Activité Biologique des Oligopeptides

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.