Peptides (from the Greek πεπτός, "digested" from πέσσειν "to digest") are short polymers of amino acids linked by peptide bonds. They have the same chemical structure as proteins, but are shorter in length. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond. Peptides have an amino end and a carboxyl end.
One definition is that those chains that are short enough to be made synthetically from the constituent amino acids are called peptides rather than proteins. However, with the advent of better synthetic techniques, peptides as long as hundreds of amino acids can be made, including full proteins like ubiquitin. Native chemical ligation has given access to even longer proteins, so this convention seems to be outdated.
Another definition places an informal dividing line at approximately 50 amino acids in length (some people claim shorter lengths). This definition is somewhat arbitrary. Long peptides, such as the amyloid beta peptide linked to Alzheimer's disease, can be considered proteins; and small proteins, such as insulin, can be considered peptides.
Here are the major classes of peptides, according to how they are produced:
Peptides in molecular biology
Peptides have recently received prominence in molecular biology for several reasons. The first and most important is that peptides allow the creation of peptide antibodies in animals without the need to purify the protein of interest. This involves synthesizing antigenic peptides of sections of the protein of interest. These will then be used to make antibodies in a rabbit or mouse against the protein.
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