Most peptides act as signals: they bind to specific receptors on the surface of cells, trigger downstream signalling cascades, and nudge cells or tissues toward certain responses. What a given peptide does depends on which receptor it fits, where that receptor is expressed, and how long the peptide survives before enzymes break it down.
Receptors and signalling pathways
Peptides work in the body the way a key works in a lock. Each peptide has a specific sequence of amino acids that folds into a shape, and that shape fits certain receptors — proteins embedded in cell membranes that are built to recognize it. When the peptide docks, the receptor changes shape on the inside of the cell and sets off a chain of chemical events called a signalling cascade.
Many peptide hormones act through a family of receptors called G-protein-coupled receptors (GPCRs). The best-studied example in the current peptide landscape is GLP-1 (glucagon-like peptide-1), a gut hormone released after meals. When GLP-1 binds its receptor, the downstream signalling augments insulin secretion and inhibits glucagon secretion, which is how it helps control post-meal blood sugar (Drucker, Cell Metabolism 2018). The same review maps GLP-1 receptor-positive cell types across the body, which explains why one peptide can influence appetite, gastric emptying, and cardiovascular physiology at the same time.
Two features make this signalling system powerful:
- Specificity — a peptide only acts where its receptor is expressed, so effects are targeted rather than body-wide by default.
- Amplification — one peptide-receptor binding event can trigger many downstream molecules, so small amounts of peptide can produce meaningful biological effects.
From cell signal to whole-body effect
A single binding event does not change how a person feels; the effect comes from the same signal firing across millions of cells in the tissues where the receptor lives. The pattern of receptor expression determines the pattern of effects:
- A peptide whose receptor sits on pancreatic islet cells shifts hormone release and blood sugar.
- A peptide whose receptor sits on appetite-regulating neurons changes hunger and food intake.
- A peptide acting on skin or connective-tissue cells may influence collagen remodelling or local repair signalling.
This receptor-based mechanism is why peptides have become such a productive drug class: they can be highly selective for one biological pathway. A 2022 review in Signal Transduction and Targeted Therapy surveys how therapeutic peptides act through specific receptor interactions across multiple organ systems, from incretin pathways to natriuretic peptide receptors in the cardiovascular system (Wang et al., 2022). By one count, over 60 peptide drugs are approved in the United States and other major markets, with more than 150 in active clinical development (Lau & Dunn, Bioorganic & Medicinal Chemistry 2018).
Metabolism, clearance, and why half-life matters
Because peptides are chains of amino acids, the body treats them much like dietary protein: proteolytic enzymes in the blood, gut, kidneys, and tissues cut them apart, and the fragments are recycled. Two practical consequences follow:
- Most natural peptides are short-lived. Native GLP-1, for example, is degraded within minutes of release — useful for a meal-to-meal signal, useless as a once-daily medicine.
- Most peptides cannot survive digestion, which is why peptide drugs are typically injected rather than swallowed (oral formulations exist but require special engineering).
Drug developers work around this by modifying the peptide backbone — swapping vulnerable amino acids, attaching fatty-acid chains, or otherwise shielding the molecule from enzymes. Semaglutide is the textbook case: chemical modifications extend its survival in the body far enough to support once-weekly injection, per its FDA labeling. Half-life is therefore not a footnote — it largely determines how a peptide can be studied and dosed in trials.
Examples from this catalog
Mapping a few peptides from this site onto the mechanism above shows how varied — and how variably supported — "peptides" are:
- Semaglutide — a GLP-1 receptor agonist; receptor, signalling pathway, and clinical effects are characterized in large randomized trials. This is the strongest mechanistic evidence tier.
- Tirzepatide — activates both GIP and GLP-1 receptors, an example of one molecule engineered to hit two receptor systems.
- BPC-157 and TB-500 — discussed as healing peptides, but their proposed mechanisms come mainly from animal and cell studies; no receptor pathway has been mapped in humans the way GLP-1 has.
- GHK-Cu — a copper-binding fragment studied mostly in skin and wound-healing models, where it is thought to act on local repair and remodelling signals.
The mechanism template — bind a target, trigger a cascade, get cleared by enzymes — is shared. The quality of evidence behind each step is what separates an approved medicine from an experimental compound.