Amino Acid Metabolism Practice Questions

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.

Q: Which amino acid is not present in creatine?

Lysine. **Explanation:** The synthesis of **Creatine** (methyl-guanidinoacetate) is a high-yield topic in biochemistry, as it involves the coordination of three specific amino acids across two organs (kidney and liver). **1. Why Lysine is the Correct Answer:** Lysine is not involved in the creatine biosynthetic pathway. While lysine is an essential amino acid involved in carnitine synthesis and collagen cross-linking, it plays no role in the formation of creatine. **2. Analysis of Incorrect Options (The Precursors):** Creatine is synthesized using three specific amino acids: * **Glycine (Option C):** Acts as the backbone. In the kidney, the enzyme *AGAT* transfers an amidino group to glycine to form guanidinoacetate (GAA). * **Arginine (Option A):** Serves as the donor of the amidino group during the first step of synthesis. * **Methionine (Option B):** Specifically in its active form, **S-adenosylmethionine (SAM)**, it provides the methyl group required to convert guanidinoacetate into creatine in the liver. **Clinical Pearls for NEET-PG:** * **Site of Synthesis:** Starts in the **Kidney** (formation of GAA) and is completed in the **Liver** (methylation). * **Storage:** 95% of creatine is stored in skeletal muscle as **Creatine Phosphate**, a high-energy reservoir used to regenerate ATP during the first few seconds of intense exercise. * **Excretion:** Creatinine (the anhydride form) is excreted in the urine. Its excretion rate is constant and proportional to total muscle mass, making it a reliable marker for GFR. * **Mnemonic:** Remember **"GAM"** (Glycine, Arginine, Methionine) to recall the three precursors.

Q: Which of the following amino acids cannot be used for glycogen synthesis?

Leucine. ### Explanation The ability of an amino acid to contribute to glycogen synthesis depends on whether its carbon skeleton can be converted into **glucose** (Gluconeogenesis). **1. Why Leucine is the Correct Answer:** Amino acids are classified as glucogenic, ketogenic, or both. **Leucine and Lysine** are the only two **purely ketogenic** amino acids. Their catabolism yields Acetyl-CoA or Acetoacetate, which enters the TCA cycle but results in the loss of two carbons as $CO_2$ before reaching Oxaloacetate. Consequently, they cannot provide a net synthesis of glucose or contribute to glycogen stores. **2. Analysis of Incorrect Options:** * **Alanine (Option A):** The most important glucogenic amino acid. It undergoes transamination to form **Pyruvate**, a direct precursor for gluconeogenesis via the Glucose-Alanine cycle. * **Threonine (Option B):** A glucogenic and ketogenic amino acid. It can be converted into **Pyruvate** or **Succinyl-CoA**, both of which can enter the gluconeogenic pathway. * **Methionine (Option D):** A purely glucogenic amino acid. Its catabolism leads to the formation of **Propionyl-CoA**, which is converted to **Succinyl-CoA**, a TCA cycle intermediate used for glucose synthesis. **3. NEET-PG High-Yield Pearls:** * **Purely Ketogenic:** Leucine and Lysine (Mnemonic: The "L"s). * **Both Glucogenic & Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, Isoleucine (Mnemonic: PITTT). * **Purely Glucogenic:** All other 14 amino acids. * **Key Concept:** Acetyl-CoA cannot be converted back to Pyruvate in humans because the Pyruvate Dehydrogenase reaction is irreversible; this is why ketogenic substrates cannot form glucose.

Q: Brain damage in phenylketonuria is due to the accumulation of which substance?

Phenylalanine. **Explanation:** **Phenylketonuria (PKU)** is an autosomal recessive disorder caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)** or its cofactor, **Tetrahydrobiopterin (BH4)**. This enzyme normally converts Phenylalanine into Tyrosine. **Why Phenylalanine is the correct answer:** The primary cause of brain damage in PKU is the **toxic accumulation of Phenylalanine** in the blood and brain. High levels of Phenylalanine saturate the Large Neutral Amino Acid Transporter (LAT1) at the blood-brain barrier. This competitively inhibits the transport of other essential amino acids (like Tyrosine and Tryptophan) into the brain, leading to: 1. **Defective Myelination:** Phenylalanine interferes with cholesterol synthesis and myelin formation. 2. **Neurotransmitter Deficiency:** Reduced brain levels of Tyrosine and Tryptophan lead to decreased synthesis of Dopamine, Norepinephrine, and Serotonin. **Why other options are incorrect:** * **Tyrosine:** In PKU, Tyrosine becomes an **essential amino acid** because it cannot be synthesized from Phenylalanine. Its levels are typically low or normal, not elevated. * **Tryptophan:** Tryptophan levels are not elevated; in fact, its entry into the brain is blocked by high Phenylalanine, contributing to pathology. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Intellectual disability, "Mousy" or "Musty" body odor (due to Phenylacetate), and hypopigmentation (fair skin/blue eyes) due to decreased melanin. * **Diagnosis:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Management:** Dietary restriction of Phenylalanine and supplementation of Tyrosine. Aspartame (artificial sweetener) must be avoided as it contains Phenylalanine. * **Maternal PKU:** If a mother with PKU doesn't maintain a strict diet during pregnancy, the fetus may develop microcephaly and congenital heart defects.

Q: Serine is produced in humans from which precursor molecule?

Glycine. **Explanation:** **1. Why Glycine is Correct:** Serine and glycine are interconvertible through a reversible reaction catalyzed by the enzyme **Serine Hydroxymethyltransferase (SHMT)**. This reaction requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor and **N5, N10-Methylene Tetrahydrofolate (THF)** as the one-carbon donor/acceptor. In this pathway, glycine receives a hydroxymethyl group to form serine. Additionally, serine can be synthesized de novo from **3-phosphoglycerate** (an intermediate of glycolysis), but among the given options, glycine is the direct precursor. **2. Why the Other Options are Incorrect:** * **B. Methionine:** This is an essential sulfur-containing amino acid. While it provides the sulfur atom for cysteine synthesis, it is not a direct precursor for serine. * **C. Homocysteine:** This is an intermediate in the methionine cycle. It reacts with serine to form cystathionine (via cystathionine β-synthase), meaning serine is a *reactant* used to metabolize homocysteine, not a product derived from it. * **D. Homoserine:** This is an intermediate in the synthesis of threonine and methionine in plants and bacteria; it is not a precursor for serine in human metabolism. **3. NEET-PG High-Yield Pearls:** * **Cofactor Alert:** Always remember that SHMT requires **Vitamin B6**. Deficiency can impair the interconversion of these amino acids. * **One-Carbon Pool:** This reaction is a major entry point for one-carbon units into the folate pool (converting THF to Methylene-THF). * **Glucogenic Status:** Both serine and glycine are **purely glucogenic** amino acids as they can ultimately be converted to pyruvate. * **Special Product:** Serine is a crucial precursor for the synthesis of **sphingosine, phospholipids (phosphatidylserine), and cysteine.**

Q: Which of the following inborn errors of metabolism presents with porphyria-like clinical features?

Tyrosinemia type I. **Explanation:** **Tyrosinemia Type I** (Hepatorenal Tyrosinemia) is caused by a deficiency of the enzyme **Fumarylacetoacetate Hydrolase (FAH)**. This deficiency leads to the accumulation of fumarylacetoacetate, which is converted into **Succinylacetone**. Succinylacetone is a potent inhibitor of **ALA Dehydratase** (Porphobilinogen Synthase), an enzyme in the heme synthesis pathway. This inhibition mimics **ADP (ALA Dehydratase Deficiency) Porphyria**, leading to the accumulation of delta-aminolevulinic acid (ALA). Consequently, patients present with "porphyria-like" neurological crises, including abdominal pain and peripheral neuropathy. **Analysis of Incorrect Options:** * **Phenylketonuria (PKU):** Caused by Phenylalanine Hydroxylase deficiency. It presents with intellectual disability, seizures, and a "mousy" odor, but does not interfere with heme synthesis. * **Alkaptonuria:** Caused by Homogentisate Oxidase deficiency. It presents with ochronosis (darkening of tissues), dark urine on standing, and arthritis, but no porphyria-like symptoms. * **Metachromatic Leukodystrophy:** A lysosomal storage disorder (Arylsulfatase A deficiency). While it involves neurological regression, the pathology is due to sulfatide accumulation in myelin, not heme pathway inhibition. **NEET-PG High-Yield Pearls:** * **Pathognomonic Marker:** Succinylacetone in urine is diagnostic for Tyrosinemia Type I. * **Clinical Triad:** Liver failure (cirrhosis/HCC), renal tubular dysfunction (Fanconi syndrome), and porphyric crises. * **Treatment:** **Nitisinone (NTBC)**, which inhibits 4-hydroxyphenylpyruvate dioxygenase to prevent the formation of toxic succinylacetone.

Question 1

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 2

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 3

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

Accepted Answer

PENMT

Answer

COMT

Answer

MAO

Answer

SAM

Question 4

Which amino acid is not present in creatine?

Accepted Answer

Lysine

Answer

Arginine

Answer

Methionine

Answer

Glycine

Question 5

Which of the following amino acids cannot be used for glycogen synthesis?

Accepted Answer

Leucine

Answer

Alanine

Answer

Threonine

Answer

Methionine

Question 6

Brain damage in phenylketonuria is due to the accumulation of which substance?

Accepted Answer

Phenylalanine

Answer

Tyrosine

Answer

Tryptophan

Answer

None of the above

Question 7

Serine is produced in humans from which precursor molecule?

Accepted Answer

Glycine

Answer

Methionine

Answer

Homocysteine

Answer

Homoserine

Question 8

Arrange the steps of the formation of creatinine from glycine in sequence.

Accepted Answer

Formation of guanidino acetic acid from arginine and glycine -> Formation of creatine by methyl transferase -> Transport of creatine from liver to muscle -> Formation of creatine phosphate -> Conversion of creatine phosphate to creatinine

Answer

Transport of creatine from liver to muscle -> Formation of creatine phosphate -> Formation of creatine by methyl transferase -> Conversion of creatine phosphate to creatinine -> Formation of guanidino acetic acid from arginine and glycine

Answer

Formation of creatine phosphate -> Transport of creatine from liver to muscle -> Conversion of creatine phosphate to creatinine -> Formation of creatine by methyl transferase -> Formation of guanidino acetic acid from arginine and glycine

Answer

Formation of creatine by methyl transferase -> Formation of creatine phosphate -> Formation of guanidino acetic acid from arginine and glycine -> Conversion of creatine phosphate to creatinine -> Transport of creatine from liver to muscle

Question 9

Which of the following inborn errors of metabolism presents with porphyria-like clinical features?

Accepted Answer

Tyrosinemia type I

Answer

Phenylketonuria

Answer

Alkaptonuria

Answer

Metachromatic leukodystrophy

Question 10

Cysteine is considered a non-essential amino acid only in the presence of dietary?

Accepted Answer

Methionine

Answer

Serine

Answer

Folate

Answer

Phenylalanine

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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