Amino acids that have been incorporated into peptides are termed residues. A water molecule is released during formation of each amide bond. All peptides except cyclic peptides have an N-terminal (amine group) and C-terminal (carboxyl group) residue at the end of the peptide (as shown for the tetrapeptide in the image).
Many kinds of peptides are known. They have been classified or categorized according to their sources and functions. 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, and blood-brain peptides.
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. 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.
Peptones are derived from animal milk or meat digested by proteolysis. In addition to containing small peptides, the resulting material includes fats, metals, salts, vitamins, and many other biological compounds. Peptones are used in nutrient media for growing bacteria and fungi.
Peptide fragments refer to fragments of proteins that are 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 that have been degraded by natural effects.
The peptide families in this section 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 signaling functions.
^Tao, Kai; Levin, Aviad; Adler-Abramovich, Lihi; Gazit, Ehud (26 Apr 2016). "Fmoc-modified amino acids and short peptides: simple bio-inspired building blocks for the fabrication of functional materials". Chem. Soc. Rev. 45 (14): 3935-3953. doi:10.1039/C5CS00889A. PMID27115033.
^Tao, Kai; Wang, Jiqian; Zhou, Peng; Wang, Chengdong; Xu, Hai; Zhao, Xiubo; Lu, Jian R. (February 10, 2011). "Self-Assembly of Short A?(16-22) Peptides: Effect of Terminal Capping and the Role of Electrostatic Interaction". Langmuir. 27 (6): 2723-2730. doi:10.1021/la1034273. PMID21309606.
^Kai Tao, Guy Jacoby, Luba Burlaka, Roy Beck, Ehud Gazit (July 26, 2016). "Design of Controllable Bio-Inspired Chiroptic Self-Assemblies". Biomacromolecules. 17 (9): 2937-2945. doi:10.1021/acs.biomac.6b00752. PMID27461453.CS1 maint: uses authors parameter (link)
^Kai Tao, Aviad Levin, Guy Jacoby, Roy Beck, Ehud Gazit (23 August 2016). "Entropic Phase Transitions with Stable Twisted Intermediates of Bio-Inspired Self-Assembly". Chem. Eur. J. 22 (43): 15237-15241. doi:10.1002/chem.201603882. PMID27550381.CS1 maint: uses authors parameter (link)
^Donghui Jia, Kai Tao, Jiqian Wang, Chengdong Wang, Xiubo Zhao, Mohammed Yaseen, Hai Xu, Guohe Que, John R. P. Webster, Jian R. Lu (June 16, 2011). "Dynamic Adsorption and Structure of Interfacial Bilayers Adsorbed from Lipopeptide Surfactants at the Hydrophilic Silicon/Water Interface: Effect of the Headgroup Length". Langmuir. 27 (14): 8798-8809. doi:10.1021/la105129m. PMID21675796.CS1 maint: uses authors parameter (link)
^Xu JY, Qin LQ, Wang PY, Li W, Chang C (October 2008). "Effect of milk tripeptides on blood pressure: a meta-analysis of randomized controlled trials". Nutrition. 24 (10): 933-40. doi:10.1016/j.nut.2008.04.004. PMID18562172.