Peptides (from the Greek word πεπτος, meaning "digestible") are polypeptide composed of two or more amino acids chained together by a peptide bond. They are "digestible" because they are commonly isolated from the digestion of proteins by proteases (enzymes that cut proteins into smaller pieces). The link between the amino acid residues are peptide bonds which consists of an amidebond.
Size of Peptides - What is a Peptide and What is a Protein?
Proteins are polypeptide molecules (or consist of multiple polypeptide subunits). The distinction is that peptides are short and polypeptides 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 Native chemical ligation has given access to even longer proteins, so this convention seems to be outdated.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 amyloid beta peptide linked to Alzheimer's disease, can be considered proteins; and small proteins, such as insulin, can be considered 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.
Here are the major classes of peptides, according to how they are produced:
Are synthesized using a modular enzyme complex (which functions much like a conveyor belt on a factory). Nonribosomal peptides are confined primarily to unicellular organisms, plants, and fungi. All of these complexes are laid out in a similar fashion, and they can contain many different modules to perform a diverse set of chemical manipulations on the developing 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.
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.
Refer to fragments of proteins which used to identify or quantify the source protein. 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.
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 antibodies in animals without the need to purify the protein of interest. One can simply make antigenic peptides of sections of the protein of interest. These will suffice in making 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
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.