Glossary

A short chain of amino acids joined by peptide bonds — larger than a single amino acid, smaller than a full protein. Peptides are the body's signaling shorthand, and the molecules this reference is built around.

A hormone that happens to be a peptide: a secreted amino-acid chain that carries a signal through the bloodstream from the tissue that makes it to the tissue that reads it.

The building block of every peptide and protein. Twenty standard amino acids, strung in a specific order, encode a peptide's identity, its fold, and what it does.

A molecule's mass, measured in daltons (Da). It quietly sorts this field: small signaling peptides run a few thousand daltons, glycoprotein hormones tens of thousands.

The larger, inactive precursor a peptide hormone is minted as, then cleaved from. Proglucagon is cut into GLP-1 and glucagon; POMC into α-MSH and ACTH.

A gut hormone released after a meal that amplifies glucose-dependent insulin release — GLP-1 and GIP are the two. The name fuses intestine and insulin.

A peptide that neurons use to signal — as a neurotransmitter or a slower-acting neuromodulator. Oxytocin and vasopressin are the classic examples.

A family of peptides cleaved from the POMC precursor — α-MSH and ACTH among them — that act at melanocortin receptors to govern pigmentation, appetite, and the stress axis.

Heart- and vessel-derived peptides (ANP, BNP, CNP) that lower blood pressure by driving sodium and water excretion — the body's counter to fluid overload.

A protein — usually anchored in the cell membrane — that recognizes one specific signaling molecule and converts its arrival into an action inside the cell. A hormone is only as specific as the receptor that reads it.

Any molecule that binds a receptor: the key to its lock. A native hormone, a drug, and a blocker can all be ligands for the same receptor.

The largest family of cell-surface receptors and the target of most peptide hormones: a seven-pass membrane protein that, on binding its ligand, activates an intracellular G protein to relay the message.

The GPCR branch most peptide hormones dock into. A large extracellular domain grips the peptide like a catcher's mitt — the feature that long made these receptors hard to switch on with a small-molecule pill.

An intracellular signal generated the moment a receptor fires, carrying the message from the membrane inward to the machinery that acts on it.

The prototypical second messenger — a small molecule that surges inside a cell when a stimulatory GPCR switches on, activating the kinases that carry out the response.

The tuning-out that follows sustained stimulation: the receptor is uncoupled or pulled inside the cell, so the same signal yields a smaller response. Why continuous agonism can, in time, resemble blockade.

When a ligand switches a receptor on toward some downstream pathways but not others — the same receptor, a selectively edited message.

A molecule that binds a receptor and switches it on — pharmacology's active voice, standing in for the body's own signal. Most peptide therapeutics here are receptor agonists.

A molecule that occupies a receptor and blocks it, muting the signal rather than sending it.

A ligand that turns a receptor on only part-way, even when every receptor is occupied — a deliberately quieter signal than a full agonist's.

A single engineered molecule that activates two or more receptors at once — the design behind tirzepatide (GIP + GLP-1) and the triple agonists.

A compound that prompts a gland to secrete. A growth-hormone secretagogue, for instance, makes the pituitary release its own GH rather than supplying it from outside.

How narrowly a molecule acts on its intended receptor versus its close relatives. Leaky selectivity is why blocking one TGF-β ligand can spill onto its cousins.

What the body does to a molecule over time — absorption, distribution, and clearance — as distinct from what the molecule does to the body.

The time for the amount of a molecule in the body to fall by half. Native peptide hormones are often measured in minutes; engineered analogs stretch that to days.

The enzyme that clips and inactivates incretins like GLP-1 within minutes — the clearance route that protease-resistant analogs are built to evade.

The fraction of an administered dose that actually reaches circulation intact. Oral peptides fare famously badly — around 1% for oral semaglutide — which is why most are injected.

The plateau reached when each dose replaces roughly what has cleared since the last, so peaks and troughs stop drifting upward.

Attaching a fatty-acid chain to a peptide so it clings to albumin in the blood — turning a molecule cleared in minutes into one that lasts days. The trick behind liraglutide and semaglutide.

The most abundant protein in blood plasma. The body reclaims it slowly, so a drug engineered to grip albumin borrows that long life.

An engineered peptide's ability to withstand the enzymes that chop peptides apart — usually bought by substituting unnatural amino acids at the cleavage sites.

Beneath the skin — the injection route most peptide therapeutics take, into the fat layer they then absorb from steadily.

Produced within the body itself. Here, the native hormones against which every engineered analog is measured.

An engineered molecule built on an endogenous hormone and tuned for potency, half-life, or receptor selectivity. Semaglutide is a GLP-1 analog.

A community-circulated peptide outside the approved-drug system, where the evidence is often thinnest and the claims loudest. This reference grades them, and says so.

This reference's rigor badge — a five-rung grade from Established to Limited marking the weight of evidence behind a molecule, kept separate from how promising it is.