Biologically Active Peptides:
Peptides happen naturally in some biological systems although synthetic peptides can be made. Biologically active peptides are those with particular biological roles.
Properties of Peptides:
As peptides are polymers of amino acids, one must expect them to show ionic and acid base properties.
i) Ionic Property:
Ionization property of peptides is consequently of contribution from N-terminal free α-amino group, C-terminal α-carboxyl group and ionisable R groups of some residues. As α-amino groups and α-carboxyl groups of all non-terminal amino acids participate in peptide bonds formation, they don't contribute to overall ionization property.
ii) Titration Curves:
Acid-base property of peptides results from their ionization property. These positive and negative groups could take part in acid-base titration. Peptides, therefore, like free amino acids, have feature titration curves and characteristic isoelectric pH (pI) at which they suppose electrical neutrality and so remain static in the electric field. When the amino acid having ionisable R group becomes the residue in a peptide, pKa value of R group can change because of a number of factors. First, is due to loss of charge in its α-amino and α-carboxyl groups. Second, is because of interaction with other peptide R groups, and lastly, other environmental factors such as salt concentration. Because of these aforementioned reasons, it is hard to pinpoint pKa values from titration curves of both peptide and proteins as is the case for free amino acids.
Examples and Functions of Biologically Active Peptides:
Peptides occur in abundance in nature. Glutathione, a tripeptide is the most abundant peptide in mammalian tissue. A number of hormones are peptides molecules.
Some of the hormone regulatory factors that are secreted by hypothalamus are peptides which administer release of hormones by other endocrine glands.
Several antimicrobial agents prepared by microorganisms are peptides which can have both D- and L-amino acid residues. Penicillin has a cyclic peptide as part of its structure. Gramicidins, the tyrocidins, the polymyxins, the subtilisins and bacitracins are other peptide antibiotics.
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