Biochemistry from Dr. Dhiraj J Trivedi SDM College of Medical Sciences and Hospital, Dharwad, India

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 NH3.
• 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 H2O2.

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 NH3.
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
NH3 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 H2O forms NH3 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 -NH2 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
NH2 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 (-CH3) 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 –CH3 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 B12 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