How research peptides are classified: origin, structure, size and receptor

Why there is more than one scheme

Each grouping was built for a different job, so every compound has a place in all four.

By origin

Endogenous peptides

An endogenous peptide is one the body makes. Several compounds in the catalogue are copies of natural peptides, or start from one.

• Review Oxytocin is a hormone of nine amino acids, made mainly in the hypothalamus, a region of the brain.⁴
• Review Alpha-melanocyte-stimulating hormone (alpha-MSH) is a peptide of 13 amino acids derived from a precursor protein, proopiomelanocortin.⁵
• Cell study MOTS-c is a 16-amino-acid peptide encoded within the mitochondrial 12S rRNA gene.⁶ The overview of mitochondrial peptide research covers its relatives.
• Chemistry study The tripeptide Gly-His-Lys (GHK) occurs naturally in blood plasma, where it binds copper.⁷ GHK-Cu is its copper complex.

Fragments of a larger molecule

A fragment is a stretch of sequence taken from a longer peptide or protein.

• Review KPV is the last three residues of alpha-MSH.⁵
• Review BPC-157 was described in 1993 as a 15-amino-acid fragment of BPC, a larger peptide isolated from gastric juice.⁸
• Chemistry study A racing laboratory’s 2012 paper describes TB-500 as a veterinary preparation whose key ingredient is a synthetic version of LKKTETQ, residues 17 to 23 of thymosin beta-4, with an acetyl group added at one end.⁹
• Review AOD-9604 is listed in a 2026 review as a fragment of growth hormone, residues 176 to 191.¹⁰

This grouping matters most when reading the literature. A study of the full-length parent is not a study of the fragment, and the two can travel under similar names. The TB-500 page sorts its sources by the molecule each one used.

Synthetic analogues

An analogue keeps most of a natural sequence and changes part of it.

Review A review of peptide drugs aimed at G protein-coupled receptors found that most are agonists, meaning activators, and put this down to the dominant strategy of modifying the sequence of the natural ligand.³

• Review Tesamorelin, sermorelin and CJC-1295 are analogues of growth hormone-releasing hormone (GHRH).¹⁰
• Review Melanotan I and Melanotan II are synthetic derivatives of alpha-MSH.¹¹
• Review A 2005 paper describes Semax and Selank as hybrid peptides, which combine sequences from different natural regulatory peptides, and places them among the peptides that contain the motif Pro-Gly-Pro.¹²

Designed sequences

Some compounds were built for a job that no single natural peptide does, or were made before a natural counterpart was known.

Review The growth hormone secretagogues are the clearest case. They are a class of synthetic peptide and non-peptide compounds that release growth hormone.¹³ Their receptor was cloned in 1996, and ghrelin, the natural hormone that acts at it, was identified only in 1999.¹³ A 2026 review lists ipamorelin among the peptides of this class.¹⁰

Cell study Retatrutide was reported in 2022, under the code LY3437943, as a single peptide that is an agonist at three receptors: those for glucagon, GIP and GLP-1.¹⁴¹⁵ The same paper went on from receptor assays in cells to obese mice and a first study in people.¹⁴ The retatrutide page explains where its sequence came from.

The line between an analogue and a designed sequence is not sharp, and a compound placed on one side of it here could fairly be placed on the other.

Entries that are not peptides

The catalogue also holds compounds that are not peptides. NAD+ is nicotinamide adenine dinucleotide, C21H27N7O14P2: a dinucleotide, not a chain of amino acids.¹⁶

By structure

Two compounds with similar sequences can still be different kinds of object. Structure is the grouping that says what an identity test must confirm beyond the order of the residues.

Linear peptides

A linear peptide is a single unbranched chain with two ends. A sequence written out letter by letter, such as the fifteen residues of BPC-157, describes a chain of this kind.

Chemistry study The ends can be capped. TB-500, as the analytical paper cited above describes it, carries an acetyl group on its first residue.⁹ The article on peptide nomenclature and modifications, listed under related reading, explains how such changes are written.

Cyclic peptides

In a cyclic peptide, part of the chain is closed into a ring.

Chemistry study A study of how oxytocin breaks down in solution describes it as a cyclic nonapeptide: one disulfide bond, a link between two sulfur atoms, joins the cysteines at positions 1 and 6.¹⁷ The ring is part of the molecule’s identity: the same nine residues left as an open chain would be a different compound.

Lipidated peptides

A lipidated peptide carries a fatty acid chain.

Review A 2023 review describes attaching fatty acids as a molecular engineering technology first applied to insulin and since applied to several other peptide and protein medicines, whose effects would otherwise be short-lived.¹⁸ How the attachment prolongs action depends on the fatty acid and on where it sits.¹⁸

Review Nature uses the same device. Ghrelin carries an eight-carbon fatty acid on its third residue, a serine, and needs it to bind its receptor.¹³

Chemistry study Among catalogue compounds, a 2025 stability study describes semaglutide, an agonist of the GLP-1 receptor, as a peptide of 31 amino acids modified with an 18-carbon fatty diacid that lets it bind albumin, a blood protein.¹⁹ The article on incretin receptor agonists describes the fatty acid chains on semaglutide, tirzepatide and retatrutide, and the article on half-life extension strategies sets out the general idea.

Review Other attachments exist. A 2026 review lists CJC-1295 in two forms, with and without a “drug affinity complex”.¹⁰ The CJC-1295 page explains the difference.

Metal complexes

A few peptides are studied as complexes with a metal ion.

Chemistry study A 2011 structural study describes the complex that GHK forms in solution: one peptide holds a copper(II) ion through three of its nitrogen atoms.⁷

For this class the name alone does not say whether a formula, a weight or a stated mass refers to the bare peptide or to the complex. The GHK-Cu page lists each form.

By size

Chain length gives peptides their everyday names. The unit is the residue, one amino acid within a chain.

The international recommendations on nomenclature define a peptide as any compound formed by amide bonds between amino acids.¹ They add that peptides of fewer than about 10 to 20 residues may be called oligopeptides and longer ones polypeptides.¹ Polypeptides of specific sequence with more than about 50 residues are usually known as proteins, “but authors differ greatly on where they start using this term”.¹

Regulators draw their own lines. A United States regulation, 21 CFR 600.3, defines a protein as an amino acid polymer of specific, defined sequence that is longer than 40 amino acids.²

Short chains are named by count, with numerical prefixes.

Size is a label, and by itself it does not say what a compound does: a tripeptide and a chain of 31 residues are both peptides.

By receptor family or pathway

Pharmacologists group compounds by what they bind. For many peptides that is a receptor on the surface of a cell.

Review Many natural peptides activate G protein-coupled receptors (GPCRs). A 2020 review counted nearly 50 approved peptide drugs acting at these receptors, most of them for metabolic disease or cancer.³

Review A receptor of this kind is a single chain that crosses the cell membrane seven times.²⁰ The first scheme proposed for sorting these receptors divided them into lettered classes by sequence similarity, among them class A, the rhodopsin-like receptors, and class B, the secretin receptor family.²⁰ Family B is also written family 2.²¹ Two of these classes hold the receptors that matter for this catalogue. The ghrelin receptor is a rhodopsin-like receptor of family A.¹³ The receptors for glucagon, secretin and their relatives belong to the other, which their nomenclature review calls the class 2 family.²²

The secretin family: incretin and GHRH receptors

Review Glucagon, GLP-1, GIP, GHRH and secretin are structurally related peptides, and each acts through its own member of a related family of receptors.²² The nomenclature committee of the International Union of Pharmacology names each receptor after its natural ligand.²²

Two groups of catalogue compounds are studied here. The GHRH analogues named above are covered in the overview of growth hormone secretagogues. Semaglutide and retatrutide, both described above, are covered with the incretin receptor agonists.

The rhodopsin family: ghrelin and melanocortin receptors

Review The ghrelin receptor, also called the growth hormone secretagogue receptor 1a, is a rhodopsin-like receptor of family A and the target of the synthetic growth hormone secretagogues.¹³ Ipamorelin is listed in that class.¹⁰

Review That puts the two components of one catalogue blend in different receptor families: CJC-1295, as a GHRH analogue, with the secretin family, and ipamorelin, as a growth hormone secretagogue, with the rhodopsin family.¹⁰¹³²²

Review The melanocortin receptors are a family of five class A receptors, and all but one of them respond to a shared set of natural agonists that includes alpha-MSH.¹¹ Melanotan I and Melanotan II serve as tool compounds, the reference molecules used to probe these receptors.¹¹ The overview of melanocortin peptide research covers this family. KPV comes from the same hormone, but whether it acts through these receptors is a separate question, which the KPV page takes up.

Compounds grouped by pathway

For several catalogue compounds, the sources read for this page name no receptor of this kind. For them the grouping is by pathway, or by what is known of the mechanism.

• Chemistry study The segment of thymosin beta-4 that TB-500 reproduces is described as the site responsible for binding actin, a structural protein inside the cell.⁹
• Chemistry study GHK is described as a high-affinity chelator of copper(II), meaning that it holds the metal ion tightly.⁷
• Cell study The 2015 report that described MOTS-c placed its cellular actions on the folate cycle and on activation of the enzyme AMPK.⁶
• Review The 1993 overview of BPC-157 called its final mechanism poorly understood.⁸
• The peptides named after the thymus gland are covered in the overview of thymic peptide research.

The research-area labels in the store’s menus, such as recovery or longevity, are a way to find things. They are not pharmacological classes.

What a class does not tell you

Human evidence

This guide describes how compounds are grouped. It reports no trial, and membership of a class is not human evidence.

Review Compounds studied on the same hormone axis can stand at opposite ends of the evidence. A 2026 review of peptides that act on the growth hormone axis sorted them into tiers running from regulatory-grade randomized trial data to a complete absence of human studies.¹⁰

Each compound page has its own “Human evidence” section, and the guide to evidence levels in peptide research explains how to weigh what is there. Products on this site are supplied for laboratory research use only.