Amino acid pathway MCQs With Answer is a focused review for B. Pharm students covering amino acid metabolism, biosynthesis and catabolism, transamination, the urea cycle, and one‑carbon (folate/B12) pathways. This concise, keyword‑rich introduction highlights essential topics: amino acid pathways, enzymes (aminotransferases, dehydrogenases, hydroxylases), cofactors (PLP, THF, BH4, B12), regulation, essential vs nonessential amino acids, and inherited metabolic disorders (PKU, MSUD, homocystinuria). Emphasis is on pathway logic, biochemical steps, and clinical correlations to prepare pharmacy students for exams and practice. Clear explanations and targeted MCQs reinforce understanding of metabolic fates, enzyme defects, and therapeutic implications.
‘Now let’s test your knowledge with 30 MCQs on this topic.’
Q1. Which enzyme catalyzes the reversible transamination between glutamate and oxaloacetate?
- Aspartate aminotransferase (AST)
- Alanine aminotransferase (ALT)
- Glutamate dehydrogenase
- Branched‑chain aminotransferase
Correct Answer: Aspartate aminotransferase (AST)
Q2. Which cofactor is essential for all aminotransferase (transaminase) reactions in amino acid metabolism?
- Biotin
- Pyridoxal phosphate (PLP)
- Nicotinamide adenine dinucleotide (NAD+)
- Tetrahydrofolate (THF)
Correct Answer: Pyridoxal phosphate (PLP)
Q3. Which amino acid is synthesized from the glycolytic intermediate 3‑phosphoglycerate?
- Serine
- Alanine
- Lysine
- Phenylalanine
Correct Answer: Serine
Q4. Among aromatic amino acids, which is considered nonessential because it can be produced from phenylalanine?
- Phenylalanine
- Tryptophan
- Tyrosine
- Histidine
Correct Answer: Tyrosine
Q5. The conversion of phenylalanine to tyrosine is catalyzed by phenylalanine hydroxylase. Which cofactor is required for this reaction?
- Tetrahydrobiopterin (BH4)
- Vitamin B12 (cobalamin)
- Pyridoxal phosphate (PLP)
- FAD (flavin adenine dinucleotide)
Correct Answer: Tetrahydrobiopterin (BH4)
Q6. What is the first committed enzyme of the urea cycle that synthesizes carbamoyl phosphate in mitochondria?
- Ornithine transcarbamylase (OTC)
- Carbamoyl phosphate synthetase I (CPS I)
- Argininosuccinate synthetase
- Arginase
Correct Answer: Carbamoyl phosphate synthetase I (CPS I)
Q7. Which of the following is NOT a branched‑chain amino acid (BCAA)?
- Valine
- Isoleucine
- Leucine
- Methionine
Correct Answer: Methionine
Q8. Which amino acid is exclusively ketogenic and cannot be converted into glucose?
- Alanine
- Leucine
- Valine
- Glutamate
Correct Answer: Leucine
Q9. Which enzyme catalyzes oxidative deamination of glutamate to release free ammonia and alpha‑ketoglutarate?
- Glutamate dehydrogenase
- Glutamine synthetase
- Glutaminase
- Aspartate aminotransferase
Correct Answer: Glutamate dehydrogenase
Q10. Remethylation of homocysteine to methionine requires which vitamin‑derived cofactor?
- Folate (THF)
- Vitamin B12 (methylcobalamin)
- Pyridoxal phosphate (PLP)
- Vitamin C (ascorbate)
Correct Answer: Vitamin B12 (methylcobalamin)
Q11. S‑adenosylmethionine (SAM) primarily functions in metabolism as:
- An amino acid transporter
- A universal methyl group donor
- An electron carrier in oxidation reactions
- A urea cycle intermediate
Correct Answer: A universal methyl group donor
Q12. The enzyme that condenses serine and homocysteine to form cystathionine (a step in cysteine biosynthesis) is:
- Cystathionine β‑synthase (CBS)
- Cystathionine γ‑lyase (CSE)
- Cystine reductase
- Methionine adenosyltransferase
Correct Answer: Cystathionine β‑synthase (CBS)
Q13. The conversion of serine to glycine involves transfer of a one‑carbon unit to tetrahydrofolate. Which enzyme catalyzes this reaction?
- Serine hydroxymethyltransferase
- Glycine dehydrogenase
- Methylene tetrahydrofolate reductase (MTHFR)
- Thymidylate synthase
Correct Answer: Serine hydroxymethyltransferase
Q14. Phenylketonuria (PKU) most commonly arises from a deficiency of which enzyme?
- Tyrosine aminotransferase
- Phenylalanine hydroxylase (PAH)
- Tryptophan dioxygenase
- Branched‑chain α‑ketoacid dehydrogenase
Correct Answer: Phenylalanine hydroxylase (PAH)
Q15. Maple syrup urine disease (MSUD) results from a defect in which enzyme complex involved in BCAA catabolism?
- Branched‑chain α‑ketoacid dehydrogenase (BCKD) complex
- Pyruvate dehydrogenase complex
- Ornithine transcarbamylase
- Glutamate dehydrogenase
Correct Answer: Branched‑chain α‑ketoacid dehydrogenase (BCKD) complex
Q16. Alanine aminotransferase (ALT) catalyzes the reversible conversion between alanine and which keto‑acid?
- Oxaloacetate
- Pyruvate
- Alpha‑ketoglutarate
- Succinyl‑CoA
Correct Answer: Pyruvate
Q17. Nitric oxide (NO) is synthesized from which amino acid by nitric oxide synthase?
- Glutamine
- Arginine
- Tryptophan
- Histidine
Correct Answer: Arginine
Q18. The enzyme that converts glutamate plus ammonia into glutamine, consuming ATP, is:
- Glutaminase
- Glutamine synthetase
- Glutamate dehydrogenase
- Glutamine transaminase
Correct Answer: Glutamine synthetase
Q19. Which amino acid serves as a precursor for niacin (NAD) synthesis and is implicated in pellagra when deficient?
- Tryptophan
- Lysine
- Phenylalanine
- Threonine
Correct Answer: Tryptophan
Q20. Which amino acid is predominantly glucogenic, yielding substrates for gluconeogenesis?
- Leucine
- Lysine
- Valine
- Phenylalanine
Correct Answer: Valine
Q21. Which amino acid is the major nitrogen donor for biosynthetic reactions including nucleotide and amino sugar synthesis?
- Aspartate
- Glutamine
- Serine
- Proline
Correct Answer: Glutamine
Q22. The enzyme that hydrolyzes glutamine to glutamate and free ammonia, important in renal ammoniagenesis, is:
- Glutamine synthetase
- Glutaminase
- Glutamate dehydrogenase
- Glutamate synthase
Correct Answer: Glutaminase
Q23. Deamination of which amino acid directly yields oxaloacetate (via transamination), linking to the TCA cycle?
- Aspartate
- Alanine
- Glutamate
- Leucine
Correct Answer: Aspartate
Q24. Homocystinuria is most commonly due to deficiency of which enzyme in methionine/cysteine metabolism?
- Methionine adenosyltransferase
- Cystathionine β‑synthase (CBS)
- Betaine‑homocysteine methyltransferase
- Methylene tetrahydrofolate reductase (MTHFR)
Correct Answer: Cystathionine β‑synthase (CBS)
Q25. Which enzyme catalyzes formation of citrulline from ornithine and carbamoyl phosphate in the urea cycle?
- Arginase
- Ornithine transcarbamylase (OTC)
- Argininosuccinate lyase
- Carbamoyl phosphate synthetase II
Correct Answer: Ornithine transcarbamylase (OTC)
Q26. Which amino acid, together with succinyl‑CoA, is the precursor for δ‑aminolevulinic acid (ALA) in heme synthesis?
- Glycine
- Cysteine
- Proline
- Tyrosine
Correct Answer: Glycine
Q27. A deficiency of which urea cycle enzyme typically causes the most severe neonatal hyperammonemia and is X‑linked?
- Carbamoyl phosphate synthetase I (CPS I)
- Ornithine transcarbamylase (OTC)
- Arginase
- Argininosuccinate synthetase
Correct Answer: Ornithine transcarbamylase (OTC)
Q28. Which amino acid is directly synthesized by transamination of pyruvate?
- Alanine
- Glutamine
- Aspartate
- Proline
Correct Answer: Alanine
Q29. The branched‑chain α‑ketoacid dehydrogenase complex (BCKD) requires which vitamin‑derived cofactor for activity?
- Thiamine pyrophosphate (TPP)
- Biotin
- Vitamin C
- Vitamin K
Correct Answer: Thiamine pyrophosphate (TPP)
Q30. In the conversion of serine to glycine, the one‑carbon unit is carried by which folate derivative?
- 5‑Methyl‑THF
- 5,10‑Methylene‑THF
- 10‑Formyl‑THF
- Dihydrofolate (DHF)
Correct Answer: 5,10‑Methylene‑THF
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