Amino Acid Metabolism Practice Questions

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.

Q: Which of the following statements is true regarding the management of Phenylketonuria?

Limiting the substrate for the deficient enzyme. **Explanation:** **Phenylketonuria (PKU)** is an autosomal recessive disorder caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**. This enzyme normally converts the amino acid Phenylalanine into Tyrosine. **1. Why Option A is Correct:** In PKU, the metabolic block leads to the toxic accumulation of **Phenylalanine (the substrate)** in the blood and brain. High levels of phenylalanine are neurotoxic and lead to intellectual disability. Therefore, the primary management strategy is **limiting the substrate** by providing a diet low in Phenylalanine. This prevents the buildup of toxic metabolites like phenylpyruvate and phenyllactate. **2. Analysis of Incorrect Options:** * **Option B:** While Tyrosine becomes an "essential" amino acid in PKU and is supplemented, the statement "supplementing missing amino acids" is too vague. The hallmark of management is restriction, not just supplementation. * **Option C:** Although research is ongoing, **gene therapy** is not yet a standard or "successfully usable" clinical treatment for PKU in routine practice. * **Option D:** While regular cow's milk is high in phenylalanine and must be restricted, the child **cannot completely avoid milk**. Infants require specialized Phenylalanine-free medical formulas to ensure they receive adequate protein for growth. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mousy/Musty Odor:** Due to phenylacetic acid in sweat and urine. * **Hypopigmentation:** Phenylalanine inhibits Tyrosinase, leading to fair skin and blue eyes. * **Guthrie Test:** A bacterial inhibition assay used for neonatal screening. * **Maternal PKU:** If a pregnant woman with PKU doesn't maintain a strict diet, the high phenylalanine levels act as a **teratogen**, causing microcephaly and congenital heart defects in the fetus. * **Sapropterin:** A synthetic form of BH4 (cofactor) used in BH4-responsive variants of PKU.

Q: Which of the following is a semi-essential amino acid?

Histidine. **Explanation:** Amino acids are classified based on their nutritional requirement into essential, non-essential, and semi-essential categories. **Why Histidine is the correct answer:** **Histidine** (along with Arginine) is classified as a **semi-essential (or conditionally essential)** amino acid. While the adult body can synthesize sufficient amounts to maintain nitrogen balance, the rate of synthesis is inadequate during periods of rapid growth, such as infancy, childhood, pregnancy, or recovery from major trauma. Therefore, dietary intake is mandatory during these specific physiological states to meet the increased demand. **Analysis of Incorrect Options:** * **A. Tyrosine:** This is a **non-essential** amino acid because it can be synthesized in the body from the essential amino acid Phenylalanine (via phenylalanine hydroxylase). * **C. Alanine:** This is a **non-essential** amino acid. It is easily synthesized via the transamination of pyruvate, a product of glycolysis. * **D. Isoleucine:** This is a strictly **essential** amino acid. It cannot be synthesized by the human body and must be obtained entirely through the diet. **High-Yield NEET-PG Pearls:** * **Mnemonic for Essential Amino Acids:** "PVT TIM HALL" (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, **Histidine**, **Arginine**, Leucine, Lysine). Note that **H** and **A** are the semi-essential ones. * **Histidine Precursor:** It is the precursor for **Histamine** (via decarboxylation), a key mediator in allergic reactions and gastric acid secretion. * **Clinical Correlation:** Histidine levels are used in the **FIGLU excretion test** to diagnose Folic Acid deficiency.

Question 1

Which amino acid is not present in creatine?

Accepted Answer

Lysine

Answer

Arginine

Answer

Methionine

Answer

Glycine

Question 2

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

Accepted Answer

Leucine

Answer

Alanine

Answer

Threonine

Answer

Methionine

Question 3

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 4

Serine is produced in humans from which precursor molecule?

Accepted Answer

Glycine

Answer

Methionine

Answer

Homocysteine

Answer

Homoserine

Question 5

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 6

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 7

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

Accepted Answer

Methionine

Answer

Serine

Answer

Folate

Answer

Phenylalanine

Question 8

Which of the following statements is true regarding the management of Phenylketonuria?

Accepted Answer

Limiting the substrate for the deficient enzyme

Answer

Supplementing the missing amino acids through diet

Answer

Gene therapy is a successfully usable treatment

Answer

The child should avoid milk consumption

Question 9

Which of the following is a semi-essential amino acid?

Accepted Answer

Histidine

Answer

Tyrosine

Answer

Alanine

Answer

Isoleucine

Question 10

In the biosynthesis of urea, one nitrogen atom is derived from ammonia while the other nitrogen atom is from:

Accepted Answer

Aspartate

Answer

Glycine

Answer

Alanine

Answer

Glutamate

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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