Peptides

Peptides (from the Greek πεπτος, "digestible"), are the family of short molecules formed from the linking, in a defined order, of various α-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.

Like proteins, peptides are polypeptide molecules. The distinction is that peptides are short and proteins are long. There are several different conventions to determine these, all of which have flaws.

One convention is that those peptide 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 Native chemical ligation has given access to even longer proteins, and so this convention seems to be outdated.

Another convention places an informal dividing line is at approximately 50 amino acids in length (some people claim shorter lengths). However, this definition is somewhat arbitrary — some peptides such as alzheimer's beta peptide come close and some proteins (such as insulin) are close to the lower limit for proteins. Because of the arbitrary nature of this definition, there is considerable movement within the scientific community to ascribe the more-specific definition that "a peptide is an amino acid molecule without secondary 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 peptides that cannot be proteins.

There are three large classes of peptides, according to how they are produced — ribosomal peptides, nonribosomal peptides, and digested peptides.


 * Ribosomal peptides are synthesized by translation of mRNA. They are often subjected to proteolysis to generate the mature form. These function, typically in higher organisms, as hormones and signaling 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, etc.  In general, they are linear, although lariat structures are not uncommon.


 * Nonribosomal peptides are synthesized using a modular enzyme complex (which functions much like a conveyor belt on a factory). Nonribosomal peptides 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.  In general, these peptides are 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 individuals that 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.  Ultimately digested peptides are ribosomal peptides, although they aren't made on the ribosome of the organism that contains them.

The Grand Peptide Families
These peptides are ribosomal peptides, usually with hormonal activity. All of these peptides are synthesized by cells as longer "propeptides" or "proproteins" and truncated prior to exiting the cell. They are released into the bloodstream where they perform their signalling functions.

Vasopressin and oxytocin

 * Vasopressin
 * Oxytocin

The Tachykinin peptides

 * Substance P
 * Kassinin
 * Neurokinin A
 * Eledoisin
 * Neurokinin B

Vasoactive intestinal peptides

 * VIP Vasoactive intestinal peptide
 * PACAP Pituitary adenylate cyclase activating peptide
 * PHI 27
 * PHM 27
 * GHRH 1-24 Growth hormone releasing hormone 1-24
 * Glucagon
 * Secretin

Pancreatic polypeptide-related peptides

 * NPY
 * PYY Peptide YY
 * APP Avian pancreatic polypeptide
 * HPP Human pancreatic polypeptide

Opioid peptides

 * Proopiomelanocortin (POMC) Peptides
 * The Enkephalin pentapeptides
 * The Prodynorphin peptides

Calcitonin peptides

 * Calcitonin
 * Amylin

Notes on terminology

 * A polypeptide is a single linear chain of amino acids.
 * A protein is one or more polypeptides more than about 50 amino acids long.
 * An oligopeptide or (simply) a peptide is a polypeptide less than 30-50 amino acids long.
 * A tripeptide has three amino acids.
 * A dipeptide has two amino acids.
 * A neuropeptide is a peptide that is active in association with neural tissue
 * A peptide hormone is a peptide that acts as a hormone.