On this pageWhat Are Peptides? Basic Chemistry
Peptides are short chains of amino acids joined by amide (peptide) bonds, usually defined as containing roughly 2 to 50 residues. Many of the signals that cells use to communicate are peptides, and synthetic peptides are among the most common tools in receptor pharmacology and cell biology.
This article groups signaling peptides into four research classes: peptide hormones, neuropeptides, antimicrobial peptides and matrikines. It then covers how research peptides are made, characterized and stored.
What Are Peptides? Basic Chemistry
A peptide has an N-terminal amino group, a C-terminal carboxyl group, and a backbone of repeating amide bonds. Its properties are set by the sequence and by modifications such as C-terminal amidation, N-terminal acetylation or pyroglutamate formation, disulfide bridges, lipidation, and incorporation of non-natural amino acids.
In cells, most signaling peptides are cut from larger precursor proteins (prepropeptides) by signal peptidases and prohormone convertases, then modified and stored in secretory vesicles. Because short peptides are degraded by peptidases, their signaling is typically local or short-lived, which is a key consideration in assay design.
Peptide Hormones and Receptor Signaling
Peptide hormones act on receptors at the cell surface. Most bind G protein-coupled receptors (GPCRs): examples include glucagon and glucagon-like peptide-1 (GLP-1) at class B GPCRs, and oxytocin and vasopressin at class A GPCRs. Receptor activation is studied through second messengers such as cAMP and intracellular calcium, β-arrestin recruitment, and receptor internalization.
Insulin is an exception that illustrates a second major mechanism: it binds a receptor tyrosine kinase, triggering autophosphorylation and the IRS–PI3K–AKT and RAS–MAPK cascades. Research on peptide hormones often focuses on structure–activity relationships, biased agonism, and how amino acid substitutions alter receptor selectivity or resistance to peptidases such as DPP-4.
Neuropeptides in Neuronal Signaling Research
Neuropeptides are released by neurons, often alongside classical small-molecule neurotransmitters, and act mainly through GPCRs. Well-studied examples include substance P (NK1 receptor), neuropeptide Y (Y1–Y5 receptors), the opioid peptides (enkephalins, endorphins and dynorphins), orexins, and oxytocin and vasopressin in their central roles.
Compared with fast neurotransmitters, neuropeptides are present at lower concentrations, act at high-affinity receptors, and are not recycled by reuptake. Research tools include synthetic agonists and antagonists, fluorescently labelled ligands for receptor localization, and knockout or knockdown of specific precursor genes in cell and tissue models.
Antimicrobial Peptides and Membrane Interactions
Antimicrobial peptides (AMPs), also called host-defense peptides, are typically short, cationic and amphipathic. Examples include the defensins, the human cathelicidin LL-37, and the magainins originally isolated from frog secretions. Their positive charge attracts them to the negatively charged membranes of bacteria, and their amphipathic structure lets them insert into and disrupt lipid bilayers.
Laboratory study of AMPs uses minimum inhibitory concentration (MIC) assays, membrane permeabilization assays with fluorescent dyes, circular dichroism to follow secondary-structure changes in membrane-mimetic environments, and model lipid vesicles. Several AMPs, LL-37 in particular, also bind host-cell receptors and modulate immune signaling, which links this class back to cell signaling research.
Matrikines: Signaling Peptides From the Extracellular Matrix
Matrikines are peptide fragments released by proteolysis of extracellular matrix proteins that act as signals to cells. Examples studied in culture systems include:
- the tripeptide GHK and its copper complex GHK-Cu, studied for effects on collagen and proteoglycan expression in fibroblast cultures;
- the pentapeptide KTTKS, a fragment of the type I procollagen propeptide;
- the elastin-derived hexapeptide VGVAPG, which binds the elastin receptor complex.
Matrikine research connects matrix turnover by proteases such as MMPs to the feedback signals that regulate further matrix synthesis.
How Research Peptides Are Made
Most research peptides are produced by solid-phase peptide synthesis (SPPS), introduced by Bruce Merrifield. The C-terminal amino acid is anchored to a resin and the chain is built one residue at a time, usually with Fmoc chemistry for temporary N-terminal protection. After cleavage from the resin, the crude peptide is purified by reversed-phase HPLC and freeze-dried. Longer or modified peptides may be produced by fragment ligation or recombinant expression.
Characterization, Storage and Documentation
Each research peptide lot should be documented with:
- HPLC purity (percent area of the main peak),
- mass spectrometry to confirm the expected molecular mass and sequence identity,
- appearance, lot number and analysis date, and, where relevant, peptide content and endotoxin.
Lyophilized peptides are best stored sealed, dry and cold, protected from light, and allowed to reach room temperature before opening. Peptides containing methionine, cysteine or tryptophan are prone to oxidation and are best kept under inert gas. Solutions are aliquoted and frozen to limit freeze–thaw. NuRev publishes lot documentation on its Certificates of Analysis page.
NuRev supplies peptides for laboratory research use only; they are not for human or veterinary use.