Biologically important peptides

• peptide bond (a type of covalent bond), i.e., short sequences of amino acid, Iinked covalently, are known as peptiiles.
• the molecules made up of 100 amino acids or less are known as peptides or polypeptides, and molecules made up of more than 100 amino acids are known as proteins. But this distinction is not hardand fast.

Important  ones  are given  below:-

A- Glutathione:

1. Glutathione  is  a  tripeptide  made  up  of glutamate,  cysteine  and glycine  (y-glutamyl-cysteinyl-glycine).
2. Glutathione  is  present  in  all mammalian  cells  except  neurons.
3. The sulphydryl (-SH) group of cysteineresidue  is the reactive portion of glutathione which can undergo oxidation and reduction.
4. glutathione may exist as the reduced (G-SH) or oxidized form (G-S-S-G) and can play a role in some oxidation-reduction reactions.
5. It helps  in  keeping  some enzyme in  active  state by preventing the oxidation of sulphydryl (-SH) group of enzyme.
6. detoxification of H₂O₂, by reducing it. Superoxide anion (O₂-) first converted to H₂O₂, by superoxide dismutase.
7. glutathione  scavenges  free  radicals  and superoxide  anion.
8.  It helps  in  detoxification  of xenobiotics  by  their  conjugation  (conjugation  reaction).
9. Reduced form of glutathione (GSH) with a free sulphydryl (-SH) group serves as a redox buffer, regulating redox state ofthe cell.
10. prevents  formation  of methemoglobina.
11.  It  is  involved  in  the  transport  of amino acids  across  the  cell membane of kidney  and  intestine.

Various hormones  are peptides,  e.g

• thyrotropin  releasing  hormones  (3  amino acids).
• oxytocin  (9  amino acids),
• bradykinin  (9  amino  acids).
• vasopressin  (9 amino acids)
• angiotensin  II (8  amino acids)
• insulin  (51  amino  acids :  30  in  A chain  and  21  in  B chain)

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