Peptides are short chains of amino acid monomers linked by peptide bonds. The covalent chemical bonds are formed when the carboxyl group of one amino acid reacts with the amino group of another; the shortest peptides are dipeptides, consisting of 2 amino acids joined by a single peptide bond, followed by tripeptides, etc. A polypeptide is a long and unbranched peptide chain. Hence, peptides fall under the broad chemical classes of biological oligomers and polymers, alongside nucleic acids and polysaccharides, etc. Peptides are distinguished from proteins on the basis of size, as an arbitrary benchmark can be understood to contain 50 or fewer amino acids. Proteins consist of one or more polypeptides arranged in a biologically functional way bound to ligands such as coenzymes and cofactors, or to another protein or other macromolecule, or to complex macromolecular assemblies. While aspects of the lab techniques applied to peptides versus polypeptides and proteins differ, the size boundaries that distinguish peptides from polypeptides and proteins are not absolute: long peptides such as amyloid beta have been referred to as proteins, smaller proteins like insulin have been considered peptides.
Amino acids that have been incorporated into peptides are termed "residues" due to the release of either a hydrogen ion from the amine end or a hydroxyl ion from the carboxyl end, or both, as a water molecule is released during formation of each amide bond. All peptides except cyclic peptides have an N-terminal and C-terminal residue at the end of the peptide. Many kinds of peptides are known, they have been categorized according to their sources and function. According to the Handbook of Biologically Active Peptides, some groups of peptides include plant peptides, bacterial/antibiotic peptides, fungal peptides, invertebrate peptides, amphibian/skin peptides, venom peptides, cancer/anticancer peptides, vaccine peptides, immune/inflammatory peptides, brain peptides, endocrine peptides, ingestive peptides, gastrointestinal peptides, cardiovascular peptides, renal peptides, respiratory peptides, opiate peptides, neurotrophic peptides, blood–brain peptides; some ribosomal peptides are subject to proteolysis.
These function in higher organisms, as hormones and signaling molecules. Some organisms produce peptides as antibiotics, such as microcins. Peptides have posttranslational modifications such as phosphorylation, sulfonation, palmitoylation and disulfide formation. In general, peptides are linear. More exotic manipulations do occur, such as racemization of L-amino acids to D-amino acids in platypus venom. Nonribosomal peptides are assembled by enzymes, not the ribosome. A common non-ribosomal peptide is glutathione, a component of the antioxidant defenses of most aerobic organisms. Other nonribosomal peptides are most common in unicellular organisms and fungi and are synthesized by modular enzyme complexes called nonribosomal peptide synthetases; these complexes are laid out in a similar fashion, they can contain many different modules to perform a diverse set of chemical manipulations on the developing product. These peptides are cyclic and can have complex cyclic structures, although linear nonribosomal peptides are common.
Since the system is related to the machinery for building fatty acids and polyketides, hybrid compounds are found. The presence of oxazoles or thiazoles indicates that the compound was synthesized in this fashion. Peptide fragments refer to fragments of proteins that are used to identify or quantify the source protein; these are the products of enzymatic degradation performed in the laboratory on a controlled sample, but can be forensic or paleontological samples that have been degraded by natural effects. Use of peptides received prominence in molecular biology for several reasons; the first is that peptides allow the creation of peptide antibodies in animals without the need of purifying the protein of interest. This involves synthesizing antigenic peptides of sections of the protein of interest; these will be used to make antibodies in a rabbit or mouse against the protein. Another reason is that techniques such as mass spectrometry enable the identification of proteins based on the peptide masses and sequence that result from their fragmentation.
Peptides have 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- see the page on Protein tags. Inhibitory peptides are used in clinical research to examine the effects of peptides on the inhibition of cancer proteins and other diseases. For example, one of the most promising application is through peptides that target LHRH; these particular peptides act as an agonist, meaning that they bind to a cell in a way that regulates LHRH receptors. The process of inhibiting the cell receptors suggests that peptides could be beneficial in treating prostate cancer, but additional investigations and experiments are required before their cancer-fighting attributes can be considered definitive; the peptide families in this section are ribosomal peptides 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. Magainin family Cecropin famil