Class 4 reactions of amino acid metabolism

1. General reactions involved in Amino acid Metabolism Dr. Dhiraj J Trivedi

2. Reactions of Amino acid metabolism 1. De amination 2. Desulphuration 3. Trans amination 4. Trans methylation

3. Deamination Short note on 5marks

4. Deamination DEFINITION: Deamination is the process by which N- atom of amino acid is removed as free NH 3 . Types : It can be of two types 1] Oxidative deamination. 2] Non-oxidative deamination.

5. 1] OXIDATIVE DEAMINATION: Site : Liver and Kidney Coenzymes : Pyridoxal phosphate Enzymes : D- amino acid oxidases and L - amino acid oxidases . Even though D- amino acids are normally not present in human body their concentration is higher than L- amino acid oxidases in the body. L- amino acid oxidases are Flavoprotein. They are reoxidized at substrate level, directly by molecular oxygen forming H 2 O 2 .

6. Step-I : The amino acid is first dehydrogenated by Flavoprotein of L- amino acid oxidase forming α Imino acid. Step-II Water molecule is added which, decomposes to form Ketoacid and N is lost as NH 3 . 1] OXIDATIVE DEAMINATION:

7. 2] Non oxidative deamination: There are certain amino acids, which can be non- oxidatively deaminated by specific enzymes . a] Amino acid dehydrase :

8. b] Amino acid desulphurase: Sulphur containing amino acid like Cysteine and homo-Cysteine are deaminated by primary desulphuration forming imino acid . It is then spontaneously hydrolyzed to α keto acid and NH 3 is made free.

9. 3] Deamination of L- Glutamic acid: L- Glutamic acid is not deaminated by L- amino acid oxidase but it require L- glutamate dehydrogenase . Reaction : L- glutamate dehydrogenase Act on L- Glutamic acid to form α imino glutaric acid . Which on addition of molecule of H 2 O forms NH 3 and α keto glutarate . Also called as Transdeamination , Reaction is reversible.

10. Trans amination Short note on 5marks

11. Transamination: Definition : Transamination is a reversible reaction α -NH 2 group of amino acid is transferred to α- keto acid, resulting in the formation of new amino acid and a new keto acid. Thus at the end of reaction donor amino acid becomes new keto acid & The recipient keto acid becomes new amino acid .

12. Transamination: Transamination is a reversible reaction . It is only for intermolecular transfer of NH 2 group. In this reaction free ammonia is not released during reaction . It requires Pyridoxal phosphate , as coenzyme.

13. Salient feature of Transamination : Site of transamination: Principally in all cells major tissues Liver , Heart, Kidney and Brain. Enzymes : called transaminases or aminotransferases. Co-enzymes: Pyridoxal Phosphate (B6-P) Substrates: Amino acids & Keto acids End product : Keto acid & non essential amino acids

14. Salient feature of Transamination : Substrates : Amino acids : Almost all amino acids undergo transfer of amino group to small or large extent. Exceptions -- Lysine , Threonine , Proline and hydroxyrpoline do not take part in transamination reaction.

15. Salient feature of Transamination : Keto acids : There are only three keto acids, Ketoglutarate , Pyruvate and Oxaloacetate which acts as recipient molecules in transamination reactions . All three are component of TCA cycle. Reversible reaction : Hence keto acid can be derived from amino acid, and amino acids can be obtained from Ketoacid.

16. Mechanism of enzyme action : Requires Pyridoxal phosphate as co-enzyme. Reaction occur in two stages 1] Amino group from amino acid is accepted by Pyridoxal phosphate to form pyridoxamine and amino acid is converted to keto acid .   2 ] In second stage, Amino group is accepted by Keto acid from pyridoxamine and gets converted to amino acid. The Pyridoxal phosphate is regenerated.

17. Mechanism of enzyme action :

18. Alanine Pyruvic acid α -Keto Glutarate Glutamate Serum Glutamate Pyruvate Transaminase Pyridoxal phosphate Examples of Transamination : Aspartate Oxalo acetate α -Keto Glutarate Glutamate Serum Glutamate Oxaloacetate Transaminase Pyridoxal phosphate 1 2

19. α -Keto Glutarate Glutamate Pyridoxal phosphate α -Amino group of Various amino acids α -Keto Glutarate acts as common acceptor of α -Amino group In transaminase reactions

20. Clinical Significance: Two transaminases having clinical significance 1. Serum glutamate Pyruvate transaminase is also named as alenine transaminase (ALT). Normal level is 5 to 35 IU/L. Its activity increases in liver diseases. 2 . Serum glutamate oxaloacetate transaminase is also named as aspartate transaminase (AST). Its normal value is 5 to 40 IU/L. The increased activity is seen in heart diseases .  

21. Functions of transamination: 1 ] Inter-conversion of amino acid to keto acid and keto acid to amino acid. This provides non-essential amino acids in the body. 2 ] Utilization of alpha keto acids when in excess. 3 ] Prevents toxicity of ammonia as it is not at all released free during reaction.

22. Trans methylation Explain the reaction of transmethylation 5 marks

23. Trans methylation An important biological transfer of methyl group with in amino acids. Process similar to transamination . The difference is transfer of methyl group instead amino group.

24. Trans methylation DEFINITION: The transfer of methyl group (-CH 3 ) from active Methionine to an acceptor molecule is known as Transmethylation. Methionine has to be activated to S- adenosyl Methionine ( SAM) to donate the methyl group.

25. Trans methylation All compounds having –CH 3 group cannot act as methyl group donor. Methyl group attached to N atom or S atom becomes active and can become methyl group donor .

26. Enzymes & Coenzymes of trans methylation Enzymes: Enzymes involved are collectively known as methyltransferases or transmethylases . Coenzymes: Tetra hydro folate (THF) is a coenzyme that actively participates in methyl group transfer . Vit B 12 is also involved in this reaction (makes folate free).

27. Significance of Trans methylation: 1 ] Transmethylation is an essential process since many biological compounds become functionally active only after methylation. 2 ] Transmethylation synthesizes Biologically important compounds like Choline, Creatine, Epinephrine, Nor-epinephrine .

28. Significance of Trans methylation: 3 ] Amino acid residues of protein on methylation control protein turn over. In fact methylation protect the proteins from immediate degradation . 4] In plants and animals Transmethylation help to synthesis certain hormone. ( Epinephrine, Nor epinephrine)

29. Significance of Trans methylation: 5 ] Decrease of Transmethylation will affect synthesis of choline and in turn phospholipid . This leads to decreased formation of lipoproteins and accumulation of lipid in liver causing fatty liver. 6] Choline synthesis is also required for the synthesis of Acetyl- choline, a neurotransmitter.

30. IMPORTANT TRANSMETHYLATION REACTIONS: S- adenosyl Methionine acts as donor of methyl group . Transmethylation is a type of one carbon metabolic reaction. Methyl group acceptors Methylated product Guanido acetate Creatine Nor-epinephrine Epinephrine Epinephrine Metanephrine Ethamolamine Choline Phosphatidyl Ethamolamine Lecithin Or Phosphatidyl choline Serine Choline Acetyl serotonin Melatonin Homo Cysteine Methionine

31. Q: What is transmethylation reaction? Give two examples. A: Transmethylation: Transfer of methyl group (-CH3) from active Methionine to an acceptor is known as Transmethylation reaction. S- Adenosyl methionine (SAM) is active methyl group donor in many transmethylation reactions. Examples are , SAM Guanidinoacetate Creatine Serine Choline Nor epinephrine Epinephrine S- adenosyl homocysteine SAM S- adenosyl homocysteine 1 2 3