peptides

peptides

1. peptides are intermediate in structural complexity between amino acids and proteins

2. they are compounds formed by linking amino acids through peptide bonds, and have unbranched chains

3. chemical hydrolysis of proteins yields peptides

4. peptides are also generated by enzymatic hydrolysis

5. amino acids are linked by peptide bonds which is a covalent linkage between the a-carboxyl group of 1 amino acid and the a-amino goup of the adjacent amino acid

6. peptide bonds may be regarded as a substituted amide linkage and may be written a -CONH-

7. nomenclature

   • number of amino acids
     • 2 - dipeptide
     • 10 - oligopeptide
     • more than 10 - polypeptide
   • an amino acid unit is called a residue
   • naming is always from the amino-terminus to the carboxyl terminus
   • with the exception of the last amino acid residue (carboxyl terminal) all the amino acid residues carry the suffix -yl
   • if a different COOH is used, its greek letter must be stated

8. the bonds

   • the four atoms of the peptide bond and the two attached a carbon atoms are all in the same spatial plane
   • the carbonyl (C=O) and imino (N-H) bond dipoles are in the opposite direction relative to the C-N bond axis

9. naturally occurring peptides

   • carnosine: (B-alanylhistidine)
     • dipeptide, found in the muscle tissue of vertebrates including human muscle
     • it is suggested, that it may function as a pH buffering agent in muscle cells
   • glutathione: (Y-glutamylcysteinylglycine)
     • responsible for transporting amino acids across cell membranes
     • acts as a scavenger of toxic oxidizing agents in the cell such as H2O2, OH free radicals etc. rendering them harmless
   • oxytocin and vasopressin
     • semi cyclic nonapeptide hormones, released by pituitary gland
     • oxytocin: stimulates the contraction of uterine muscles in pregnant females and ejection of milk from mammary glands in lactating females
     • vasopressin: produces potent antidiuretic effects by stimulating the readsorption of H2O by the kidney. also responsible for contraction of smooth muscles in blood vessels thus regulating blood pressure
     • the residues at 3 and 8 are especially important, confers different properties to peptides.
     • the amide modification of the C-terminus ie. glycinamide is necessary for the full biological activity of these peptides
   • gramicidin-S
     • cyclic decapeptide antibiotic, produced by bacteria
     • contains 2 nonprotein amino acids ie D-phe and L-orn (ornithine)
     • used as a chemotherapeutic agent against disease

10. biuret reaction

    • is the only reaction that is specific for peptides and proteins and not for amino acids
    • alkaline CuSO4 reacts with compounds containing two or more peptide bonds to give a purple/violet coloured complex that absorbs maximally at 570nm.
    • the intensity of colour obtained is a measure of the number of peptide bonds present

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