Reactions of amino acids

General and color reactions of amino acids

1. Reactions of amino acids Namrata Chhabra M.D., Biochemistry 7/5/2012 Namrata Chhabra M.D., Biochemistry 1

2. Learning objectives At the end of this presentation, the learners will be able to understand: Reactions of amino acids due to: Amino group Carboxyl group Both amino and carboxyl groups, Side chain of amino acids, and The specific color reactions of amino acids 7/5/2012 Namrata Chhabra M.D., Biochemistry 2

3. Reactions of amino acids Reactions due to amino group Reactions due to carboxyl group Reactions due to both amino and carboxyl groups Reaction due to side chain 7/5/2012 Namrata Chhabra M.D., Biochemistry 3

4. Reactions due to amino group i) Oxidative deamination- Α n amino group is removed and corresponding α -keto acid is formed. α -keto acid produced is either converted to glucose or ketone bodies or is completely oxidized. 7/5/2012 Namrata Chhabra M.D., Biochemistry 3

5. Reactions due to amino group (contd.) ii) Transamination- Transfer of an α amino group from an amino acid to an α keto acid to form a new amino acid and a corresponding keto acid. 7/5/2012 Namrata Chhabra M.D., Biochemistry 5

6. Reactions due to amino group (contd.) iii) Formation of carbamino compound CO 2 binds to α amino acid on the globin chain of hemoglobin to form carbamino hemoglobin The reaction takes place at alkaline pH and serves as a mechanism for the transfer of Carbon dioxide from the tissues to the lungs by hemoglobin. 7/5/2012 Namrata Chhabra M.D., Biochemistry 6

7. 2) Reactions due to carboxyl group i) Decarboxylation- Amino acids undergo alpha decarboxylation to form corresponding amines. Examples- Glutamic acid GABA Histidine Histamine Tyrosine Tyramine 7/5/2012 Namrata Chhabra M.D., Biochemistry 7

8. 2) Reactions due to carboxyl group (contd.) ii) Formation of amide linkage Non α carboxyl group of an acidic amino acid reacts with ammonia by condensation reaction to form corresponding amides Aspartic acid Asparagine Glutamic acid Glutamine 7/5/2012 Namrata Chhabra M.D., Biochemistry 8

9. 3) Reactions due to both amino & carboxyl groups Formation of peptide bond- Carboxyl group of an amino acid binds with amino group of another amino acid forming a peptide bond with the loss of one molecule of water. 7/5/2012 Namrata Chhabra M.D., Biochemistry 9

10. Reactions due to side chains 1) Ester formation OH containing amino acids e.g. serine, threonine can form esters with phosphoric acid in the formation of phosphoproteins (figure-1) OH group containing amino acid can also form: Glycosides – by forming O- glycosidic bond with carbohydrate residues (figure-2) 7/5/2012 Namrata Chhabra M.D., Biochemistry 10 Figure-1 Figure-2

11. Reactions due to side chains 2) Reactions due to SH group (Formation of sulphide bonds) Cysteine has a sulfhydryl group( SH) group and can form a disulphide (S-S) bond with another cysteine residue. The dimer is called Cystine 7/5/2012 Namrata Chhabra M.D., Biochemistry 11

12. Formation of disulphide bond 7/5/2012 Namrata Chhabra M.D., Biochemistry 12 Two cysteine residues can connect two polypeptide chains by the formation of interchain disulphide chains.

13. Reactions due to side chains (contd.) 3)Transmethylation The methyl group of Methionine can be transferred after activation to an acceptor for the formation of important biological compounds. For example (figure) conversion of norepinephrine to epinephrine by S Adenosyl methionine (active form of methionine 7/5/2012 Namrata Chhabra M.D., Biochemistry 13

14. Colour reactions of amino acids 7/5/2012 Namrata Chhabra M.D., Biochemistry 14 S.No. Test Significance 1) Ninhydrin reaction All Alpha amino acids 2) Xanthoproteic test Aromatic amino acids 3) Millon’s test Confirmatory test of Tyrosine 4) Biuret test Not given by free amino acids, and dipeptides 5) Sakaguchi test Arginine 6) Hopkins Cole reaction Confirmatory test of Tryptophan 7) Lead acetate test cysteine and cysteine, not by Methionine 8) Nitroprusside reaction SH group containing amino acids

15. Biuret test 7/5/2012 Namrata Chhabra M.D., Biochemistry 15 Positive Control

16. Ninhydrin test- For alpha amino acids 7/5/2012 Namrata Chhabra M.D., Biochemistry 3

17. Xanthoproteic test- For Aromatic amino acids 7/5/2012 Namrata Chhabra M.D., Biochemistry 2

18. Millon’s test- For Tyrosine 7/5/2012 Namrata Chhabra M.D., Biochemistry 2

19. Sakaguchi test for Arginine 7/5/2012 Namrata Chhabra M.D., Biochemistry 2

20. Hopkin Cole reaction for Tryptophan 7/5/2012 Namrata Chhabra M.D., Biochemistry 3

21. Lead acetate test for Cysteine and Cystine 7/5/2012 Namrata Chhabra M.D., Biochemistry 3

22. 7/5/2012 Namrata Chhabra M.D., Biochemistry 22 Thank you