Peptide

Conventions

One convention is that those peptide chains that are short enough to be made synthetically from the constituent ubiquitin. Native chemical ligation has given access to even longer proteins, so this convention seems to be outdated.

Another convention places an informal dividing line at approximately 50 amino acids in length (some people claim shorter lengths). However, this definition is somewhat arbitrary. Long peptides, such as the insulin, can be considered peptides.

Peptide classes

Here are the major classes of peptides, according to how they are produced:

Ribosomal peptides 

Are synthesized by disulfide formation. In general, they are linear, although lariat structures have been observed.^[2] More exotic manipulations do occur, such as racemization of L-amino acids to D-amino acids in platypus venom.^[3]

thiazoles often indicate that the compound was synthesized in this fashion.^[7]

Peptones 

Are derived from animal meat digested by proteolytic digestion. In addition to containing small peptides, the resulting spray-dried material includes fats, metals, salts, vitamins and many other biological compounds. Peptone is used in nutrient media for growing bacteria and fungi.^[8]

Peptide Fragments 

Refer to fragments of proteins which used to identify or quantify the source protein.^[9] Often these are the products of enzymatic degradation performed in the laboratory on a controlled sample, but can also be forensic or paleontological samples which have been degraded by natural effects.^[10][11]

Peptides in molecular biology

Peptides have received prominence in molecular biology in recent times 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.^[12] 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.

Another reason is that peptides have become instrumental in mass spectrometry, allowing the identification of proteins of interest based on peptide masses and sequence.

Peptides have recently been used in the study of protein structure and function. For example, synthetic peptides can be used as probes to see where protein-peptide interactions occur.

Inhibitory peptides are also used in clinical research to examine the effects of peptides on the inhibition of cancer proteins and other diseases.

Well-known peptide families in humans

The peptide families in this section are all 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.

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

• POMC) Peptides
• The Enkephalin pentapeptides
• The Prodynorphin peptides

Calcitonin peptides

• Calcitonin
• Amylin
• AGG01

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 dipeptide has two amino acids.
• A tripeptide has three amino acids.
• A pentapeptide has five amino acids.
• A nonapeptide has nine amino acids (e.g., oxytocin).
• A neuropeptide is a peptide that is active in association with neural tissue.
• A hormone.