LESSON 2 OF 3
How peptides send signals
By the end you can
- Describe how a peptide talks to a cell
- Explain receptors, agonists and antagonists
- Define half-life
Messengers and receptors
Most signaling peptides never enter the cell they talk to. Instead they bind to a receptor, a protein on the cell's surface shaped to fit that specific messenger.
When the peptide binds, the receptor changes shape and sets off a chain of events inside the cell. Scientists call this a signaling pathway. The end result might be a hormone being released, a gene being switched on, or a cell starting to move.
Agonists and antagonists
A molecule that switches a receptor on is an agonist. Semaglutide, for example, is a GLP-1 receptor agonist: it binds the same receptor natural GLP-1 does.
A molecule that blocks a receptor without switching it on is an antagonist. Some peptides act on more than one receptor at once, like tirzepatide (two receptors) or retatrutide (three).
Half-life: how long a signal lasts
The body clears peptides quickly. Enzymes in the blood cut them apart, and the kidneys filter small molecules out. The time it takes for half of a dose to be cleared from the body is its half-life.
Natural GLP-1 has a half-life of about two minutes. That short life is one of the main reasons scientists redesign peptides, which is the topic of the next lesson.
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Key terms
- Receptor
- A protein, usually on a cell's surface, that a specific messenger binds to.
- Signaling pathway
- The chain of events inside a cell after a receptor is switched on.
- Agonist
- A molecule that binds a receptor and activates it.
- Antagonist
- A molecule that binds a receptor and blocks it.
- Half-life
- The time it takes for half of a dose to be cleared from the body.
Remember
- Peptides work by binding receptors, like keys in locks.
- Agonists switch receptors on; antagonists block them.
- Natural peptides usually have short half-lives.