Peptide
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.
Proteins are
polypeptide molecules (or consist of multiple polypeptide subunits). 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Native chemical ligation has given access to even longer proteins, and so this convention seems to be outdated.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 â€" some peptides such as
alzheimer's beta peptide can be considered proteins and some proteins (such as
insulin) are close to the upper limit for peptides. 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 : 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 common.
;
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.
Peptides have received prominence in molecular biology in recent times. There are several reasons for this. The first and most important is that peptides allow the creation of antibodies in animals without the need to purify your
protein of interest. You can simply make antigenic peptides of sections of your protein. These will suffice in making antibodies in a rabbit or mouse against your 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.
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*
Substance P*
Kassinin*
Neurokinin A*
Eledoisin*
Neurokinin B*
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*
NPY*
PYY Peptide YY*
APP Avian pancreatic polypeptide*
HPP Human pancreatic polypeptide*
Proopiomelanocortin (
POMC) Peptides
* The
Enkephalin pentapeptides* The
Prodynorphin peptidesCalcitonin peptides
*
Calcitonin*
Amylin*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.
*
Peptides Peptide protocols and bioinformatics, peptide synthesis services, and information on peptides.
*
The Journal: Peptide *
Peptide Synthesis:Tips & Resources*
Peptidomimetics (such as
peptoids and
β-peptides) are molecules related to peptides, but with different properties.
*
Peptide synthesis*
Translation*
Ribosome
