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Peptides are naturally occuring short chains of amino acid monomers connected by amide bonds. Peptides include many antibiotics, hormones and other substances that involve in the biological functions of living beings. There are 9 groups of peptides that have been classified according to their sources and 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).
Ribosomes in cell 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.
