What Is a Peptide? Definition and Examples

A peptide is a molecule consisting of two or more amino acids linked together by peptide bonds. The general structure of an amino acid is: R-CH(NH₂)COOH. Each amino acid is a monomer that forms a peptide polymer chain with other amino acids when the carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH₂) of another amino acid, forming a covalent bond between the amino acid residues and releasing a molecule of water.

Functions

Peptides are biologically and medically important molecules. They naturally occur within organisms, plus lab-synthesized compounds are active when introduced into a body. Peptides act as structural components of cells and tissues, hormones, toxins, antibiotics, and enzymes. Examples of peptides include the hormone oxytocin, glutathione (stimulates tissue growth), melittin (honey bee venom), the pancreatic hormone insulin, and glucagon (a hyperglycemic factor).

Synthesis

Ribosomes in cells construct many peptides, as RNA is translated into an amino acid sequence and the residues are linked together. There are also nonribosomal peptides, which are constructed by enzymes rather than ribosomes. In either case, once amino acids have been linked, they undergo posttranslational modifications. These may include hydroxylation, sulfonation, glycosylation, and phosphorylation. While most peptides are linear molecules, some form rings or lariat structures. Less often, L-amino acids undergo racemization to form D-amino acids within peptides.

Peptide Versus Protein

The terms "peptide" and "protein" are commonly confused. Not all peptides form proteins, but all proteins consist of peptides. Proteins are large peptides (polypeptides) containing 50 or more amino acids or molecules that consist of multiple peptide subunits. Also, proteins typically display more complex structure than simpler peptides.

Classes of Peptides

Peptides may be classified either by their function or by their source. The Handbook of Biologically Active Peptides lists groups of peptides, including:

• Antibiotic peptides
• Bacterial peptides
• Brain peptides
• Cancer and anticancer peptides
• Cardiovascular peptides
• Endocrine peptides
• Fungal peptides
• Gastrointestinal peptides
• Invertebrate peptides
• Opiate peptides
• Plant peptides
• Renal peptides
• Respiratory peptides
• Vaccine peptides
• Venom peptides

Naming Peptides

Peptides are named according to how many amino acid residues they contain or according to their function:

• Monopeptide: consists of one amino acid
• Dipeptide: consists of two amino acids
• Tripeptide: has three amino acids
• Tetrapeptide: has four amino acids
• Pentapeptide: has five amino acids
• Hexapeptide: has six amino acids
• Heptapeptide: has seven amino acids
• Octapeptide: has eight amino acids
• Nonapeptide: has nine amino acids
• Decapeptide: has ten amino acids
• Oligopeptide: consists of between two and twenty amino acids
• Polypeptide: linear chain of many amino acids linked by amide or peptide bonds
• Protein: either consists of more than 50 amino acids or multiple polypeptides
• Lipopeptide: consists of a peptide bonded to a lipid
• Neuropeptide: any peptide active in neural tissue
• Peptidergic agent: chemical that modulates the functioning of peptides
• Proteose: peptides produced by the hydrolysis of proteins

Sources

• Abba J. Kastin, ed. (2013). Handbook of Biologically Active Peptides (2nd ed.). ISBN 978-0-12-385095-9.
• Ardejani, Maziar S.; Orner, Brendan P. (2013-05-03). "Obey the Peptide Assembly Rules". Science. 340 (6132): 561–562. doi:10.1126/science.1237708
• Finking R, Marahiel MA; Marahiel (2004). "Biosynthesis of Nonribosomal Peptides". Annual Review of Microbiology. 58 (1): 453–88. doi:10.1146/annurev.micro.58.030603.123615
• IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). ISBN 0-9678550-9-8.