typically take hours to days.
At cell level, a drug's onset (how quickly it acts) follows its receptor superfamily. Salbutamol is a bronchodilator (a drug that relaxes airway smooth muscle) and acts at a G-protein coupled receptor, so its cellular response takes seconds. Prednisolone is a corticosteroid, a drug that mimics the body's own glucocorticoids (steroid hormones) and reduces inflammation. It acts slowly through a nuclear receptor that changes gene transcription, with a response in hours to days.
Key facts
- Ligand-gated ion channels
- respond in milliseconds, because the receptor and the ion channel are one protein.
- G-protein coupled receptors
- respond in seconds, using a G-protein and second messengers.
- Kinase-linked receptors
- respond in minutes to hours, by switching on an enzyme inside the cell.
- Nuclear (intracellular) receptors
- respond in hours to days, since their action alters gene transcription.
- Voltage-gated ion channels
- also act within milliseconds, but a change in membrane potential opens them rather than a ligand binding, so they are not one of the four receptor superfamilies.
- Rule
- the fewer the steps between binding and effect, the faster the response.
Exam traps
- Voltage-gated ion channels also act within milliseconds, but a change in membrane potential opens them rather than a ligand binding, so they are not ligand-gated ion channels and are not one of the four receptor superfamilies.
- Kinase-linked receptors typically respond in minutes to hours, which overlaps at its upper end with the hours to days of nuclear receptors, but their timescale is shorter and they are not defined by gene transcription.
- G-protein coupled receptors are the next fastest of the four receptor superfamilies, after ligand-gated channels, but they pass the signal through a G-protein and second messengers, so responses take seconds rather than milliseconds.
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