General Index
Oligopeptides
The Peptide Bond
Condensation of the a-carboxyl group of one aminoacid with the a-amino of another,
with loss of one molecule of water, makes both aminoacids to be united by a Peptide bond or Amide Bond. The resulting compounds
are called Peptides, with a preffix indicating the number of aminoacids entering in the molecule. Thus, we say dipeptides
(2 aminoacids), tripeptides (3 aminoacids), tetrapeptides (4 aminoacids), and so on. In general, we define as Oligopeptides
those having a relatively small number of aminoacids (less than 50), and Polypeptides when the degree of polymerization is much higher (even
thousands of aminoacids)
All proteins have polypeptide structure, but these terms are not to be confused. A protein may be formed by one or more
polypeptides.
The peptide bond has interesting properties. As an example, we'll see the structure of the tripeptide Alanyl-alanyl-alanine:
Peptide bond has polarity, because it is formed from two different groups (amino and carboxyl). Thus, in one end of the peptide
remains a free amino (-NH2) group: it is the N-terminus or Amino-terminus:
is the C-terminus o Carboxyl terminus
By convention, the polypeptide chains are numbered from the N-terminus. In the case of this tripeptide, we have:
Residue 1:
(When forming a peptide, aminoacids are called residues).
Let's see the structure of the peptide bond:
The side chain determines largely the general configuration of the peptide.
Index
Some oligopeptides of interest
Apart from proteins, that will be studied in the following modules, oligopeptides have very often roles of chemical signals.
Let's see some examples:
The Thyrotropin releasing hormone (TRH), a hypothalamic peptide that promotes liberation of Thyrotropin (TSH)
from the anterior hypophysis:
Is a hypothalamic factor of sequence (Pyroglutamyl-histidyl-prolineamide). Note that the -NH2 terminus appears
blocked by the side chain of glutamic acid and the C-terminus is also blocked, appearing as an amide -CONH2.
Many other hormones have a peptide structure. Thus, Oxytocin, a neurohypophysis nonapeptide that causes the
contraction of uterine muscle:
Note that hydrogen atoms are not represented.
Insulin has also a peptide nature:
Is a pancreatic hormone formed by two peptide chains:
linked by two disulfide bonds:
Insulin appears involved in glucose homeostasis, like other pancreatic hormone, Glucagon:
Many Neurotransmitters are also oligopeptides, like the Enkephalins:
Whose respective sequences are YGGFL and YGGFM. they are the physiological agonists of the opioid receptors. Opioid drugs
of abuse (morphine, heroin, etc.) owe their mode of action to being structural analogs on enkephalins.
Other important oligopeptide is Glutathion:
Whose reduced form (GSH) is the tripeptide g-glutamyl-cysteinyl-glycine. The reducing power
of GSH is due to the free -SH group of the central cysteine. When oxidized, two molecules of GSH are linked to form Oxidized Glutathion (GSSG):
Index