Reaction of AMINO ACIDS

1. GENERAL REACTIONS OF AMINO ACIDS
Dr.K.Durga sowmithri
Dept of biochemistry

2. Questions -LAQ &SHORT NOTES
• Enumerate general reactions of amino acids giving one example each
• Transamination reactions
• Decarboxylation of amino acids

3. Reactions due to
• 1.Carboxyl group
• Decarboxylation
• Amide formation
• 2.Amino group
• Transamination
• Oxidative deamination
• Formation of carbamino compounds

4. • 3.Side chains
• Transmethylation
• Ester formation by OH group
• Reactions of amide group
• Reactions of sulfhydryl group

5. Due to Carboxyl Group-Decarboxylation
• The amino acids will undergo alpha decarboxylation to form the
corresponding amine . Thus, some important amines are produced
from amino acids. For example,
• Histidine → Histamine + CO2(mediator of allergy and anaphylactic
reaction)
• Tyrosine → Tyramine + CO2
• Tryptophan → Tryptamine + CO2
• Lysine → Cadaverine + CO2
• Glutamic acid → excitatory neurotransmitter Gamma amino
butyric acid (GABA) inhibitory NT+ CO2

7. Amide Formation
• The-COOH group of dicarboxylic amino acids (other than alpha
carboxyl) can combine with ammonia to form the corresponding amide.
For example,
• Aspartic acid + NH3 → Asparagine
• Glutamic acid + NH3 → Glutamine .
These amides are also components of protein structure.
The amide group of glutamine serves as the source of nitrogen for nucleic
acid synthesis.

9. Due to Amino Group-Transamination
• The alpha amino group of
amino acid can be transferred
to alpha keto acid to form the
corresponding new amino acid
and alpha keto acid .This is
an important reaction in the
body for the interconversion of
amino acids and for synthesis of
non-essential amino acids.

10. Oxidative Deamination
• The alpha amino group is removed from the amino acid to
form the corresponding keto acid and ammonia.
• In the body, Glutamic acid is the most common amino acid to
undergo oxidative deamination.

12. Formation of Carbamino Compound
• Carbon dioxide adds to the alpha amino group of amino
acids to form carbamino compounds. The reaction
occurs at alkaline pH and serves as a mechanism for
the transport of carbon dioxide from tissues to the lungs by
hemoglobin
• Hb—NH2 + CO2 → Hb—NH—COOH (Carbamino-Hb)

13. Due to Side Chains
• Transmethylation
The methyl group of Methionine, after activation, may be
transferred to an acceptor, which becomes methylated
Methionine + Acceptor → Methylated Acceptor
+Homocysteine

14. Ester Formation by the OH Group
The hydroxy amino acids can form esters with phosphoric
acid.
In this manner, the Serine and Threonine residues of proteins
are involved in the formation of phosphoproteins.
Similarly, these hydroxyl groups can form O-glycosidic
bonds with carbohydrate residues to form glycoproteins.

15. Reactions of the amide group
• The amide group of glutamine and asparagine can form N
glycosidic bonds with carbohydrate residues to form
glycoproteins.

16. Reactions of SH Group
Cysteine has a sulfhydryl (SH) group and it can form a
disulfide (S-S) bond with another cysteine residue.
The two cysteine residues can connect two polypeptide chains by
the formation of interchain disulfide bonds or links
The dimer formed by two cysteine residues is sometimes
called Cystine or Dicysteine.

18. Summary

19. Transamination is the exchange of the alpha amino group
between one alpha amino acid and another alpha keto acid,
forming a new alpha amino acid. As an example, amino group is
interchanged between alanine and glutamic acid.
In almost all cases, the amino group is accepted by alpha
ketoglutaric acid so that glutamic acid is formed.
iii. The enzymes catalyzing the reaction as a group are known as
amino transferases. These enzymes have pyridoxal phosphate
as prosthetic group.
The reaction is readily reversible.

20. • Ammoniagenesis helps in excretion of hydrogen ions. The
pyridoxal phosphate is held in Schiff base linkage with the
epsilon amino group of the lysine residue of the enzyme
protein.
• This forms an aldimine link with the alpha amino group of the
reacting amino acid.
• Then the linkage shifts to a ketimine linkage followed by
hydrolysis, the products being an alpha keto acid and
pyridoxamine phosphate. During the 2nd phase of the reaction,
the reaction is reversed, the new amino acid is formed and
pyridoxal phosphate is regenerated.

21. • Biological Significance of Transamination
• First Step of Catabolism
• In this first step, ammonia is removed, and the carbon
skeleton of the amino acid enters into catabolic pathway.

22. Synthesis of Non-Essential Amino Acids
By means of transamination, all non-essential amino acids can be
synthesized by the body from keto acids available from other
sources. Pyruvate can be transaminated to synthesize alanine.
Similarly oxaloacetate produces aspartic acid. Alpha keto
glutarate is transaminated to form glutamic acid.
Those amino acids, which cannot be synthesized in this manner,
are therefore essential; they should be made available in the food

23. Interconversion of Amino Acids
If amino acid no.1 is high and no.2 is low; the amino group
from no.1 may be transferred to a keto acid to give amino acid
no.2 to equalize the quantity of both. This is called
equalization of quantities of non-essential amino acids.

24. • Exceptions
• Lysine, threonine and proline are not transaminated. They follow
direct degradative pathways.

25. Clinical Significance of Transamination
Aspartate amino transferase (AST) and Alanine amino transferase
(ALT) are induced by glucocorticoids, which favor
gluconeogenesis. AST is increased in myocardial infarction and
ALT in liver diseases.

26. Transdeamination
1. The amino group of most of the amino acids is released by a
coupled reaction, trans deamination, that is transamination
followed by oxidative deamination.
2. Transamination takes place in the cytoplasm of all the cells of
the body; the amino group is transported to liver as glutamic
acid, which is finally oxidatively deaminated in the
mitochondria of hepatocytes.
3. Thus, the two components of the reaction are physically far
away, but physiologically they are coupled, the term
transdeamination

27. Oxidative Deamination of Glutamate
Only liver mitochondria contain glutamate dehydrogenase
(GDH) which deaminates glutamate to alpha keto glutarate plus
ammonia.
So, all amino acids are first transaminated to glutamate,
which is then finally deaminated (transdeamination) .
Amino acids are deaminated at the rate of about 50 – 70 gram per
day.

28. During the transamination reaction the amino group of all other
amino acids is funnelled into glutamate.
Hence, the glutamate dehydrogenase reaction is the final
reaction, which removes the amino group of all amino acids .
It needs NAD+ as coenzyme. (NADP can also act as a co-
enzyme). It is an allosteric enzyme; it is activated by ADP and
inhibited by GTP.
The hydrolysis of glutamine also yields NH3 but this occurs
mainly in the kidney where the NH4+ excretion is required for
acid-base regulation

29. Minor Pathways of Deamination
1. L-amino acid oxidase can act on all amino acids except hydroxy amino
acids and dicarboxylic amino acids. It uses FMN as coenzyme. The
peroxide formed in this reaction is decomposed by catalase in the
peroxisomes.
2. D-amino acid oxidase can oxidize glycine and any D amino acid that
may be formed by bacterial metabolism. It uses FAD as co-enzyme.
3. Ammonia may be formed in the body through minor reactions like
oxidation of monoamines by MAO (mono amine oxidase) .

31. Non-oxidative Deaminations
1. Dehydratases act on hydroxy amino acids to remove ammonia from
the following amino acids:
a. Serine will give rise to pyruvate b. Threonine is converted to alpha keto
butyric acid.
2. Desulfhydrase: Cysteine undergoes deamination and simultaneous
transsulfuration to form pyruvate.
3. Histidine also undergoes non-oxidative deamination to form urocanic
acid; catalyzed by histidase.
Ammonia may also be produced in the gastrointestinal tract by bacterial
putrefaction.

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