Amino Acid Metabolism Practice Questions

Q: Vanillylmandelic acid (VMA) is a metabolite of:

Adrenaline. **Explanation:** **Why Adrenaline is correct:** Vanillylmandelic acid (VMA) is the end-stage metabolic byproduct of **catecholamines**, specifically **Adrenaline (Epinephrine)** and **Noradrenaline (Norepinephrine)**. The metabolic pathway involves two key enzymes: **COMT** (Catechol-O-methyltransferase) and **MAO** (Monoamine oxidase). Adrenaline is first converted to Metanephrine and subsequently oxidized to VMA, which is then excreted in the urine. **Why the other options are incorrect:** * **Thyroxine (T4):** This is a thyroid hormone derived from Tyrosine. Its metabolism involves deiodination (conversion to T3 or rT3) and conjugation in the liver, not VMA production. * **Serotonin:** The major end-metabolite of serotonin (5-hydroxytryptamine) is **5-HIAA** (5-Hydroxyindoleacetic acid). Elevated 5-HIAA is a marker for Carcinoid syndrome. * **Growth Hormone:** This is a peptide hormone. Like all proteins, it is degraded into its constituent amino acids by proteolysis. **Clinical Pearls for NEET-PG:** 1. **Pheochromocytoma:** Urinary VMA levels are measured in a **24-hour urine collection** to diagnose Pheochromocytoma (a catecholamine-secreting tumor of the adrenal medulla). 2. **Neuroblastoma:** VMA is also a critical tumor marker for Neuroblastoma in children. 3. **Dietary Restrictions:** Before a VMA test, patients must avoid "VMA-rich" foods like vanilla, chocolate, coffee, and bananas, as they can cause false-positive results. 4. **Metanephrines:** Note that urinary metanephrines are generally considered more sensitive than VMA for screening purposes.

Q: Which of the following amino acids is involved in the synthesis of thyroxine?

Tyrosine. **Explanation:** **Tyrosine** is the correct answer because it serves as the direct structural precursor for thyroid hormones (**T3 and T4/Thyroxine**). Within the thyroid follicle, tyrosine residues on the protein **thyroglobulin** undergo iodination (forming Monoiodotyrosine and Diiodotyrosine) and subsequent coupling to produce thyroxine. **Analysis of Options:** * **Tyrosine (Correct):** Beyond thyroxine, it is also the precursor for **Catecholamines** (Dopamine, Norepinephrine, Epinephrine) and **Melanin**. * **Glycine (Incorrect):** While a versatile precursor, it is involved in the synthesis of **Heme, Creatine, Purines, and Glutathione**, but not thyroid hormones. * **Methionine (Incorrect):** An essential sulfur-containing amino acid primarily involved in **methylation reactions** (via S-adenosylmethionine/SAM) and the initiation of protein synthesis. * **Threonine (Incorrect):** An essential amino acid primarily involved in the structure of O-linked glycoproteins and mucins; it does not contribute to hormone synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Precursor Rule:** Remember the "T" rule: **Tyrosine** leads to **T**hyroxine and **T**hree catecholamines. * **Enzyme Deficiency:** A defect in the conversion of Phenylalanine to Tyrosine (via Phenylalanine Hydroxylase) leads to **Phenylketonuria (PKU)**. * **Iodination:** The enzyme **Thyroid Peroxidase (TPO)** is responsible for both the iodination of tyrosine residues and the coupling reaction. * **Other Precursors:** * **Tryptophan:** Precursor for Serotonin, Melatonin, and Niacin (B3). * **Histidine:** Precursor for Histamine. * **Arginine:** Precursor for Nitric Oxide (NO) and Urea.

Q: Serotonin is a derivative of which amino acid?

Tryptophan. **Explanation:** **1. Why Tryptophan is correct:** Serotonin (5-hydroxytryptamine or 5-HT) is synthesized from the essential amino acid **Tryptophan**. The synthesis occurs in a two-step pathway: * **Step 1:** Tryptophan is hydroxylated to 5-hydroxytryptophan by the enzyme *Tryptophan hydroxylase* (the rate-limiting step), which requires **Tetrahydrobiopterin ($BH_4$)** as a cofactor. * **Step 2:** 5-hydroxytryptophan is decarboxylated to Serotonin by *Aromatic L-amino acid decarboxylase*, requiring **Pyridoxal Phosphate (Vitamin $B_6$)**. **2. Why the other options are incorrect:** * **Tyrosine:** This is the precursor for Catecholamines (Dopamine, Norepinephrine, Epinephrine), Thyroid hormones ($T_3, T_4$), and Melanin. * **Phenylalanine:** This is the precursor for Tyrosine. A deficiency in the enzyme converting Phenylalanine to Tyrosine leads to Phenylketonuria (PKU). * **Melatonin:** This is not an amino acid; it is a hormone synthesized **from** Serotonin in the pineal gland. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Carcinoid Syndrome:** Tumors (usually in the ileum) secrete excessive serotonin. This "diverts" tryptophan from Niacin synthesis, leading to **Pellagra-like symptoms** (Dermatitis, Diarrhea, Dementia). * **Metabolism:** Serotonin is degraded by Monoamine Oxidase (MAO) into **5-HIAA** (5-Hydroxyindoleacetic acid), which is measured in 24-hour urine samples to diagnose Carcinoid syndrome. * **Melatonin Synthesis:** Tryptophan $\rightarrow$ Serotonin $\rightarrow$ Melatonin. * **Niacin Synthesis:** Tryptophan is also a precursor for $NAD^+/NADP^+$ (60 mg of Tryptophan = 1 mg of Niacin).

Q: Increased levels of alanine in serum after fasting suggest what?

Increased release of alanine from muscle. ### Explanation **1. Why Option A is Correct: The Glucose-Alanine Cycle (Cahill Cycle)** During fasting, the body shifts to a catabolic state to maintain blood glucose levels. Skeletal muscle begins to break down proteins into amino acids. While many amino acids are oxidized locally, **alanine** and **glutamine** serve as the primary carriers of nitrogen to the liver. In the muscle, pyruvate (from glycolysis or amino acid carbon skeletons) undergoes transamination to form alanine. This alanine is released into the bloodstream and transported to the liver, where it serves as a major substrate for **gluconeogenesis**. Therefore, increased serum alanine during fasting is a direct physiological marker of increased muscle proteolysis and the operation of the Cahill Cycle. **2. Why the Other Options are Incorrect:** * **Option B:** During fasting, amino acid utilization for gluconeogenesis is actually **increased**, not reduced, to prevent hypoglycemia. * **Option C:** While membrane leakage can occur in pathological states (like rhabdomyolysis), the rise of alanine in fasting is a regulated metabolic response, not a result of structural cell damage. * **Option D:** In a fasting state, the liver **increases** its uptake of alanine to convert it into glucose. Decreased uptake would impair glucose homeostasis. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **The Cahill Cycle:** Muscle sends Alanine to the Liver $\rightarrow$ Liver converts Alanine to Glucose $\rightarrow$ Glucose returns to Muscle. * **Key Enzyme:** **ALT (Alanine Transaminase)** requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. * **Nitrogen Transport:** Alanine is the primary transporter of nitrogen from muscle to liver; **Glutamine** is the primary transporter from most other peripheral tissues. * **Glucogenic vs. Ketogenic:** Alanine is a purely **glucogenic** amino acid.

Q: Which enzyme is involved in the cleavage of glycine in liver mitochondria?

Dihydrolipoyl dehydrogenase. ### Explanation The cleavage of glycine in liver mitochondria is catalyzed by the **Glycine Cleavage System (GCS)**, also known as the glycine synthase complex. This multienzyme complex is structurally and functionally similar to the Pyruvate Dehydrogenase (PDH) and α-Ketoglutarate Dehydrogenase complexes. **Why Dihydrolipoyl dehydrogenase is correct:** The GCS consists of four protein components: 1. **P-protein:** A pyridoxal phosphate-dependent decarboxylase. 2. **H-protein:** A lipoic acid-containing protein. 3. **T-protein:** A tetrahydrofolate-dependent aminomethyltransferase. 4. **L-protein (Dihydrolipoyl dehydrogenase):** This enzyme is responsible for the re-oxidation of the dihydrolipoyl group on the H-protein, using NAD+ as an electron acceptor. Since the L-protein is identical to the E3 subunit found in the PDH complex, **Dihydrolipoyl dehydrogenase** is the correct enzyme involved in this process. **Why other options are incorrect:** * **Pyruvate dehydrogenase (Option A):** This is the E1 subunit of the PDH complex specifically involved in the decarboxylation of pyruvate, not glycine. * **Dihydrolipoyl transacetylase (Option B):** This is the E2 subunit of the PDH complex. While the GCS has a similar lipoic acid-bearing protein (H-protein), it does not involve a transacetylase reaction. **Clinical Pearls for NEET-PG:** * **Non-ketotic Hyperglycinemia:** A deficiency in any of the GCS components (most commonly the P-protein) leads to high glycine levels in the blood and CSF, causing severe neurological distress and seizures in neonates. * **Common Subunit:** Remember that **Dihydrolipoyl dehydrogenase (E3)** is a shared component among three major complexes: PDH, α-Ketoglutarate Dehydrogenase, and Branched-chain α-keto acid Dehydrogenase. * **Cofactors:** The GCS requires PLP, Lipoic acid, THF, and NAD+.

Q: Tetrahydrobiopterin is used in the management of which amino acid defect?

Phenylalanine. **Explanation:** **Tetrahydrobiopterin (BH4)** is a vital cofactor for several hydroxylase enzymes. Its primary clinical application is in the management of **Phenylketonuria (PKU)**, specifically cases involving **Phenylalanine Hydroxylase (PAH)** deficiency or defects in BH4 synthesis/regeneration. 1. **Why Phenylalanine is Correct:** The enzyme **Phenylalanine Hydroxylase** converts Phenylalanine to Tyrosine. This reaction requires BH4 as a co-substrate. In "Sapropterin-responsive PKU," pharmacological doses of BH4 (Sapropterin) can enhance the residual activity of a mutated PAH enzyme, effectively lowering blood phenylalanine levels. Furthermore, in rare cases of **Malignant PKU** (caused by DHPR deficiency), BH4 supplementation is the mainstay of treatment. 2. **Why Other Options are Incorrect:** * **Alanine:** Alanine undergoes transamination via ALT (requiring Vitamin B6/PLP), not hydroxylation requiring BH4. * **Tyrosine:** While Tyrosine hydroxylase requires BH4 to produce DOPA, "Tyrosinemia" (the primary defect of tyrosine metabolism) is managed by dietary restriction or Nitisinone, not BH4. * **Tryptophan:** Tryptophan hydroxylase (for serotonin synthesis) does require BH4; however, BH4 is not used as a standard clinical management strategy for primary tryptophan metabolic defects in the same way it is for PKU. **High-Yield Clinical Pearls for NEET-PG:** * **BH4-dependent enzymes:** Phenylalanine hydroxylase, Tyrosine hydroxylase, Tryptophan hydroxylase, and Nitric Oxide Synthase (NOS). * **Malignant PKU:** Caused by a deficiency of **Dihydropteridine reductase (DHPR)**. Unlike classic PKU, it presents with neurological deterioration despite a phenylalanine-restricted diet because neurotransmitter synthesis (Dopamine/Serotonin) is also impaired. * **Guthrie Test:** A classic bacterial inhibition assay used for neonatal screening of PKU.

Q: Which cofactor is involved in the metabolism of sulfur-containing amino acids?

Vitamin B12. **Explanation:** The metabolism of sulfur-containing amino acids (Methionine and Cysteine) is heavily dependent on the **Methionine Cycle**. The correct answer is **Vitamin B12 (Cobalamin)** because it acts as an essential cofactor for the enzyme **Methionine Synthase**. This enzyme catalyzes the remethylation of Homocysteine back to Methionine, utilizing Methyl-tetrahydrofolate as a donor. A deficiency in B12 leads to the "Methyl-folate trap," resulting in secondary folate deficiency and hyperhomocysteinemia. **Analysis of Incorrect Options:** * **A. Folic Acid:** While Folate (as N5-methyl THF) is a co-substrate in this reaction, Vitamin B12 is the specific prosthetic group/cofactor required for the enzyme's catalytic activity. * **B. Biotin (B7):** Involved in carboxylation reactions (e.g., Pyruvate carboxylase, Acetyl-CoA carboxylase). It is not directly involved in the sulfur amino acid pathway. * **C. Vitamin B1 (Thiamine):** Acts as a cofactor for oxidative decarboxylation (e.g., Pyruvate dehydrogenase, Alpha-ketoglutarate dehydrogenase). **High-Yield Clinical Pearls for NEET-PG:** * **The "Triple B" Rule:** Metabolism of sulfur amino acids (specifically Homocysteine) requires three B-vitamins: **B6** (for Cystathionine $\beta$-synthase), **B9** (Folate), and **B12**. * **Homocystinuria:** Deficiency of Vitamin B12 or the enzyme Cystathionine $\beta$-synthase leads to elevated homocysteine, which is a significant risk factor for early-onset atherosclerosis and venous thrombosis. * **Propionyl-CoA Pathway:** Vitamin B12 is also a cofactor for **Methylmalonyl-CoA mutase**, which converts Methylmalonyl-CoA to Succinyl-CoA (the catabolic pathway for Isoleucine, Valine, Threonine, and Methionine).

Q: Transfer of the carbamoyl moiety of carbamoyl phosphate to ornithine is catalysed by a liver mitochondrial enzyme?

Ornithine transcarbamoylase. ### Explanation The question describes the second step of the **Urea Cycle**, which occurs within the **mitochondrial matrix** of hepatocytes. **Correct Option: B. Ornithine transcarbamoylase (OTC)** The enzyme **Ornithine transcarbamoylase** catalyzes the transfer of the carbamoyl group from carbamoyl phosphate to the amino acid ornithine. This reaction produces **citrulline**, which is then transported out of the mitochondria into the cytosol. This is a critical step in the disposal of toxic ammonia. **Analysis of Incorrect Options:** * **A. Carbamoyl phosphate synthetase I (CPS-I):** This is the rate-limiting enzyme of the urea cycle. It catalyzes the formation of carbamoyl phosphate from $NH_4^+$, $CO_2$, and 2 ATP. It does not transfer the moiety to ornithine. * **C. N-acetyl glutamate synthetase (NAGS):** This enzyme produces N-acetylglutamate (NAG) from acetyl-CoA and glutamate. NAG is an essential allosteric activator of CPS-I, not a catalyst for the ornithine reaction. * **D. N-acetyl glutamate hydrolase:** This enzyme breaks down NAG; it is not involved in the primary steps of the urea cycle. **High-Yield Clinical Pearls for NEET-PG:** * **OTC Deficiency:** This is the **most common** urea cycle disorder. Unlike other urea cycle enzyme deficiencies (which are autosomal recessive), OTC deficiency is **X-linked recessive**. * **Biochemical Marker:** In OTC deficiency, carbamoyl phosphate accumulates and is diverted to the pyrimidine synthesis pathway, leading to increased levels of **Orotic acid** in the urine (Orotic aciduria). * **Localization:** Remember that the first two steps (CPS-I and OTC) occur in the **mitochondria**, while the remaining steps occur in the **cytosol** (Mnemonic: **CO** = **C**PS-I and **O**TC are in the mitochondria).

Q: Which of the following statements is TRUE regarding a 3-week-old child with phenylketonuria, EXCEPT?

Phenylalanine should be completely stopped in the diet.. **Explanation:** **Why Option D is the correct answer (The Exception):** Phenylalanine is an **essential amino acid** required for normal growth, protein synthesis, and tissue repair. In Phenylketonuria (PKU), the goal is to **restrict** phenylalanine intake to maintain blood levels within a safe range (2–6 mg/dL), not to eliminate it entirely. Complete removal from the diet would lead to severe protein deficiency, growth failure, and even death. **Analysis of other options:** * **Option A:** Provocative protein meal tests (or phenylalanine loading tests) can be used to differentiate between classical PKU and transient hyperphenylalaninemia by observing the rate of clearance. * **Option B:** In PKU, the enzyme **Phenylalanine Hydroxylase (PAH)** is deficient. Since tyrosine is normally synthesized from phenylalanine via this enzyme, it becomes a **conditionally essential amino acid** that must be supplemented in the diet. * **Option C:** Due to the metabolic block, phenylalanine accumulates in the blood (Hyperphenylalaninemia). It is then diverted to alternative pathways, forming phenylketones like **phenylpyruvate**, which is excreted in the urine (giving it a characteristic "mousy" odor). **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficiency:** Most commonly Phenylalanine Hydroxylase; rarely **Dihydropteridine reductase** (malignant PKU). * **Cofactor:** Tetrahydrobiopterin (**BH4**). * **Clinical Features:** Intellectual disability, seizures, "mousy" body odor, and hypopigmentation (due to decreased melanin synthesis from tyrosine). * **Screening:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Management:** "Diet for life" (low phenylalanine) and avoiding **Aspartame** (which contains phenylalanine).

Q: Which enzyme is responsible for the conversion of norepinephrine to epinephrine?

PENMT. **Explanation:** The conversion of norepinephrine to epinephrine is the final step in the catecholamine biosynthetic pathway. **1. Why PENMT is correct:** The enzyme **Phenylethanolamine N-methyltransferase (PNMT)** catalyzes the transfer of a methyl group from S-adenosylmethionine (SAM) to norepinephrine to form epinephrine. This reaction occurs primarily in the **adrenal medulla**. A key regulatory feature is that **cortisol** (transported via the intra-adrenal portal system) induces the expression of PNMT, ensuring high epinephrine production during stress. **2. Why the other options are incorrect:** * **COMT (Catechol-O-methyltransferase):** This enzyme is involved in the **degradation** (metabolism) of catecholamines, not their synthesis. It transfers a methyl group to the hydroxyl group of the catechol ring. * **MAO (Monoamine Oxidase):** This enzyme is also involved in the **degradation** of catecholamines via oxidative deamination. * **SAM (S-adenosylmethionine):** While SAM is required for this reaction, it is a **co-factor (methyl donor)**, not the enzyme itself. **Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Tyrosine hydroxylase (converts Tyrosine to DOPA). * **Vitamin C requirement:** Dopamine $\beta$-hydroxylase (converts Dopamine to Norepinephrine) requires Vitamin C and Copper. * **Localization:** PNMT is found in the cytosol; therefore, norepinephrine must exit the chromaffin granules to be methylated and then re-enter for storage. * **Pheochromocytoma:** Tumors arising outside the adrenal gland (paragangliomas) often lack PNMT and thus secrete norepinephrine rather than epinephrine.

Question 1

Vanillylmandelic acid (VMA) is a metabolite of:

Accepted Answer

Adrenaline

Answer

Thyroxine

Answer

Serotonin

Answer

Growth hormone

Question 2

Which of the following amino acids is involved in the synthesis of thyroxine?

Accepted Answer

Tyrosine

Answer

Glycine

Answer

Methionine

Answer

Threonine

Question 3

Serotonin is a derivative of which amino acid?

Accepted Answer

Tryptophan

Answer

Tyrosine

Answer

Phenylalanine

Answer

Melatonin

Question 4

Increased levels of alanine in serum after fasting suggest what?

Accepted Answer

Increased release of alanine from muscle

Answer

Reduced amino acid utilization for gluconeogenesis

Answer

Break in continuity of plasma membrane resulting in leakage of amino acids

Answer

Decreased uptake of alanine by liver

Question 5

Which enzyme is involved in the cleavage of glycine in liver mitochondria?

Accepted Answer

Dihydrolipoyl dehydrogenase

Answer

Pyruvate dehydrogenase

Answer

Dihydrolipoyl transacetylase

Answer

None of these

Question 6

Tetrahydrobiopterin is used in the management of which amino acid defect?

Accepted Answer

Phenylalanine

Answer

Alanine

Answer

Tyrosine

Answer

Tryptophan

Question 7

Which cofactor is involved in the metabolism of sulfur-containing amino acids?

Accepted Answer

Vitamin B12

Answer

Folic acid

Answer

Biotin

Answer

Vitamin B1

Question 8

Transfer of the carbamoyl moiety of carbamoyl phosphate to ornithine is catalysed by a liver mitochondrial enzyme?

Accepted Answer

Ornithine transcarbamoylase

Answer

Carbamoyl phosphate synthetase

Answer

N-acetyl glutamate synthetase

Answer

N-acetyl glutamate hydrolase

Question 9

Which of the following statements is TRUE regarding a 3-week-old child with phenylketonuria, EXCEPT?

Accepted Answer

Phenylalanine should be completely stopped in the diet.

Answer

Provocative protein meal tests help in the diagnosis.

Answer

Tyrosine becomes an essential amino acid in the diet.

Answer

Serum phenylalanine is increased and urinary phenyl pyruvate level is elevated.

Question 10

Which enzyme is responsible for the conversion of norepinephrine to epinephrine?

Accepted Answer

PENMT

Answer

COMT

Answer

MAO

Answer

SAM

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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