Peptide: Types and functions
Peptide (peptide bond) is amide linkage formed by the reaction between α-carboxyl group of one amino acid
and α-amino group of another amino acid with the elimination of water molecule.
Peptide bond has partial double bond character so it is shorter than single bond and longer than double bond.
Peptide bond is rigid and planner.
The partial double bond character of peptide bond, prevent free rotation of polypeptide chain.
The peptide bond is ‘trans’ It never occurs in ‘cis’ configuration due to steric hindrance.
-COO and -NH group of peptide bond does not ionize but is polar, so it can form hydrogen bond during
formation of secondary structure of proteins.
Types of peptides
1. Dipeptides:
Compound formed when two amino acids linked by 1 peptide bond.
Examples:
Carnosine ( β-alanyl-L-histidine)
Anserine (β-alanyl-N-methylhistidine)
Aspartame (Asparagine-phenylalanine)
2. Tripeptides
Compound formed when three amino acids linked by 2 peptide bond.
Examples;
Glutathione ( Glutamyl-cystinyl-glycine)
Opthalmic acid (L-γ-Glutamyl-α-L-amino butyrl-glycine)
3. Oligopeptides
Compound formed when more than 2 and less than 20 amino acids are linked by peptide bonds.
Examples;
Tetrapeptide; Tulfsin ( thrionine-lysine-proline-Arginine)
Endomorphin-1 ( Tyrosine-proline-tryptophan-phenylalanine)
Amanitin ( Decapeptide)
Netropsin
4. Polypeptides
Compound formed when more than 20 amino acids are linked by peptide bond.
Examples:
Insulin
Growth hormone
Functions of peptides:
i. Precursor of protein: Peptides are precursor of protein.
ii. As alkaloids: Peptides are also the constituents of alkaloids (Alkaloids are group of secondary metabolites
such as Nicotin, Caffeine, Terpentine, Ergotamine etc).
iii. As Antimicrobial agent: Peptides possess antibacterial properties. Secondary metabolites of bacteria and
fungi have antimicrobial activity. Eg. Penicillin G ( valine-cystein-phenylacetic acid)
iv. As Hormones: Peptides acts as hormones eg. Insulin, Somatostatin, vasopressin etc
v. Peptides also acts as growth factors. Eg. Ascorbic acid (vit. C)
vi. As anti-oxidant: Peptide functions as anti-oxidant. They scavenge free radicals. Eg. Carnosine
vii. Clinical diagnosis: hyper secretion of peptide in urine is indicator for mental state of disturbance like
depression, schizophrenia etc.
viii. As structural component: peptides form long chains creating structural protein which provides support
to body. Eg. Keratin, collagen