General Metabolism of Amino Acids – MCQs for GPAT, NIPER, AIIMS Pharmacist, Railway Pharmacist, SSC, ESIC & State Pharmacist Exams

What is amino acid metabolism?

Amino acids are the building blocks of proteins. When proteins are broken down, amino acids are either:

• Used to synthesize new proteins and other nitrogen-containing compounds, or

• Catabolised (broken down) to produce energy.

Amino acid metabolism involves two important parts:

• Removal of the amino (–NH₂) group (nitrogen metabolism)

• Utilization of the carbon skeleton (energy metabolism)

1. Transamination

Definition

Transamination is the transfer of an amino group (–NH₂) from one amino acid to an α-keto acid without releasing free ammonia.

General Reaction

Amino Acid + α-Ketoglutarate ⇌ α-Keto Acid + Glutamate

Enzymes

• Aminotransferases (Transaminases)

  • Alanine aminotransferase (ALT/SGPT)

  • Aspartate aminotransferase (AST/SGOT)

Coenzyme

• Pyridoxal phosphate (PLP)

• Active form of Vitamin B₆

Importance

• First step in amino acid breakdown.

• Collects amino groups into glutamate.

• Helps synthesize non-essential amino acids.

• Does not release free ammonia.

Important Points for MCQs

✔ Requires Vitamin B₆ (PLP).
✔ α-Ketoglutarate is the major amino-group acceptor.
✔ Produces glutamate.
✔ Reversible reaction.
✔ Lysine and threonine generally do not undergo transamination.

2. Deamination

Deamination removes the amino group from amino acids, producing free ammonia (NH₃).

There are two major types.

A. Oxidative Deamination

Removal of an amino group with simultaneous oxidation, producing free ammonia.

Major Enzyme

Glutamate dehydrogenase (GDH)

Reaction

Glutamate → α-Ketoglutarate + NH₃

Cofactors

• NAD⁺ or NADP⁺

Site

• Liver mitochondria

Importance

• Produces free ammonia for the urea cycle.

• Regenerates α-ketoglutarate.

B. Non-Oxidative Deamination

Removal of an amino group without oxidation.

Examples

• Serine → Pyruvate + NH₃

• Threonine → α-Ketobutyrate + NH₃

Enzyme

• Serine dehydratase

• Threonine dehydratase

Difference Between Oxidative and Non-Oxidative Deamination

• Feature Oxidative Deamination Non-Oxidative Deamination

• Oxidation involved Yes No

• Main enzyme Glutamate dehydrogenase Dehydratases

• Cofactor NAD⁺/NADP⁺ Usually PLP

• Main substrate Glutamate Serine, Threonine

• Ammonia released Yes Yes

MCQ Tip

Transamination transfers NH₂.
Deamination removes NH₂.

3. Decarboxylation

Removal of the carboxyl group (CO₂) from an amino acid.

Enzyme

Amino acid decarboxylases

Coenzyme

Vitamin B₆ (PLP)

Importance

Produces many biologically active amines.

Examples

• Amino Acid Product

• Histidine Histamine

• Glutamate GABA

• DOPA Dopamine

• 5-Hydroxytryptophan Serotonin

• Tyrosine Tyramine

Significance

• Produces neurotransmitters.

• Produces hormones.

• Important in nervous system function.

MCQ Tip

Histidine → Histamine
Glutamate → GABA

These are commonly asked.

4. Urea Cycle (Nitrogen Disposal)

The urea cycle converts toxic ammonia into urea, which is safely excreted by the kidneys.

Site

• Liver

• Partly mitochondrial

• Partly cytosolic

Why is it necessary?

Ammonia is highly toxic, especially to the brain.

The body converts ammonia into urea, which is

• Non-toxic

• Water-soluble

• Easily excreted in urine

Steps of Urea Cycle (Easy Trick)

Step 1

Ammonia + CO₂

↓

Carbamoyl phosphate

Enzyme: Carbamoyl phosphate synthetase-I (CPS-I)

Activator: N-Acetylglutamate

Step 2

Carbamoyl phosphate + Ornithine

↓

Citrulline

Step 3

Citrulline + Aspartate

↓

Argininosuccinate

Step 4

Argininosuccinate

↓

Arginine + Fumarate

Step 5

Arginine

↓

Urea + Ornithine

Important Facts

• Occurs only in the liver.

• First, nitrogen comes from ammonia.

• Second nitrogen comes from aspartate.

• Fumarate links the urea cycle with the TCA cycle.

• Urea is excreted through the kidneys.

MCQ points

• Rate-limiting enzyme = CPS-I

• Activator = N-Acetylglutamate

• Hyperammonaemia occurs if the urea cycle is defective.

5. Fate of Carbon Skeletons

After removal of the amino group, the remaining carbon skeleton enters energy metabolism.

It may form:

• Pyruvate

• Acetyl-CoA

• Acetoacetate

• Oxaloacetate

• α-Ketoglutarate

• Succinyl-CoA

• Fumarate

Glucogenic Amino Acids

These produce intermediates that can form glucose through gluconeogenesis.

Examples

• Alanine

• Glycine

• Serine

• Valine

• Methionine

• Histidine

• Aspartate

End Products

• Pyruvate

• Oxaloacetate

• Succinyl-CoA

• Fumarate

• α-Ketoglutarate

Ketogenic Amino Acids

These produce:

• Acetyl-CoA

• Acetoacetate

They cannot produce glucose.

Exclusively Ketogenic

• Leucine

• Lysine

Mnemonic

LL = Leucine & Lysine = Lipid Lovers

Both Glucogenic and Ketogenic

• Isoleucine

• Phenylalanine

• Tyrosine

• Tryptophan

• Threonine

High-Yield Exam Facts

• Vitamin B₆ is required for transamination and decarboxylation.

• Glutamate is the central amino acid in nitrogen metabolism.

• ALT and AST are markers of liver injury.

• Glutamine transports ammonia safely in blood.

• Alanine transports nitrogen from muscle to liver (Glucose–Alanine Cycle).

• Only Leucine and Lysine are purely ketogenic.

• Most amino acids are glucogenic.

• Urea is synthesized only in the liver.

• Hyperammonemia causes neurological symptoms.

Memory Tricks

Transamination

Transfer NH₂

(No free ammonia)

Deamination

Delete NH₂

(Free ammonia produced)

Decarboxylation

CO₂ removed

(Biogenic amines formed)

Urea Cycle

Ammonia → Urea → Urine

Ketogenic Amino Acids

LL = Leucine & Lysine

Quick Revision Table

• Process Main Enzyme Product Key Point

• Transamination ALT, AST Glutamate No free NH₃

• Oxidative deamination Glutamate dehydrogenase NH₃ Uses NAD⁺/NADP⁺

• Non-oxidative deamination Dehydratases NH₃ No oxidation

• Decarboxylation Decarboxylase Biogenic amines PLP dependent

• Urea cycle CPS-I Urea Liver only

Practice MCQs

1. Which coenzyme is required for both transamination and decarboxylation reactions?

A. FAD
B. NAD⁺
C. Pyridoxal phosphate (PLP)
D. Biotin

Answer: C. Pyridoxal phosphate (PLP)

2. The principal amino-group collector during amino acid metabolism is:

A. Pyruvate
B. Glutamate
C. Oxaloacetate
D. Citrate

Answer: B. Glutamate

3. Which amino acids are exclusively ketogenic?

A. Valine and Methionine
B. Leucine and Lysine
C. Phenylalanine and Tyrosine
D. Alanine and Glycine

Answer: B. Leucine and Lysine

4. Which enzyme catalyzes oxidative deamination?

A. ALT
B. AST
C. Glutamate dehydrogenase
D. Arginase

Answer: C. Glutamate dehydrogenase

5. The rate-limiting enzyme of the urea cycle is:

A. Arginase
B. Ornithine transcarbamylase
C. Carbamoyl phosphate synthetase-I
D. Argininosuccinate lyase

Answer: C. Carbamoyl phosphate synthetase-I

6. Histidine undergoes decarboxylation to form:

A. GABA
B. Histamine
C. Dopamine
D. Serotonin

Answer: B. Histamine

7. Which statement is correct?

A. Transamination releases free ammonia.
B. Oxidative deamination requires glutamate dehydrogenase.
C. Urea is synthesized in skeletal muscle.
D. Leucine is glucogenic.

Answer: B. Oxidative deamination requires glutamate dehydrogenase.

Exam Tip

For competitive pharmacy examinations, remember the sequence:

Transamination → Deamination → Urea Cycle → Carbon Skeleton Utilisation.

MCQs

1. Which enzyme requires pyridoxal phosphate (PLP) as a coenzyme and transfers an amino group without releasing free ammonia?

A. Glutamate dehydrogenase
B. Alanine aminotransferase (ALT)
C. Carbamoyl phosphate synthetase I
D. Arginase

Answer: B. Alanine aminotransferase (ALT)

Explanation:
ALT catalyses transamination, transferring an amino group from alanine to α-ketoglutarate, producing pyruvate and glutamate. PLP (vitamin B6) is the essential coenzyme. Glutamate dehydrogenase performs oxidative deamination, CPS-I catalyses the first step of the urea cycle, and arginase hydrolyses arginine to urea.

2. During amino acid catabolism, most amino groups are first transferred to:

A. Oxaloacetate
B. Pyruvate
C. α-Ketoglutarate
D. Succinyl-CoA

Answer: C. α-Ketoglutarate

Explanation:
Most transaminases transfer amino groups to α-ketoglutarate, forming glutamate, which acts as the major collector of amino groups before ammonia is released through oxidative deamination.

3. Which statement best explains the physiological importance of transamination?

A. It directly produces urea.
B. It converts amino acids into fatty acids.
C. It funnels amino groups to glutamate before ammonia release.
D. It produces ATP directly.

Answer: C. It funnels amino groups to glutamate before ammonia release.

Explanation:
Transamination safely transfers amino groups to glutamate, preventing accumulation of toxic free ammonia. Urea is formed later through the urea cycle.

4. Glutamate dehydrogenase catalyses:

A. Decarboxylation of glutamate
B. Oxidative deamination of glutamate
C. Transamination of glutamate
D. Amidation of glutamate

Answer: B. Oxidative deamination of glutamate

Explanation:
Glutamate dehydrogenase removes the amino group from glutamate as free ammonia using NAD⁺ or NADP⁺, producing α-ketoglutarate.

5. Which amino acid cannot undergo transamination?

A. Alanine
B. Aspartate
C. Lysine
D. Glutamate

Answer: C. Lysine

Explanation:
Lysine (along with threonine) generally does not undergo transamination because unstable cyclic intermediates would be formed.

6. Which vitamin deficiency most severely impairs transamination reactions?

A. Vitamin B₁
B. Vitamin B₂
C. Vitamin B₆
D. Vitamin B₁₂

Answer: C. Vitamin B₆

Explanation:
PLP, the active form of vitamin B₆, is the coenzyme required by all aminotransferases.

7. The major source of free ammonia entering the urea cycle is:

A. Alanine aminotransferase
B. Glutamate dehydrogenase
C. Arginase
D. Aspartate aminotransferase

Answer: B. Glutamate dehydrogenase

Explanation:
Oxidative deamination of glutamate by glutamate dehydrogenase releases free ammonia, which enters the urea cycle.

8. Which enzyme catalyses the first committed step of the urea cycle?

A. Carbamoyl phosphate synthetase I
B. Carbamoyl phosphate synthetase II
C. Ornithine transcarbamylase
D. Argininosuccinate synthetase

Answer: A. Carbamoyl phosphate synthetase I

Explanation:
CPS-I is located in mitochondria and uses free ammonia to synthesise carbamoyl phosphate.

9. Carbamoyl phosphate synthetase I is activated by:

A. ATP
B. Citrulline
C. N-Acetylglutamate
D. Arginine

Answer: C. N-Acetylglutamate

Explanation:
N-Acetylglutamate is the obligatory allosteric activator of CPS-I.

10. Which amino acid directly donates the second nitrogen atom of urea?

A. Glutamine
B. Glutamate
C. Aspartate
D. Alanine

Answer: C. Aspartate

Explanation:
One nitrogen of urea comes from free ammonia, while the second originates from aspartate.

11. Ornithine participates in the urea cycle by:

A. Being converted to glutamate
B. Acting as a catalytic carrier of carbamoyl groups
C. Donating ammonia directly
D. Producing ATP

Answer: B. Acting as a catalytic carrier of carbamoyl groups

Explanation:
Ornithine combines with carbamoyl phosphate to form citrulline and is regenerated at the end of the cycle.

12. Which step of the urea cycle occurs in the cytosol?

A. Formation of carbamoyl phosphate
B. Formation of citrulline
C. Formation of argininosuccinate
D. Activation of CPS-I

Answer: C. Formation of argininosuccinate

Explanation:
Only the first two reactions occur in mitochondria; the remaining reactions occur in the cytosol.

13. The immediate product formed after ornithine combines with carbamoyl phosphate is:

A. Arginine
B. Citrulline
C. Argininosuccinate
D. Fumarate

Answer: B. Citrulline

Explanation:
Ornithine transcarbamylase catalyzes the formation of citrulline.

14. Which intermediate links the urea cycle with the TCA cycle?

A. Citrulline
B. Fumarate
C. Ornithine
D. Carbamoyl phosphate

Answer: B. Fumarate

Explanation:
Fumarate enters the TCA cycle, forming the aspartate-argininosuccinate shunt.

15. Hyperammonemia primarily affects the brain because:

A. Ammonia inhibits glycolysis.
B. Excess glutamine causes osmotic swelling of astrocytes.
C. It blocks fatty acid oxidation.
D. It inhibits glycogen synthesis.

Answer: B. Excess glutamine causes osmotic swelling of astrocytes.

Explanation:
Ammonia is detoxified to glutamine in astrocytes. Excess glutamine causes cerebral edema and neurological dysfunction.

16. Which amino acid is exclusively ketogenic?

A. Phenylalanine
B. Tyrosine
C. Leucine
D. Isoleucine

Answer: C. Leucine

Explanation:
Only leucine and lysine are exclusively ketogenic.

17. Which amino acid is both glucogenic and ketogenic?

A. Lysine
B. Leucine
C. Isoleucine
D. Alanine

Answer: C. Isoleucine

Explanation:
Isoleucine yields acetyl-CoA (ketogenic) and succinyl-CoA (glucogenic).

18. Which amino acid is exclusively glucogenic?

A. Valine
B. Leucine
C. Lysine
D. Acetyl-CoA

Answer: A. Valine

Explanation:
Valine forms succinyl-CoA and contributes to gluconeogenesis.

19. The carbon skeleton of alanine is converted into:

A. Oxaloacetate
B. Pyruvate
C. Acetyl-CoA
D. Fumarate

Answer: B. Pyruvate

Explanation:
ALT converts alanine into pyruvate through transamination.

20. Which amino acid directly forms oxaloacetate?

A. Aspartate
B. Leucine
C. Lysine
D. Histidine

Answer: A. Aspartate

Explanation:
Aspartate is transaminated to oxaloacetate.

21. Which enzyme catalyzes non-oxidative deamination?

A. Glutamate dehydrogenase
B. Serine dehydratase
C. ALT
D. AST

Answer: B. Serine dehydratase

Explanation:
Serine dehydratase removes ammonia without oxidation, producing pyruvate.

22. Decarboxylation of histidine produces:

A. Histamine
B. Dopamine
C. GABA
D. Serotonin

Answer: A. Histamine

Explanation:
Histidine decarboxylase forms histamine, an important inflammatory mediator.

23. Which neurotransmitter is formed by decarboxylation of glutamate?

A. Dopamine
B. Serotonin
C. GABA
D. Histamine

Answer: C. GABA

Explanation:
Glutamate decarboxylase converts glutamate to γ-aminobutyric acid (GABA).

24. Which amino acid is the major transporter of ammonia from peripheral tissues to the liver?

A. Glycine
B. Glutamine
C. Serine
D. Methionine

Answer: B. Glutamine

Explanation:
Glutamine safely transports ammonia through the bloodstream.

25. During prolonged fasting, skeletal muscle transports amino nitrogen mainly as:

A. Glycine
B. Arginine
C. Alanine
D. Histidine

Answer: C. Alanine

Explanation:
The glucose-alanine cycle transports nitrogen to the liver while providing pyruvate for gluconeogenesis.

26. Which reaction directly generates free ammonia in kidney tissue to help maintain acid-base balance?

A. ALT reaction
B. Glutaminase reaction
C. AST reaction
D. Hexokinase reaction

Answer: B. Glutaminase reaction

Explanation:
Renal glutaminase liberates ammonia, which buffers urinary H⁺ ions as NH₄⁺.

27. The urea cycle primarily functions in:

A. Kidney cortex
B. Liver hepatocytes
C. Skeletal muscle
D. Brain

Answer: B. Liver hepatocytes

Explanation:
The liver is the principal site of urea synthesis.

28. Which pair of amino acids is exclusively ketogenic?

A. Leucine and Lysine
B. Valine and Leucine
C. Isoleucine and Lysine
D. Phenylalanine and Tyrosine

Answer: A. Leucine and Lysine

Explanation:
Only leucine and lysine produce ketone body precursors exclusively.

29. A deficiency of ornithine transcarbamylase would most likely result in:

A. Hypoglycemia without hyperammonemia
B. Hyperammonemia due to impaired urea synthesis
C. Increased ketone body formation only
D. Increased glycogen storage

Answer: B. Hyperammonemia due to impaired urea synthesis

Explanation:
OTC deficiency blocks the urea cycle, causing ammonia accumulation.

30. Which statement regarding amino acid metabolism is correct?

A. All amino acids are glucogenic.
B. All amino acids undergo transamination.
C. Glutamate occupies a central role in nitrogen metabolism.
D. Urea is synthesised mainly in skeletal muscle.

Answer: C. Glutamate occupies a central role in nitrogen metabolism.

Explanation:
Glutamate acts as the principal amino-group collector through transamination and releases ammonia via oxidative deamination. Not all amino acids are glucogenic; lysine and threonine do not normally undergo transamination, and urea is synthesised in the liver.

Dr. Alok Singh