Polypeptide

Peptide refers to a compound made from the dehydration and further condensation of two or more amino acids. The simplest peptide is composed of two amino acids, called di-peptide. Peptide consisting of multiple amino acids is called polypeptide. Polypeptide consisting of more than 50 amino acids is usually referred to as protein. Peptide is widely distributed in animals, plants and micro-organisms and has important physiological roles. It have various specific effects on growth and development, cell differentiation, brain activity, tumor diseases, immune defense, reproduction control, and anti-aging and other aspects of biological activity. For example, the glutathione is the coenzyme of certain enzymes, playing an important role in the redox processes in vivo. Many kinds of hormones belong to the polypeptide such as oxytocin, vasopressin, glucagon, and adrenocorticotropic hormone. They can regulate the body's metabolism. The hormone regulatory factors secreted by the hypothalamus adjustment factor are also polypeptides. They control the secretion of hormone by other kinds of endocrine glands. In recent years, it has found that the enkephalin (a pentapeptide) in the brain of higher animals has analgesic effect. Many antibiotics produced by microorganisms such as polymyxin, gramicidin, etc. are all peptides with a strong antibacterial effect.

Polypeptide refers to a long chain consisting of a number of amino acid units connected with each other by peptide bonds. There is no strict boundary between the definition of peptides and proteins. We generally refer to a polypeptide comprising only dozens or fewer amino acid units as peptides while referring to the polypeptide containing lot of amino acid units as protein. We often call the amino acid unit contained in polypeptide amino acid residue because these units connected to each other have lost part of the molecule, rather than being as the intact amino acid. Only amino acid residues at both ends of the peptide chain contain free α- amino group or a free α- carboxyl group. The terminal amino acid residue containing free α- amino-terminal residues is called amino group terminal (or N-terminal) residue while the terminal residue containing freedom α- carboxyl- is called the carboxy terminus (or C-terminal) residue. The naming of the polypeptide is according to the order form N terminal residues to C-terminal residues. We also use this order for writing.
For example, the pentapeptide Ser • Gly • Try • Ala • Leu consists of five amino acid residues and four peptide linkages, called glycyl-seryl-tyrosyl-alanyl-leucine. Peptide is widely found in plant and animal tissues with a number of peptides in vivo having special features, collectively known bioactive peptides.

In recent years, it has been found that: almost all the major theories of life sciences, such as immune defense, reproduction control, tumor lesions, and anti-aging all have related bioactive peptides. These theoretical issues are all closely related to the clinical practice, thus the biologically active peptide is also important in practical applications. There are many kinds of bioactive peptides. For example, the adrenocorticotropic hormone that can stimulate the development of adrenal cortex is a kind of 39 peptides; the oxytocin that can cause the contraction of the uterus belongs to nonapeptide; the two kinds of enkephalins having morphine activity are both pentapeptides. Many kinds of antibiotics are also peptides. Another example is the growth factor that is controlled by genes, playing regulatory and controlling effect in cell development. This kind of important material also belongs to polypeptide. Epidermal growth factor contains 50 amino acid residues; the nerve growth factor contains 118 amino acid residues.

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