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. Key Takeaways: Peptides A peptide is a polymer formed by linking amino acid subunits.A peptide molecule may be biologically active on its own or it may act as a subunit for a larger molecule.Proteins are essentially very large peptides, often consisting of multiple peptide subunits.Peptides are important in biology, chemistry, and medicine because they are building blocks of hormones, toxins, proteins, enzymes, cells, and body tissues. 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 peptidesBacterial peptidesBrain peptidesCancer and anticancer peptidesCardiovascular peptidesEndocrine peptidesFungal peptidesGastrointestinal peptidesInvertebrate peptidesOpiate peptidesPlant peptidesRenal peptidesRespiratory peptidesVaccine peptidesVenom peptides Naming Peptides This is an example of a tetrapeptide, with the N-terminus in green and the C-terminus in blue. JÃ ¼ Peptides are named according to how many amino acid residues they contain or according to their function: Monopeptide: consists of one amino acidDipeptide: consists of two amino acidsTripeptide: has three amino acidsTetrapeptide: has four amino acidsPentapeptide: has five amino acidsHexapeptide: has six amino acidsHeptapeptide: has seven amino acidsOctapeptide: has eight amino acidsNonapeptide: has nine amino acidsDecapeptide: has ten amino acidsOligopeptide: consists of between two and twenty amino acidsPolypeptide: linear chain of many amino acids linked by amide or peptide bondsProtein: either consists of more than 50 amino acids or multiple polypeptidesLipopeptide: consists of a peptide bonded to a lipidNeuropeptide: any peptide active in neural tissuePeptidergic agent: chemical that modulates the functioning of peptidesProteose: peptides produced by the hydrolysis of proteins Peptides in Sports Two types of peptides are classified as Schedule 2 (S2) prohibited substances on the World Anti-Doping Agency (WADA) Prohibited List, United States Anti-Doping Agency (USADA) Prohibited List, and by the Australian Sports Anti-Doping Authority. Peptide hormones and secretagogue peptides are banned for use by professional athletes, whether or not they are in competition, because the chemicals act as performance enhancers. The banned peptides are growth hormones, those that increase blood oxygenation, ones that affect muscle growth and repair, and those that cause endocrine system organs (e.g., ovaries, testes, thyroid) to secrete hormones. The substances are banned not only because they can give athletes an unfair advantage over peers, but because their use can increase the risk of hypertension, water intoxication, heart and liver damage, and cancer. 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.1237708Finking R, Marahiel MA; Marahiel (2004). Biosynthesis of Nonribosomal Peptides. Annual Review of Microbiology. 58 (1): 453–88. doi:10.1146/annurev.micro.58.030603.123615IUPAC. 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.