2. (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.
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be outdated.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). 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.

3. Peptide classes
Here are the major classes of peptides, according to how they are produced:
• Milk peptides - Milk peptides are formed from milk proteins by enzymatic
breakdown by digestive enzymes or by the proteinases formed by lactobacilli
during the fermentation of milk. Several milk peptides have been shown to
have antihypertensive effects in animal and in clinical studies.
• Ribosomal 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 organisms produce peptides as antibiotics, such as microcins.[1]
Since they are translated, the amino acid residues involved are restricted to
those utilized by the ribosome. However, these peptides frequently have
posttranslational modifications, such as phosphorylation, hydroxylation,
sulfonation, palmitylation, glycosylation and disulfide formation. In general, they
are linear, although lariat structures have been observed.[

4. • Nonribosomal peptides - These peptides are assembled by enzymes that are
specific to each peptide, rather than by the ribosome. The most common non-
ribosomal peptide is glutathione, which is a component of the antioxidant defenses
of most aerobic organisms.[4] Other nonribosomal peptides are most common in
unicellular organisms, plants, and fungi and are synthesized by modular enzyme
complexes called nonribosomal peptide synthetases.[5] These complexes are often
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.[6]
These peptides are often cyclic and can have highly-complex cyclic structures,
although linear nonribosomal peptides are also common. Since the system is
closely related to the machinery for building fatty acids and polyketides, hybrid
compounds are often found. The presence of oxazoles or thiazoles often indicates
that the compound was synthesized in this fashion.[7]
• Peptones - Peptones are derived from animal milk or 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]

5. • Peptide fragments - Peptide fragments refer to fragments of proteins that are 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 that have been degraded by natural effects.
• 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.[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. In this case the peptides are most often generated by in-gel digestion after
electrophoretic separation of the proteins.
• 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.