• 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

• what is a polypeptide?
  • 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

Peptide: Types and functions

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