Introduction:
Mastering general reactions of amino acid metabolism is essential for B. Pharm students preparing for pharmacology, biochemistry, and clinical coursework. This focused guide covers transamination, deamination, oxidative deamination, PLP-dependent reactions, urea cycle regulation, and connections to gluconeogenesis and ketogenesis. Key concepts include aminotransferases (ALT/AST), glutamate dehydrogenase, carbamoyl phosphate synthetase I, and one-carbon transfers via SAM and folate pathways. Practical clinical links—hyperammonemia, phenylketonuria, maple syrup urine disease, and diagnostic enzyme markers—are emphasized to aid application in therapeutics. Clear MCQs reinforce mechanism-focused learning and exam readiness. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. Which reaction best describes transamination in amino acid metabolism?
- Removal of amino group as free ammonia
- Transfer of amino group to alpha-ketoglutarate
- Oxidative decarboxylation of amino acids
- Formation of urea from two ammonia molecules
Correct Answer: Transfer of amino group to alpha-ketoglutarate
Q2. Which cofactor is essential for aminotransferase (transaminase) activity?
- Flavin adenine dinucleotide (FAD)
- Nicotinamide adenine dinucleotide (NAD+)
- Pyridoxal phosphate (PLP)
- Coenzyme A (CoA)
Correct Answer: Pyridoxal phosphate (PLP)
Q3. Oxidative deamination of glutamate primarily produces which pair of products?
- Glutamine and carbon dioxide
- Alpha-ketoglutarate and ammonium ion
- Ornithine and urea
- Aspartate and fumarate
Correct Answer: Alpha-ketoglutarate and ammonium ion
Q4. Which enzyme catalyzes the oxidative deamination of glutamate?
- Alanine aminotransferase (ALT)
- Aspartate aminotransferase (AST)
- Glutamate dehydrogenase
- Glutamine synthetase
Correct Answer: Glutamate dehydrogenase
Q5. Which amino acid is the primary nitrogen donor for most transamination reactions?
- Glycine
- Alanine
- Glutamate
- Serine
Correct Answer: Glutamate
Q6. Which pair of enzymes are commonly used as clinical markers of liver damage?
- Glutamine synthetase and carbamoyl phosphate synthetase I
- Alanine aminotransferase (ALT) and aspartate aminotransferase (AST)
- Phenylalanine hydroxylase and tyrosine aminotransferase
- Ornithine transcarbamylase and arginase
Correct Answer: Alanine aminotransferase (ALT) and aspartate aminotransferase (AST)
Q7. In the glucose-alanine cycle, alanine carries nitrogen from muscle to liver and is converted to which compound in the liver?
- Pyruvate
- Oxaloacetate
- Fumarate
- Acetoacetate
Correct Answer: Pyruvate
Q8. Which enzyme initiates the urea cycle by synthesizing carbamoyl phosphate?
- Ornithine transcarbamylase
- Arginase
- Carbamoyl phosphate synthetase I (CPS I)
- Argininosuccinate synthetase
Correct Answer: Carbamoyl phosphate synthetase I (CPS I)
Q9. What is the allosteric activator required for carbamoyl phosphate synthetase I activity?
- ATP
- Arginine
- N-acetylglutamate (NAG)
- Fumarate
Correct Answer: N-acetylglutamate (NAG)
Q10. Which amino acid is exclusively ketogenic (cannot be converted to glucose)?
- Leucine
- Isoleucine
- Phenylalanine
- Alanine
Correct Answer: Leucine
Q11. Phenylalanine is hydroxylated to tyrosine by which enzyme?
- Phenylalanine hydroxylase
- Tyrosine aminotransferase
- Phenylalanine decarboxylase
- Monoamine oxidase
Correct Answer: Phenylalanine hydroxylase
Q12. A deficiency of phenylalanine hydroxylase leads to accumulation of which metabolite?
- Tyramine
- Phenylpyruvate (phenyl ketones)
- Homogentisate
- Succinyl-CoA
Correct Answer: Phenylpyruvate (phenyl ketones)
Q13. Which disorder results from branched-chain α-ketoacid dehydrogenase deficiency?
- Phenylketonuria (PKU)
- Maple syrup urine disease (MSUD)
- Alkaptonuria
- Homocystinuria
Correct Answer: Maple syrup urine disease (MSUD)
Q14. The trans-sulfuration pathway converts homocysteine to which amino acid?
- Methionine
- Cysteine
- Serine
- Glycine
Correct Answer: Cysteine
Q15. S-adenosylmethionine (SAM) functions primarily as which type of donor in metabolism?
- Acetyl donor
- Methyl donor
- Phosphate donor
- Ammonia donor
Correct Answer: Methyl donor
Q16. Which amino acid provides the one-carbon units for folate-dependent reactions after conversion to N5,N10-methylene-THF?
- Alanine
- Glycine
- Serine
- Tryptophan
Correct Answer: Serine
Q17. Which enzyme converts glutamine to glutamate and free ammonia in peripheral tissues?
- Glutamate dehydrogenase
- Glutamine synthetase
- Glutaminase
- Glutamate decarboxylase
Correct Answer: Glutaminase
Q18. Which amino acid is the immediate precursor for nitric oxide synthesis?
- Arginine
- Ornithine
- Citrulline
- Glutamate
Correct Answer: Arginine
Q19. In the urea cycle, ornithine combines with carbamoyl phosphate to form which product?
- Arginine
- Ornithine transcarbamylate
- Ornithine carbamate
- Citrulline
Correct Answer: Citrulline
Q20. Which reaction type removes an amino group as free ammonia rather than transferring it?
- Transamination
- Reductive amination
- Deamination
- Amidation
Correct Answer: Deamination
Q21. Which amino acid is the major carrier of ammonia in the bloodstream to the liver?
- Alanine
- Glutamate
- Glutamine
- Leucine
Correct Answer: Glutamine
Q22. Which cofactor is required by serine hydroxymethyltransferase for one-carbon metabolism?
- PLP (pyridoxal phosphate)
- Biotin
- FAD
- Thiamine pyrophosphate
Correct Answer: PLP (pyridoxal phosphate)
Q23. Which product from tyrosine catabolism accumulates in alkaptonuria?
- Homogentisic acid
- Fumarylacetoacetate
- Melanin
- S-adenosylmethionine
Correct Answer: Homogentisic acid
Q24. Which enzyme converts phenylalanine to phenylpyruvate in untreated phenylketonuria?
- Phenylalanine hydroxylase
- Transaminase (aminotransferase)
- Phenylalanine decarboxylase
- Tyrosine aminotransferase
Correct Answer: Transaminase (aminotransferase)
Q25. The aminotransferase reaction mechanism involves formation of which intermediate between PLP and amino acid?
- Thioester
- Schiff base (aldimine)
- Carbamate
- Enamine
Correct Answer: Schiff base (aldimine)
Q26. Which amino acids yield acetyl-CoA or acetoacetate and are therefore ketogenic?
- Leucine and lysine
- Glutamate and aspartate
- Alanine and serine
- Glycine and proline
Correct Answer: Leucine and lysine
Q27. Which TCA cycle intermediate is directly formed from the catabolism of aspartate?
- Alpha-ketoglutarate
- Oxaloacetate
- Succinyl-CoA
- Fumarate
Correct Answer: Oxaloacetate
Q28. Which enzyme converts ammonia and glutamate to glutamine, consuming ATP?
- Glutaminase
- Glutamine synthetase
- Glutamate dehydrogenase
- Carbamoyl phosphate synthetase I
Correct Answer: Glutamine synthetase
Q29. Which amino acid is the primary precursor for creatine synthesis?
- Arginine
- Glycine
- Methionine (as SAM)
- All of the above
Correct Answer: All of the above
Q30. In hyperammonemia, which enzyme deficiency causes an X-linked disorder with high orotic acid levels?
- Carbamoyl phosphate synthetase I deficiency
- Ornithine transcarbamylase deficiency
- Arginase deficiency
- Argininosuccinate lyase deficiency
Correct Answer: Ornithine transcarbamylase deficiency
Q31. Which amino acid is converted to pyruvate by serine dehydratase?
- Serine
- Threonine
- Glycine
- Alanine
Correct Answer: Serine
Q32. Which enzyme catalyzes the last step of the urea cycle to release urea?
- Argininosuccinate synthetase
- Argininosuccinate lyase
- Arginase
- Ornithine transcarbamylase
Correct Answer: Arginase
Q33. Which amino acid serves as the nitrogen donor for asparagine synthesis in most tissues?
- Glutamine
- Glutamate
- Ammonia
- Asparagine synthase uses aspartate and glutamine
Correct Answer: Asparagine synthase uses aspartate and glutamine
Q34. Which reaction links amino acid catabolism to gluconeogenesis by producing oxaloacetate?
- Transamination of alanine to pyruvate
- Transamination of aspartate to oxaloacetate
- Oxidative deamination of glutamate to alpha-ketoglutarate
- Decarboxylation of leucine
Correct Answer: Transamination of aspartate to oxaloacetate
Q35. The branched-chain amino acids (leucine, isoleucine, valine) are primarily metabolized in which tissue?
- Liver
- Kidney
- Muscle
- Brain
Correct Answer: Muscle
Q36. Which cofactor is required for branched-chain α-ketoacid dehydrogenase complex activity?
- Biotin
- Thiamine pyrophosphate (TPP)
- PLP
- Folate
Correct Answer: Thiamine pyrophosphate (TPP)
Q37. Which amino acid catabolic product enters the TCA cycle as succinyl-CoA?
- Methionine via propionyl-CoA
- Phenylalanine directly as fumarate
- Leucine as acetyl-CoA
- Alanine as pyruvate
Correct Answer: Methionine via propionyl-CoA
Q38. Which enzyme deficiency leads to elevated homocysteine and risk of vascular disease?
- Cystathionine β-synthase deficiency (homocystinuria)
- Phenylalanine hydroxylase deficiency
- Arginase deficiency
- Ornithine transcarbamylase deficiency
Correct Answer: Cystathionine β-synthase deficiency (homocystinuria)
Q39. Which amino acid is both gluconeogenic and a precursor for heme synthesis?
- Glycine
- Leucine
- Lysine
- Tyrosine
Correct Answer: Glycine
Q40. The enzyme alanine aminotransferase (ALT) catalyzes the reversible transfer between alanine and which keto acid?
- Oxaloacetate
- Alpha-ketoglutarate
- Pyruvate
- Succinyl-CoA
Correct Answer: Pyruvate
Q41. Which amino acid degradation yields fumarate as an intermediate?
- Phenylalanine and tyrosine
- Leucine and lysine
- Methionine and threonine
- Alanine and glycine
Correct Answer: Phenylalanine and tyrosine
Q42. During fasting, increased proteolysis supplies amino acids for gluconeogenesis primarily through which mechanism?
- Direct conversion of amino acids to ketone bodies
- Transamination to form TCA cycle intermediates and pyruvate
- Storage as glycogen
- Conversion to triglycerides for energy
Correct Answer: Transamination to form TCA cycle intermediates and pyruvate
Q43. Which enzyme catalyzes the reversible interconversion of glutamate and glutamine?
- Glutamate dehydrogenase and glutamine synthetase/glutaminase pair
- Alanine aminotransferase only
- Carbamoyl phosphate synthetase I
- Arginase
Correct Answer: Glutamate dehydrogenase and glutamine synthetase/glutaminase pair
Q44. Nitrogen from aspartate enters the urea cycle at which intermediate?
- Carbamoyl phosphate
- Arginase
- Argininosuccinate
- Ornithine
Correct Answer: Argininosuccinate
Q45. Which reaction in amino acid metabolism requires biotin as a cofactor?
- Carboxylation of pyruvate to oxaloacetate
- Transamination of alanine
- Methylation by SAM
- Oxidative deamination of glutamate
Correct Answer: Carboxylation of pyruvate to oxaloacetate
Q46. In liver mitochondria, which amino acid-derived intermediate provides nitrogen for urea synthesis via transamination to glutamate?
- Alanine to pyruvate
- Aspartate to oxaloacetate
- Branched-chain amino acids to branched-chain keto acids
- None; only free ammonia is used
Correct Answer: Aspartate to oxaloacetate
Q47. Which enzyme deficiency causes accumulation of fumarylacetoacetate leading to liver failure and dark urine?
- Homogentisate oxidase deficiency (alkaptonuria)
- Fumarylacetoacetate hydrolase deficiency (tyrosinemia type I)
- Tyrosine aminotransferase deficiency
- Phenylalanine hydroxylase deficiency
Correct Answer: Fumarylacetoacetate hydrolase deficiency (tyrosinemia type I)
Q48. Which amino acid is the precursor for porphyrin (heme) synthesis along with succinyl-CoA?
- Glycine
- Glutamate
- Cysteine
- Proline
Correct Answer: Glycine
Q49. Which pathway regenerates tetrahydrofolate derivatives used for nucleotide and amino acid metabolism?
- Folate cycle (one-carbon metabolism)
- Urea cycle
- Glycolysis
- Fatty acid β-oxidation
Correct Answer: Folate cycle (one-carbon metabolism)
Q50. Which amino acid is converted to oxaloacetate via transamination and can be used for gluconeogenesis?
- Alanine
- Aspartate
- Leucine
- Lysine
Correct Answer: Aspartate
