Activité Biologique des Oligopeptides
An oligopeptide, already defined is simply a short polypeptide. It is composed of a small number of amino acids, sometimes arbitrarily stated as ten or fewer residues, joined by peptide bonds. Two amino acids joined by a peptide bond form a dipeptide; a peptide containing three amino acids is a tripeptide, and so on. Sometimes the term peptide is substituted for
"oligopeptide." We discuss a chemical property of peptides and then the structures of some biologically active peptides.
Peptides have roles as hormones, antibiotics, toxins, and metabolic intermediates. An example of a metabolically important peptide is glutathione. Glutathione is a tripeptide that is ubiquitious in nature and in mammals is required to prevent oxidative damage to red blood cells. It has a structure that deviates from that of a standard peptide and that we use to illustrate the procedure for naming a simple peptide (Structure 3.4). The chemical name for glutathione is gama- gluta-mylcysteinyl glycine. The suflix -yl signifies the amino acid residue whose carboxyl group is linked in peptide linkage to the amino group of the next amino acid in the peptide. In the case of peptides containing glutamic (or aspartic) acid, the carboxyl group involved in the peptide linkage must be identified. In glutathione, it is the gama-carboxyl that is bound in the peptide linkage. This obviously is an unusual situation, for when glutamic acid (or aspanic acid) occurs in proteins, it is the alpha-carboxyl that is bound in peptide linkage. When no prefix is given, the alpha linkage is understood.
A very important group of naturally occurring peptides is that of the peptide hormones, which are responsible for intracellular and interorgan communication. Tens of groups of peptide hormones are recognized. Vasopressin and oxylocin are nonapeptides that are formed in the pituitary gland and can assume a cyclical structure by forming disulfide linkages between the amino-terminal cysteine and a cysteine in the interior of the peptide. Seven of the nine amino acids in the two peptides are identical. Nevertheless, the physiological effects are quite different.
Oxylocin causes the contraction of smooth muscle; vasopressin causes a rise in blood pressure by constricting the peripheral blood vessels. The structures of these peptide hormones follow. The studient may find it instructive to identify the amino terminus and trace the course of the peptide chains of these hormones. Note that the carboxyl terminal residue of these peptides is the amide of glycine. Carboxyl terminal amidation is almost unknown among proteins, but occurs frequently in naturally occurring peptides. Protection against proteinases of the carboxypeptidase type is a possible function, insulin, produced by the pancreas, is a hormone consisting of two polypeptide chains containing a total of 51 amino acid residues. The synthesis of the peptide hormones discussed in this paragraph proceeds by proteolysis of polypeplide precursors.
Several antibiotics are peptides of comparatively simple structure and are synthesized by mechanisms that are distinct from those of protein synthesis. Gramidicin and tyrocidin are examples of such compounds. Penicillin, another antibiotic, contains the valine and cysteine residues, but these are not linked by peptide bonds. Rather, a strained four-membered ring and a sulfure-containing ring are found.
Synthetic peptides also may have biological activtiy. An especialiy simple example is the artificial sweetener Aspartame. Aspartame is L-aspartyl-L-phen-ylalanyl-methyl ester. It is approximately 200 times as sweet as sucrose and is considered to Iack some of the unpleasant aftertaste associated with other synthetic sweeteners.
