What Is a Peptide?
- By NUPEPS Peptides
- Dec 4, 2025
The word peptide originates from the Greek “πέσσειν” (péssein), meaning to digest. Peptides are compounds formed when amino acids link together through peptide bonds—a type of amide bond created during a dehydration condensation reaction between the α-carboxyl group of one amino acid and the α-amino group of another.
Peptides are categorized by size:
- Oligopeptides: 2–20 amino acids
- Polypeptides: 20–50 amino acids
- Proteins: 50+ amino acids with defined three-dimensional structures
Typically under 10 kDa, peptides may adopt simple secondary structures such as α-helices and participate in a wide range of biological processes including signaling, metabolic regulation, and immune responses. Their structural diversity and functional versatility make them valuable in fields such as biochemistry, biotechnology, and food science.
How Are Peptides Formed?
Peptides form when amino acids join through peptide bonds, either naturally in living organisms or artificially in laboratory settings.
1. Ribosomal Synthesis (In Vivo)
Messenger RNA (mRNA) guides the ribosome to assemble amino acids in a precise sequence. Transfer RNAs (tRNAs) deliver amino acids, and enzymatic reactions catalyze the condensation steps that form peptide bonds. This process creates oligopeptides and polypeptides based on genetic templates.
2. Non-Ribosomal Synthesis
Certain microorganisms use large enzyme complexes called non-ribosomal peptide synthetases (NRPS) to produce unique peptides such as antimicrobial or biologically active compounds. These often contain non-standard amino acids or complex ring structures.
3. In Vitro Chemical Synthesis
The most widely used laboratory method is Solid-Phase Peptide Synthesis (SPPS). In SPPS, protected amino acids are sequentially attached to a resin, with cycles of coupling and deprotection enabling precise, stepwise chain elongation. Peptides can also be produced by enzymatic digestion of natural proteins.
Peptide bond formation is central to all peptides and proteins, serving as the chemical backbone of their primary structure.
Peptide Nomenclature
Peptides are often named according to the number of amino acids they contain—e.g., dipeptide, tripeptide, up to decapeptide (10). Peptides with more than ten residues may be labeled as “11-peptide,” “20-peptide,” and so on.
However, numerous exceptions exist:
- Cyclic peptides (e.g., cyclosporine) often use the prefix cyclo- or unique names due to head-to-tail linkages or side-chain cyclization.
- Non-standard linkages, such as the γ-peptide bond in glutathione, do not follow standard α-peptide naming conventions.
- Functional names are used for biological roles, such as antimicrobial peptides or neuropeptides.
- Non-ribosomal peptides like actinomycin incorporate modified amino acids and are named for their origin or function.
Overall, peptide nomenclature reflects their structural complexity, biosynthetic route, and biological roles.
Classification of Peptides
Peptides can be classified in several ways:
By Amino Acid Count
- Oligopeptides (2–20 residues): Highly active, often cell-permeable.
- Polypeptides (20–50 residues): May adopt simple structures.
- Proteins (>50 residues): Complex structures and extensive functions.
By Structure
- Linear peptides: Standard α-peptide bonds (e.g., enkephalin).
- Cyclic peptides: Ring-forming linkages, often more stable (e.g., cyclosporine).
- Modified peptides: Contain D-amino acids or unusual residues (e.g., actinomycin).
- Special linkages: Such as γ-peptide bonds in glutathione.
By Mode of Synthesis
- Ribosomal peptides: Gene-encoded (e.g., endorphins).
- Non-ribosomal peptides: Microbial enzymatic products (e.g., certain antibiotics).
- Synthetic peptides: Produced via chemical or biotechnological methods (e.g., octreotide).
- Special linkages: Such as γ-peptide bonds in glutathione.
By Biological Function
- Signaling peptides: e.g., thyrotropin-releasing hormone
- Antimicrobial peptides: e.g., bee venom peptides
- Neuropeptides: e.g., endorphins
- Therapeutic or functional food peptides
By Source
- Natural peptides: Found in organisms or foods (e.g., casein peptides).
- Synthetic peptides: Designed or engineered for research or industry.
By Biological Origin
- Animal-derived, plant-derived, or microbial-derived peptides.
These categories overlap, illustrating the remarkable diversity of peptide chemistry and applications.
Key Terms in Peptide Science
- Peptide Bond: A covalent amide bond formed by dehydration between the α-carboxyl group of one amino acid and the α-amino group of another.
- Oligopeptide: A peptide of 2–20 amino acids, often bioactive and easily transported across cell membranes.
- Polypeptide: A chain of 20–50 amino acids, possibly containing simple secondary structure elements.
- Primary Structure: The linear amino acid sequence that defines the peptide’s chemical identity and higher-order structure.
- Secondary Structure: Local folding patterns such as α-helices, β-sheets, and β-turns formed by hydrogen bonding.
- Cyclic Peptides: Peptides with ring structures offering high stability and resistance to degradation.
- Ribosome-Synthesized Peptides: Peptides produced through genetic translation within living organisms.
- Non-Ribosomal Peptides: Peptides assembled by specialized microbial enzymes and often containing non-standard building blocks.
- Solid-Phase Peptide Synthesis (SPPS): A foundational in vitro technique enabling controlled, stepwise peptide construction on a solid support.
Important Notice
All articles and product information provided by NUPEPS Peptides are for educational and informational purposes only. Our products are intended exclusively for in vitro research use. In vitro research (Latin: “in glass”) refers to experiments conducted outside a living organism. NUPEPS Peptides’ products are not pharmaceuticals, are not approved by the U.S. Food and Drug Administration (FDA), and must not be used to diagnose, treat, prevent, or cure any disease or medical condition. It is strictly prohibited to introduce these products into the human or animal body in any form.