A peptide is an organic molecule consisting of two or more amino acids linked through an amide linkage called a peptide bond. Peptides are formed when the amino group (-NH2) of one amino acid, links with the carboxyl group (-COOH) of an adjacent amino acid. An amide linkage (-CO-NH-) results. If two amino acids are linked, the compound is called a dipeptide, if three, a tripeptide, and so on. Longer chains containing a few amino acids are often called oligopeptides or polypeptides if they contain as many as 50. Still longer chains are called proteins. Peptides may have many different properties depending on the nature of the amino acids they contain, and the order in which they are linked. Some peptides function as hormones--a few as antibiotics and others as important participants in metabolism.
For example, an octapeptide with a hormone-like function, called angiotensin, is formed in the blood and targets the circulatory system, causing an increase blood pressure. Oxytocin, useful in obstetrics because it promotes uterine contraction, is an unusual form because it contains nine amino acids arranged in a ring structure. Two peptides, made by bacteria, with antibiotic activity, include gramicidin, which inhibits oxidative phosphorylation (the synthesis of ATP that obtains energy from electron transport occurring as a result of aerobic respiration), and bacitracin, used for treatment of bacterial infections of the eye.
Several peptides have neuroactive (stimulating neural tissues) properties and function similarly to other neurotransmitters but with some important differences. Classic transmitters like acetylcholine act on synaptic receptors (a cellular entity that is the mechanism for genetic cross over) for only milliseconds, while neuropeptides may act for seconds, hours, or even days. Neuropeptides are released in lower concentrations than other transmitters, and they have higher potency. Examples of neuropeptides include neurotensin, somatostatin, cholecystokinin, and the opioid peptides. Opioid peptides, including endorphins, enkephalins and dynorphins, have been widely studied because opiate drugs like morphine bind (latch on) to the same receptor sites and mimic their pain-killing action.
Peptides are formed routinely in the digestive tract of animals when protein is broken down. As peptide bonds in the protein are hydrolyzed, the complex molecule is first converted to a number of smaller peptides. The peptides are subsequently converted to individual amino acids. Two enzymes involved in this digestive process are pepsin, released in the acid environment of the stomach, and trypsin, secreted by the pancreas and active in the duodenum (part of the small intestine) and upper jejunum (part of the small intestine just beyond the duodenum).
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