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PeptideHormone

The primer

What a peptide hormone actually is

A peptide hormone is a short chain of amino acids — from three residues to around fifty — that one set of cells releases to signal another. Here is what that means: how they are made, how they work, and the families they fall into.

The short version

A peptide hormone is a short chain of amino acids — from three residues to around fifty — that one set of cells releases to change what another set of cells does. That is the whole definition. Insulin, glucagon, GLP-1, oxytocin, growth hormone: all of them are a handful of amino acids, strung in a specific order, carrying a specific instruction.

The order is the message. Change one residue and you can change the receptor it fits, how long it lasts, or whether it says anything at all. And because they are built from amino acids rather than fat, peptide hormones are water-soluble — which, as it turns out, decides almost everything about how they work.

How a peptide hormone is made

A peptide hormone is not translated in its final form. The gene is read into a longer, inactive precursor called a preprohormone. A leading signal peptide routes it into the endoplasmic reticulum and is then clipped off, leaving a prohormone. Enzymes called convertases cut the prohormone at specific sites to release the mature, active peptide, which is packaged into vesicles and released on demand by exocytosis.

The clever part is that one precursor can carry several messages. The same proglucagon chain is cut one way in the pancreas to make glucagon and another way in the gut to make GLP-1 — same source, different scissors, different hormone.

1 · PREPROHORMONEsignalproglucagon chainsignal peptide removed (in the ER)2 · PROHORMONEproglucagonconvertases cut at paired basic residues3 · ACTIVE PEPTIDESGLP-1glucagonGLP-2
One gene, one precursor, several hormones. Proglucagon is cut one way in the pancreatic alpha cell to release glucagon, and another in the intestinal L-cell to release GLP-1 and GLP-2 — which is why the same chain can both raise and lower the body’s response to a meal.

How peptide hormones work

Water-solubility is the fork in the road. A peptide hormone cannot slip through the fatty cell membrane, so it never goes inside the cell. Instead it docks onto a receptor on the surface — usually a G-protein-coupled receptor — which relays the message inward through second messengers like cAMP. The effect is fast, and it is meant to be brief: most peptide hormones are taken apart within minutes, on purpose, so the next instruction can differ from the last.

This is exactly where a steroid hormone diverges. Steroids are lipid-soluble, so they cross the membrane, bind receptors inside the cell, and change which genes are transcribed — a slower, longer-lasting kind of instruction. Same word, “hormone”; two entirely different mechanisms. That short half-life is also why the native molecules make poor drugs, and why so much of the modern field is engineering around it — the subject of two minutes to seven days.

The one-line version

A peptide hormone is a water-soluble amino-acid chain that signals from the cell surface, fast and briefly. A steroid is a lipid-soluble molecule that signals from inside the cell, slow and long. Everything else follows from that.

Examples of peptide hormones, by family

There are dozens, and they are easiest to hold in mind grouped by the signaling system they belong to. Across this reference that is 76 molecules in 12 families — here is the whole map, each opening onto the molecules that matter and the evidence behind them.

Common questions

What is a peptide hormone?
A peptide hormone is a hormone made of a short chain of amino acids — typically from three residues to about fifty — that is released by one group of cells to change the behavior of another. Insulin, glucagon, GLP-1, oxytocin, and growth hormone are all peptide hormones. Being water-soluble, they act on receptors at the cell surface rather than entering the cell.
How are peptide hormones made?
A gene is transcribed and translated into a larger inactive precursor called a preprohormone. Its signal peptide is removed in the endoplasmic reticulum to give a prohormone, which enzymes called convertases cut at specific sites to release the mature, active peptide. The products are packaged into secretory vesicles and released by exocytosis. One precursor can yield several hormones: proglucagon is cut into GLP-1, glucagon, and GLP-2.
How do peptide hormones differ from steroid hormones?
Peptide hormones are chains of amino acids and are water-soluble, so they cannot cross the cell membrane and instead bind receptors on the cell surface, triggering fast second-messenger signaling. Steroid hormones are lipid-soluble, cross the membrane, and act on receptors inside the cell to change gene transcription — a slower, longer-lasting effect.
What are some examples of peptide hormones?
Examples include insulin and glucagon (blood-sugar control), GLP-1 and GIP (the incretins), growth hormone and IGF-1, oxytocin and vasopressin, the melanocortins, PTH and calcitonin, and the natriuretic peptides ANP and BNP. They are usually grouped by the signaling system they belong to.

Education, not a protocol

This page explains what peptide hormones are and how they work. It is not medical advice and not a recommendation to take anything. For how this reference is sourced, graded, and bounded, see methodology & standards.