Amino acid metabolism.

1. AMINO ACID METABOLISM
QUESTIONS AND ANSWERS

2. INTRODUCTION
• Proteins are the most abundant organic compounds and constitute a
major part of the body (10-12 kg in adults).
• About half of the body protein (collegen) is present in the supportive
tissue (skeleton and connective) while the other half is intercellular.
• Proteins are nitrogen containing macromolecule consisting L- alpha-
amino acids . of the 20 amino acids found in proteins, half can be
synthesized by the body (non-essential) while the rest have to
provided in diet (essential amino acids).
• The protein on degradation (proteolysis) release amino acids.

3. AMINO ACID POOL
An adult has about 100 g of free
amino acids which represent the
amino acid pool of the body.
• Glutamate and glutamine
together constitute about 50% ,
and essential amino acids about
10 % of the body pool.

4. 1.Sources of amino acid [AA] pool
-protein turnover (daily 300-400g of protein degraded to
Amino acid)
-dietary protein
-synthesis of non-essential Amino acids.
2.Utilization of AA from body pool
-Amino acids are converted into carbohydrates and fats.
-generally, about 10-15% of body energy requirements are
gained from the Amino acids.
-many important nitrogenous compounds (porphyrins,
purins, pyrimidins) are produced from Amino acids
-most of body proteins (300-400 g/daily) are synthesized
from Amino acid pool.

5. General Aspects of Amino Acids Metabolism.
There is a primitive pathways of AA fate
degradation:
1.fate of α-amino group is convertation into
ammonium ion (by oxidative deamination
Glutamate)
2.fate of carbon atoms which mostly turn into
energy:
-the C3 family of AA (Alanine, Serine, and
Cysteine) are converted into Pyruvate;
-the C4 family of AA (Aspartate and Asparagine)
are converted into Oxaloacetate;
-the C5 family of AA (Glutamine, Proline,
Arginine, Histidine) into α-ketoglutarate
throught Glutamate;

6. The amino acids are classified into two groups based on the
nature of metabolic end products of carbon skeleton

7. the Amino acid undergo certain common reactions
transamination followed by
deamination for the liberation of
ammonia.
The amino group of the amino
acids is utilized for the formation
of urea which is an excretory end
product of protein metamolism.

8. Transamination
Transamination is a transfer of an amino (-NH2 ) group
from an amino acid to a keto acid by transaminase
(recently, aminotransferases).
• All transaminases require pyridoxal phosphate (PLP)
• Specific transaminases exist for each pair of amino and
keto acids. aspartate and alanine transaminase make
significant contribution for transamination.
• There is no free NH3.
• Reversible.
• Production of non-essential amino group.
• Diverts excess amino acids towards energy generation.
• The amino acid undergo transamination to finally
concentrate nitrogen in glutamate.
Glutamate is the only amino acid that undergoes oxidative
deamination to liberate free NH3 for urea synthesis.

9. MECHANISM OF TRANSAMINATION
PING PONG mechanism:
Transamination occurs in two stages:
A) Transfer of amino group to the
coenzyme pyridoxal phosphate
(bound to the coenzyme) to form
pyridoxamine phosphate.
B) The amino group of
pyridoxamine phosphate is then
transfer to a keto acid to produce
a new amino acid and the
enzyme with PLP is regenerated.

10. DEAMINATION
• The removal of amino group from the amino acids as NH3 is deamination.

11. OXIDATIVE DEAMINATION
1. Oxidative deamination- it is the liberation of free ammonia from the
amino group of amino acids coupled with oxidation.
-it takes place mostly in liver and kidney.
-to provide ammonium for urea cycle.
-to provide alpha keto acid for energy generation.

12. Non-oxidative deamination
1. Non-oxidative deamination- some of the
amino acids can be deaminated to
liberate NH3 without undergo oxidation.
a. amino acids dehydrases (serine, threonine and
homoserine – are hydroxy AA deamination of
which is catalysed by pyridoxal phosphate [PLP])
b. sulfur amino acids (cystein, homocystein) undergo
deamination coupled with desulfhydrases
c. dehydratation of histidine is catalised by histidase

13. DECARBOXYLATION

14. BIOSYNTHESIS OF NITRIC OXIDE (NO)
• NO is synthesized from Arginine.
• The enzyme nitric oxide synthase cleaves the nitrogen from the arginine to form NO.
• This reaction require NADPH, FMN, FAD, heme and tetrahydrobiopterin.

15. TRANSMETHYLATION
The transfer of methyl group from active methionine to an acceptor is
known as transmethylation.
Methionine has to be activated to S-adenosylmethionine(SAM) or active
methionine to donate the methyl group.
Formation of active-methionine:-
1. the synthesis of S-adenosylmethionine occurs by the transfer of an
adenosyl group from ATP to sulfer atom of methionine. This reaction
is catalysed by methionine S-adenosylmethionine transferase.
2. The activation of methionine is unique as the sulfer becomes a
sulfonium atom (SAM is a sulfonium compound) by the addition of a
third carbon.
3. This reaction is also unusual since all the three phosphates of ATP
are elimined as pyrophosphate (PPi) and inorganic phosphate (Pi) .
4. Three high energy phosphate (3 ATP) are consumed in the formation
of SAM.

16. Amino acid metabolism
1. What is general amino acid pool? (1/2015)(1/2013)
2. What is transmethylation? Give one example. (2/2015)(2/2013)
3. Discuss the biosynthesis of nitric oxide. Mention its physiological significance.(2/2015)(4/2013) (3+2/2022)
4. Mention two important amino acids decarboxylation reactions. (2/2015)
5. Discuss the role of PLP in transamination reaction. (3/2014)(3/2012)
6. How is ‘active-methionine’ formed? (2/2021)
7. What are glucogenic aminoacids? Give example. (1+1/2014)
8. Why in deamination reaction the enzymes are termed as deaminase? (2/2011)
9. Describe the reaction catalyzed by L-glutamate dehydrogenase and mention its significance. (4/2021)
10. Transamination process of amino acid. (5/2022)
11. Name two amino acids which are both glucogenic and ketogenic in nature. (1/2022)

17. Q.Discuss the role of PLP in transamination reaction. (3/2014)(3/2012)
• All the transaminases require pyridoxal phosphate (PLP), a derivative of vitamin B6.
• the aldehyde group of PLP is linked with ε- amino acid of lysine residue, at the active site of the enzyme
forming a Schiff base (imine linkage).
• When an amino acid comes in contact with the enzyme, it displaces lysine and a new Schiff base
linkage is formed.
• The amino-acid-PLP-shiff base tightly binds with the enzyme by non-covalent forces.
Q. Why in deamination reaction the enzymes are termed as deaminase?
• deaminases help to remove α amino group from amino acid.
• Amino group is converted into ammonia while the amino acid itself converts into its corresponding keto acid.
This Enzymes catalyse this reaction that’s why they are called deaminases.

18. reaction catalyzed by L-glutamate dehydrogenase
significance
• GDH catalysed reaction is important as it
reversibly links up glutamate metabolism with
TCA cycle through alpha-ketoglutarate.
• GDH is involved in boyh catabolic and anabolic
reactions.
Q. Describe the reaction catalyzed by L-glutamate dehydrogenase and mention its significance. (4/2021)

19. METABOLISM OF GLYCINE:
Q. Discuss the synthesis glycine from choline.
• Betaine, formed by two successive oxidation of choline.
• It may transfer one of its labile methyl groups to
homocysteine to change itself to dimethylglycine.
• The latter is then oxidized through sarcosine to glycine.
Q. Discuss the role of folate in glycine metabolism.
• Glycine is synthesized from serine by the enzyme serine
hydroxymethyl transferase which is depend on
tetrahydrofolate (THF).
• Glycine synthase can convert a one carbon unit (N^5,N^10-
methylene THF), co2 and NH3 to glycine.

20. METABOLISM OF TRYPTOPHAN
Q. Discuss how L-tryptophan is converted to pyruvate. (5/2015)
• Tryptophan 2,3- dioxygenase, oxidizes tryptophan to N-formylkunurine
• N-formylkunurine is deformylated by kynurine formylase to kynurenine.
• Kynurenine is hydroxylated by kynurenine hydroxylase to 3-hydroxykynurenine.
• 3-hydroxykynurenine is cleaved by PLP-dependent kynureninase into 3-hydroxyanthranilate and alanine.
• Alanine transaminated to pyruvate.

21. Q. Discuss the catabolism of phenylalanine leading to the formation of fumerate and acetoacetate.
(5/2013)

22. METABOLISM OF S-CONTAINING AMINO ACID:
Q. Discuss the synthesis and significance of cysteine. (4/2013)
Synthesis of cysteine:-
• Mammals synthesize cysteine from two amino acids: methionine
furnishes the sulfur atom, and serine furnishes the carbon
skeleton.
• Methionine is first converted to Sadenosylmethionine, which can
lose its methyl group to any of a number of acceptors to form S-
adenosylhomocysteine (adoHcy).
• This demethylated product is hydrolyzed to free homocysteine,
which undergoes a reaction with serine, catalyzed by
cystathionine β-synthase, to yield cystathionine.
• Finally, cystathionine γ-lyase, a PLP-requiring enzyme, catalyzes
removal of ammonia and cleavage of cystathionine to yield free
cysteine.
significance of cysteine:-
• Cysteine is a non-essential amino acid important for making protein, and for other metabolic
functions.
• It's found in beta-keratin.
• This is the main protein in nails, skin, and hair.
• Cysteine is important for making collagen.

23. Q.Describe the catalytic pathway of cysteine.(6/2010)

24. UREA SYNTHESIS:
Q. Discuss the role of carbamoyl synthase- I in
relation to urea formation.
• carbamoyl phosphate synthase I (CPS I) of
mitochondria catalyses the condensation of
NH4+ ions with C02 to form carbamoyl
phosphate.
• This step consumes two ATP and is irreversible
and rate limiting.
• CPS I requires N- acetylglutamate for its activity.

25. GLUTATHIONE SYNTHESIS:
Q. Discuss the synthesis and significance of
glutathione. (4/2012)(5/2011)
Glutathione is a tripeptide consist with ( γ-glutamyl-
cysteinyl-glycine) and require three amino acids for
its formation.