Introduction to Peptides

By Peptide Information      May 1, 2025

ALL ARTICLES AND PRODUCT INFORMATION PROVIDED ON THIS WEBSITE ARE SOLELY FOR INFORMATION DISSEMINATION AND EDUCATIONAL PURPOSES.

The products provided on this website are intended exclusively for in vitro research. In vitro research (Latin: *in glass*, meaning in glassware) is conducted outside the human body. These products are not pharmaceuticals, have not been approved by the U.S. Food and Drug Administration (FDA), and must not be used to prevent, treat, or cure any medical condition, disease, or ailment. It is strictly prohibited by law to introduce these products into the human or animal body in any form.

What is a Peptide?

The term "peptide" originates from the Greek word "πέσσειν" (péssein), meaning "to digest." A peptide is a compound formed by amino acids linked through peptide bonds. Peptide bonds, which are amide bonds, arise from the dehydration condensation between the α-carboxyl group of one amino acid and the α-amino group of another. Classified by amino acid count, peptides with 2 to 20 amino acids are termed oligopeptides, those with 20 to 50 are polypeptides, and sequences exceeding 50 amino acids with specific spatial structures are generally categorized as proteins. Peptides typically have a molecular weight below 10 kDa. Their primary structure consists of a linear amino acid sequence, and some peptides can form secondary structures such as α-helices. Exhibiting diverse functions, peptides are widely involved in biological processes including signal transduction, metabolic regulation, and immune responses, rendering them important biomolecules with both structural diversity and biological functionality in applications across medicine, food science, and other fields.

How are Peptides Formed?

Peptide formation occurs through the linkage of amino acids via peptide bonds, occurring naturally in vivo or artificially in vitro. In biological systems, ribosomal synthesis involves mRNA carrying genetic information to direct ribosomes in arranging activated amino acids in a specific sequence. Transfer RNAs (tRNAs) deliver amino acids to the ribosome, where enzymatic catalysis drives dehydration condensation between the α-carboxyl group of one amino acid and the α-amino group of another, forming an amide (peptide) bond and releasing a water molecule; this process repeats to generate oligopeptides or polypeptides. Non-ribosomal synthesis relies on specialized enzyme complexes like peptide synthetases, commonly observed in microorganisms producing bioactive peptides such as antibiotics. In vitro synthesis primarily employs solid-phase peptide synthesis (SPPS), where protected amino acids are sequentially coupled to a resin support; deprotection and condensation reactions extend the peptide chain stepwise. Alternatively, target peptides can be obtained via enzymatic hydrolysis of natural proteins. Peptide bond formation represents a covalent bonding process, serving as the core linkage in the primary structure of peptides and proteins. Further details on peptide bonds are discussed in the section "Peptide Bonds."

Nomenclature of Peptides

The nomenclature of peptides typically derives from the number of amino acids they contain, following the convention of “numeral + peptide”. For instance, a dipeptide comprises two amino acids, a tripeptide three, and this continues up to a decapeptide with ten amino acids. Peptides with more than ten amino acids are directly named as “11 - peptide”, “20 - peptide”, etc. This naming system generally applies to conventional linear peptides connected by α - peptide bonds. However, exceptions exist:

Some cyclic peptides, such as cyclosporine and gramicidin, are named with a “cyclo -” prefix or a proprietary name due to head-to-tail connection or side - chain cyclization, and may contain non-natural components like D-amino acids.

Peptides with unique connectivity, such as glutathione, which has a “γ-peptide bond” formed between the γ-carboxyl group and the α-amino group, do not follow the linear α-peptide bond numbering convention.

Functional peptides, such as antimicrobial and neuropeptides, are named based on their biological functions rather than amino acid count. For example, mellitin, an antimicrobial peptide with 26 amino acids.

Non-ribosomally synthesized peptides from microorganisms, such as bacitracin and actinomycin, contain modified amino acids (e.g., methylated or cyclized) and are named after their source or function due to their unique biosynthetic pathways, rather than amino acid count.

In summary, these exceptions reflect the diverse nature of peptide nomenclature, which must take into account structural features, synthetic routes, and functional characteristics.

Classification of peptides  

Peptides can be categorized in multiple dimensions:

By the number of amino acids, oligopeptides consisting of 2-20 peptides such as dipeptide and glutathione are highly active; polypeptides with 20-50 peptides can form simple spatial structures such as insulinogen fragments; peptides with more than 50 peptides and complex functions are usually categorized as proteins, such as insulin which contains 51 amino acids.  

By chemical structure, linear peptides are linked by α-peptide bonds such as enkephalin, cyclic peptides form a ring through the first and last tail or side chain such as cyclosporine, modified peptides contain unnatural components such as the D-amino acids of actinomycin, and specially linked peptides such as glutathione are linked by γ-peptide bonds.  

By mode of synthesis, ribosomal synthetic peptides are encoded by genes such as endorphins, non-ribosomal synthetic peptides are dependent on microbial enzyme complexes such as mycopeptides, and synthetic peptides are prepared by chemical or biological means such as medicinal octreotide.  

By function, signaling peptides are involved in transmission such as thyrotropin-releasing hormone, antibacterial peptides destroy bacterial membranes such as bee venom peptides, neuropeptides regulate nerves such as endorphins for analgesia, and there are also medicinal peptides and functional food peptides.  

By source, natural peptides exist in organisms or food, such as dairy casein peptides, and synthetic peptides break through natural limitations through artificial intervention, such as cosmetic oligopeptides.  

By biological sources, they can be categorized into animal sources such as tensin, plant sources such as soybean peptides, and microbial sources such as Mycobacterium avium peptides.  

These classifications intertwine to reflect the rich diversity of peptides in structure, function and application.

Important terms related to peptides  

Peptide bond: The amide bond -CO-NH- formed by the dehydration condensation of the α-carboxy group of one amino acid with the α-amino group of another amino acid, which is the basic covalent bond connecting amino acid residues and constituting peptides and proteins.  

Oligopeptide: usually refers to low molecular weight peptide compounds, such as dipeptide, tripeptide, etc., which are formed by connecting 2-20 amino acids through peptide bond, and have high biological activity and cell membrane permeability.  

Polypeptide: peptide composed of 20-50 amino acids, molecular weight is usually less than 10 kDa, can form simple spatial structure such as α-helix, is a functional molecule between oligopeptide and protein.  

Primary structure: the amino acid sequence of peptide, determined by genetic information or artificial design, is the basic chemical structure of peptide, which directly affects its advanced structure and biological function.  

Secondary structure: the ordered conformation formed by hydrogen bonding in the local area of the peptide chain, such as α-helix, β-folding, β-turning, etc., which is not as stable as that of proteins, but is involved in the formation of functional sites.  

Cyclic peptides: peptides that form a cyclic structure by connecting the first and last amino acids or side chain groups, such as cyclosporine and short peptide, with high stability and anti-enzymatic features, often containing the prefix “cyclic” in the name.  

Ribosome synthesized peptides: peptides synthesized by the ribosomes of living organisms through mRNA template translation, and their sequences are encoded by genes, such as the hormone insulin and the neurotransmitter endorphin.  

Non-ribosomal synthetic peptides: peptides synthesized by special enzyme complexes such as peptide synthetases, commonly found in microorganisms such as bacteria and fungi, and may contain unnatural amino acids such as D-amino acids, e.g., the antibiotic Mycobacterium avium peptide.  

Solid Phase Peptide Synthesis (SPPS): In vitro artificial peptide synthesis technology, through the sequential coupling of protected amino acids on the resin carrier, through the deprotection, condensation reaction to gradually extend the peptide chain, suitable for short peptide synthesis precision.