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Peptides (from the Greek ЦŦЦӦ?, "digestible"), are the family of molecules formed from the linking, in a defined order, of various amino acids. The link between one amino acid residue and the next is an amide bond, and is sometimes referred to as a peptide bond. An amide bond is somewhat shorter than a typical carbon-nitrogen single bond, and has a partial double-bond character, because the participating carbon atom is doubly bonded to an oxygen atom and the nitrogen has a lone pair of electrons available for bonding.

There are three large classes of peptides -- ribosomal peptides, nonribosomal peptides, and digested peptides.

Ribosomal peptides are synthesized by translation of mRNA and (often) proteolysis to generate the mature form. These function, typically in higher organisms, as hormones and signalling molecules. Some lower organisms produce peptides as antibiotics (such as microcin J25). Since they are translated, the amino acid residues involved are restricted to the 20 amino acids (plus selenomethionine and pyrrolysine), and posttranslational modifications thereof, such as phosphorylation, hydroxylation, sulfonation, disulfide formation, et cetera. They are also generally linear, although lariat structures are also common. Tacrolimus, FK506

Nonribosomal peptides are synthesized using a modular enzyme complex (which functions much like a conveyor belt on a factory), and are confined primarily to unicellular organisms, plants, and fungi. There is a common core structure to all of these complexes, and they can contain many different modules to perform chemical manipulations on the evolving product. These peptides are generally cyclic (often with highly complex cyclic structures), although linear nonribosomal peptides are common. Since the system is modular and closely related to the machinery for building fatty acids and polyketides, hybrid compounds are often found. Oxazoles, thiazoles, and their reduced counterparts often indicate that the compound was synthesized in this fashion.

Digested peptides are the result of nonspecific proteolysis as part of the digestive cycle. It has also been documented that when certain food proteins such as gluten, casein, egg protein and spinach protein are broken down, opioid peptides are formed. These peptides mimic the effects of morphine, and those who are unable to break them down will experience mental illness. These peptides are quite short and are given names such as casomorphine, gluten exorphine and dermorphine.

Peptides differ from proteins, which are also long chains of amino acids, by virtue of their size. Traditionally, those peptide chains that are short enough to make synthetically from the constituent amino acids are called peptides rather than proteins. The informal dividing line is at approximately 50 amino acids in length (some people claim shorter lengths), since naturally-occurring proteins tend, at their smallest, to be hundreds of residues long. So, in essence, a peptide is a small protein. Because of the arbitrary nature of this definition, there is considerable movement to redefine this arbitrary distinction such that a peptide is an amino acid molecule without structure; on gaining defined structure it is a protein. Thus the same molecule can be either a peptide or a protein depending on its environment; though there are often peptides which cannot be proteins. However, this definition breaks down when describing nonribosomal peptides (which are usually structured).

Peptidomimetics (such as peptoids and -peptides) are molecules related to peptides, but with different properties.