What are peptides? Classification, mechanism and receptors

Peptides are one of the languages the body uses to communicate. The code written in DNA is transcribed into RNA, RNA serves as a template for building proteins, and proteins — under the action of enzymes — are sometimes cut into shorter fragments. These fragments are precisely the peptides: chains built from amino acids, usually shorter than complete proteins. In the preclinical literature, peptides are the subject of intense study, because many of them behave like precise signals: they fit specific structures on the surface of cells and trigger well-defined pathways. This makes them a rewarding research tool — they act more like a screwdriver than a hammer.

This article is the starting point for our knowledge base. We explain what peptides are from a chemical point of view, along which axis we classify them, why the receptor is so important, and what the designation research use only means in practice. We describe the individual compounds in separate write-ups — from BPC-157 to GHK-Cu — and this text ties them together into a whole.

Definition: amino acids, bonds and the scale of size

Amino acids can be imagined as Lego bricks: there is a limited set of standard elements, and function is determined by the order in which they are connected. Individual amino acids join through a peptide bond, forming a chain. By convention it is assumed that:

• Oligopeptides — short chains, from a few to a dozen or so amino acids.
• Polypeptides — longer chains, usually several dozen amino acids.
• Proteins — the longest and most complex, often folded into elaborate spatial structures.

The boundary between a "long peptide" and a "small protein" is conventional and depends on the convention used. For a researcher, more important than length itself is the sequence — it is the sequence that gives the molecule its three-dimensional shape, and the shape decides what a given molecule fits.

The lock-and-key mechanism: why shape is everything

The specificity of peptides is best described by the analogy of a key and lock. The surface of the cell is studded with receptors — protein "locks" of a defined geometry. A peptide that has a matching shape slots into the receptor and triggers a cascade of signals inside the cell. A molecule of a different shape simply does not fit. This is why changing even a single amino acid in the sequence can completely change what is observed in studies: the new "key" may fit a different "lock" or none at all.

This precision is the reason peptides are such a valued object of mechanistic research — they allow single signaling pathways to be tested in isolation, under in vitro conditions and in animal models.

The key axis of classification: peptides with a receptor vs without a receptor

Scientifically, the most useful division runs not by "effect" but by whether a specific receptor has been identified for a given peptide. This axis organizes the way a compound is studied.

Peptides with an identified receptor

For some peptides, the literature points to a specific receptor to which they bind, as well as the pathway that is then activated. Examples of such well-defined receptor axes studied in vitro and in animal models are:

• Growth hormone secretagogues — peptides studied in terms of their interaction with the GHRH axis and secretagogue receptors.
• Analogues in the melanocortin pathway — compounds studied at the receptor level in the context of the MC3R/MC4R receptors.

When the receptor is known, research focuses on affinity, selectivity toward receptor subtypes, and on the signaling cascade triggered after binding.

Peptides without an unambiguously identified receptor

There is also a numerous group of peptides for which no single, classical receptor has been identified. This does not mean they are "passive" — in the preclinical literature their activity is described through the lens of processes rather than a single lock. They are studied mechanistically in a different way: the influence on processes such as angiogenesis, expression of VEGF, remodeling of the extracellular matrix, organization of the actin cytoskeleton, or collagen synthesis is observed. For this group the research question is often: "which pathway is modulated?", not "which receptor is bound?". This distinction has real consequences for experimental design and the interpretation of results.

Forms and route of administration: why usually not oral

Research peptides most often occur in the form of a lyophilisate — a dried powder obtained through lyophilization, which is more stable in storage than a solution. It is dissolved only for the needs of a specific laboratory protocol.

The fundamental obstacle to studying peptides via the oral route is chemical in nature: the digestive tract is designed to break down proteins and peptides into amino acids. Digestive enzymes and the environment of the stomach cut most peptide chains before they could reach the bloodstream in an intact form. That is why, in research models, many peptides are analyzed in forms that bypass the digestive system — injectable or intranasal — which allows the integrity of the molecule to be preserved for the purpose of observing the mechanism. This is a methodological choice dictated by the pharmacology of the molecule, not a recommendation for use.

What "research use only" means

All compounds described in our knowledge base have the status of research use only. In practice this means that:

• they are intended for laboratory work and experiments in vitro and in models, and not for consumption by humans or animals;
• we describe exclusively what is the subject of research and in which models, never presumed effects in a specific individual;
• the quality standard is the declared HPLC purity and documentation of the identity and purity of the batch — this information can be found in the safety data sheets.

This distinction is the foundation of honest scientific communication: a peptide can be intensively studied while, at the same time, data concerning humans may be limited or non-existent.

Level of evidence: how to read the literature on peptides

A mature approach to peptides requires distinguishing levels of evidence. An in vitro result (e.g. on a cell line) says something different than a result from an animal model, and both differ from limited human data. For most of the compounds described, the available data come mainly from in vitro studies and animal models, while conclusions concerning humans remain preliminary and require further research. Treat every write-up in our base as a map of the state of knowledge, and not as a promise of a result.

How to navigate our knowledge base

From this pillar, paths lead to write-ups of the individual compounds — with a description of the sequence, the known (or unknown) receptor, and the pathways studied in the preclinical literature. If you want to organize the selection of compounds for your own research project in terms of the mechanisms of interest to you, use the Build a set tool, and you can check the quality specifications in the safety data sheets.

Research use only. Not for consumption by humans or animals.