Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 421: Glycine is useful in the synthesis of all of the following except?

A. Purine synthesis
B. Creatine synthesis
C. Spermine synthesis (Correct Answer)
D. Heme synthesis

Explanation: **Explanation:** Glycine is the simplest non-essential amino acid and serves as a vital precursor for several specialized biomolecules. The correct answer is **Spermine synthesis** because polyamines like spermine and spermidine are synthesized from **methionine** (via S-adenosylmethionine) and **ornithine** (via putrescine), not glycine. **Why the other options are incorrect:** * **Purine Synthesis:** Glycine is a major contributor to the purine ring. It provides the entire **C4, C5, and N7** atoms of the purine nucleus. * **Creatine Synthesis:** Glycine is the first substrate in creatine synthesis. It reacts with Arginine to form guanidinoacetate (catalyzed by AGAT), which is later methylated to form creatine. * **Heme Synthesis:** Glycine is the fundamental building block of heme. It condenses with Succinyl CoA to form **$\delta$-aminolevulinic acid (ALA)**, the rate-limiting step catalyzed by ALA synthase. **High-Yield NEET-PG Clinical Pearls:** 1. **Glutathione:** Glycine is one of the three amino acids (along with Glutamate and Cysteine) required for the synthesis of the antioxidant glutathione. 2. **Conjugation:** Glycine is used for the conjugation of bile acids (Glycocholic acid) and the detoxification of benzoic acid to form **Hippuric acid**. 3. **Inhibitory Neurotransmitter:** Glycine acts as an inhibitory neurotransmitter in the spinal cord and brainstem. 4. **Non-Chiral:** It is the only amino acid that does not have an asymmetric carbon atom and is therefore not optically active.

Question 422: Alkaptonuria, an inherited metabolic disorder, is due to the deficiency of which enzyme?

A. Homogentisate oxidase (Correct Answer)
B. Cystathionase
C. Phenylalanine hydroxylase
D. Tyrosine transaminase

Explanation: **Explanation:** **Alkaptonuria** is an autosomal recessive disorder of phenylalanine and tyrosine metabolism. The correct answer is **Homogentisate oxidase** (also known as homogentisate 1,2-dioxygenase). This enzyme is responsible for converting homogentisic acid into maleylacetoacetic acid. When deficient, homogentisic acid accumulates in the blood and is excreted in the urine. **Why the other options are incorrect:** * **Cystathionase:** Deficiency leads to Cystathioninuria; it is involved in the sulfur-containing amino acid pathway (methionine metabolism). * **Phenylalanine hydroxylase:** Deficiency causes **Phenylketonuria (PKU)**, characterized by intellectual disability and a "mousy" odor. * **Tyrosine transaminase:** Deficiency leads to **Tyrosinemia Type II** (Richner-Hanhart syndrome), which presents with palmoplantar keratosis and corneal ulcers. **Clinical Pearls for NEET-PG:** 1. **Triad of Alkaptonuria:** * **Urine turns black** on standing (due to oxidation and polymerization of homogentisic acid). * **Ochronosis:** Bluish-black pigmentation of connective tissues (sclera, ear cartilage). * **Ochronotic Arthritis:** Large joint arthritis due to pigment deposition in cartilage. 2. **Diagnosis:** Ferric chloride test (urine turns transiently deep blue) or silver nitrate test. 3. **Management:** Low protein diet (restricted Phenylalanine/Tyrosine) and **Nitisinone**, which inhibits 4-hydroxyphenylpyruvate dioxygenase to prevent homogentisic acid formation.

Question 423: Accumulation of homogentisic acid causes which condition?

A. Ochronosis (Correct Answer)
B. Tyrosinemia
C. Albinism
D. Tyrosinosis

Explanation: **Explanation:** The correct answer is **Ochronosis**. This condition is a hallmark clinical feature of **Alkaptonuria**, an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase**. 1. **Why Ochronosis is correct:** In Alkaptonuria, the metabolic breakdown of Tyrosine and Phenylalanine is interrupted, leading to the accumulation of **Homogentisic Acid (HGA)**. When HGA is excreted in urine and exposed to air, it oxidizes to form a black pigment. Within the body, HGA undergoes oxidation and polymerization into a melanin-like substance that deposits in connective tissues (cartilage, sclera, and heart valves). This bluish-black discoloration of tissues is termed **Ochronosis**. 2. **Why other options are incorrect:** * **Tyrosinemia:** Refers to a group of disorders (Type I, II, III) caused by deficiencies in enzymes like Fumarylacetoacetate hydrolase or Tyrosine aminotransferase. While related to the same pathway, they lead to the accumulation of Tyrosine or Succinylacetone, not HGA. * **Albinism:** Caused by a deficiency of the enzyme **Tyrosinase**, leading to a failure in melanin synthesis from Tyrosine. It results in hypopigmentation, the opposite of ochronosis. * **Tyrosinosis:** An older term for Tyrosinemia Type III (deficiency of p-hydroxyphenylpyruvate hydroxylase). **NEET-PG Clinical Pearls:** * **Classic Triad of Alkaptonuria:** 1. Homogentisic aciduria (urine turns black on standing/alkalinization), 2. Ochronosis (pigmentation of ear cartilage/sclera), and 3. Ochronotic arthritis (large joint involvement). * **Diagnosis:** Ferric chloride test (turns urine deep blue/green) or Benedict’s test (gives a brown-black precipitate). * **Management:** Low protein diet and **Nitisinone** (inhibits 4-hydroxyphenylpyruvate dioxygenase to prevent HGA formation).

Question 424: Alkaptonuria, an inherited metabolic disorder, is due to the deficiency of which enzyme?

A. Homogentisate oxidase (Correct Answer)
B. Cystathionase
C. Phenylalanine hydroxylase
D. Tyrosine transaminase

Explanation: **Explanation:** **Alkaptonuria** is an autosomal recessive disorder involving the catabolism of aromatic amino acids (Phenylalanine and Tyrosine). 1. **Why Option A is Correct:** The condition is caused by a deficiency of **Homogentisate oxidase** (also known as homogentisate 1,2-dioxygenase). This enzyme normally converts homogentisic acid (HGA) into maleylacetoacetic acid. When deficient, HGA accumulates in the blood and tissues and is excreted in the urine. Upon exposure to air, HGA undergoes oxidation and polymerization to form a brownish-black pigment. 2. **Why Incorrect Options are Wrong:** * **Option B (Cystathionase):** Deficiency leads to Cystathioninuria, characterized by elevated levels of cystathionine in the urine. * **Option C (Phenylalanine hydroxylase):** Deficiency causes **Phenylketonuria (PKU)**, leading to intellectual disability and a "mousy" body odor. * **Option D (Tyrosine transaminase):** Deficiency leads to **Tyrosinemia Type II** (Richner-Hanhart syndrome), which presents with painful corneal erosions and palmoplantar keratosis. **Clinical Pearls for NEET-PG:** * **The Classic Triad:** 1. Blackening of urine on standing (alkalinization), 2. **Ochronosis** (bluish-black pigmentation of connective tissues like the sclera and ear cartilage), and 3. Large joint **arthritis** (due to pigment deposition in cartilage). * **Diagnosis:** Confirmed by detecting Homogentisic acid in urine using Gas Chromatography-Mass Spectrometry (GC-MS). * **Biochemical Test:** Benedict’s test is positive (HGA is a reducing agent), and the Ferric Chloride test gives a transient deep blue/green color. * **Management:** High doses of Vitamin C (prevents oxidation of HGA) and Nitisinone.

Question 425: HHH syndrome is due to a defect in which of the following?

A. Tryptophan metabolism
B. Histidine transport
C. Branched chain amino acid metabolism
D. Ornithine transport (Correct Answer)

Explanation: **Explanation:** **HHH Syndrome** (Hyperammonemia-Hyperornithinemia-Homocitrullinuria) is an autosomal recessive urea cycle disorder caused by a mutation in the **SLC25A15 gene**. This gene encodes the **mitochondrial ornithine transporter (ORNT1)**, which is responsible for transporting ornithine from the cytosol into the mitochondrial matrix. **Why the correct answer is right:** In the urea cycle, ornithine must enter the mitochondria to combine with carbamoyl phosphate to form citrulline. A defect in the **ornithine transporter** leads to: 1. **Hyperornithinemia:** Ornithine accumulates in the cytosol. 2. **Hyperammonemia:** The urea cycle is stalled due to a lack of mitochondrial ornithine, leading to toxic ammonia buildup. 3. **Homocitrullinuria:** Accumulated carbamoyl phosphate reacts with lysine (instead of ornithine), forming homocitrulline, which is excreted in the urine. **Why the incorrect options are wrong:** * **A. Tryptophan metabolism:** Defects here lead to conditions like Hartnup disease (neutral amino acid transporter defect) or Pellagra-like symptoms. * **B. Histidine transport:** Histidine metabolism defects typically lead to Histidinemia (Histidase deficiency); there is no major clinical "transport" syndrome associated with it in this context. * **C. Branched chain amino acid metabolism:** Defects here lead to **Maple Syrup Urine Disease (MSUD)**, characterized by a burnt sugar odor in urine, not HHH syndrome. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Episodic hyperammonemia, developmental delay, and spastic paraplegia. * **Diagnosis:** Elevated plasma ornithine and urinary homocitrulline. * **Management:** Protein-restricted diet and citrulline/arginine supplementation. * **Key Enzyme Link:** It mimics Ornithine Transcarbamylase (OTC) deficiency but is distinguished by the presence of hyperornithinemia.

Question 426: Which of the following is a key gluconeogenic amino acid?

A. Alanine (Correct Answer)
B. Serine
C. Valine
D. Tyrosine

Explanation: **Explanation:** **Alanine** is the correct answer because it is the primary and most important gluconeogenic amino acid in the body. This is due to the **Cahill Cycle (Glucose-Alanine Cycle)**. During fasting or exercise, muscle protein is broken down; the resulting amino groups are transferred to pyruvate to form Alanine. Alanine is then transported to the liver, where it is deaminated back into pyruvate, serving as a direct substrate for gluconeogenesis. **Analysis of Options:** * **Serine (Option B):** While Serine is a glucogenic amino acid (converted to pyruvate), it is not considered the "key" or primary transporter of carbon skeletons from muscle to liver like Alanine. * **Valine (Option C):** Valine is a purely glucogenic branched-chain amino acid (BCAA) that enters the TCA cycle via Succinyl-CoA. Though important, it is not the predominant gluconeogenic precursor. * **Tyrosine (Option D):** Tyrosine is **both glucogenic and ketogenic**. It is a precursor for catecholamines and thyroid hormones but is not a primary substrate for glucose synthesis. **High-Yield NEET-PG Pearls:** * **Purely Ketogenic Amino Acids:** Leucine and Lysine (The "L"s). They cannot be converted to glucose. * **Both Glucogenic & Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, Isoleucine (Mnemonic: **PITTT**). * **Key Enzyme:** Alanine Aminotransferase (ALT/SGPT) requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor for the interconversion of Alanine and Pyruvate. * **Clinical Fact:** In states of starvation, Alanine is the first amino acid to be depleted from the muscle to maintain blood glucose levels.

Question 427: Which of the following amino acids is not ketogenic?

A. Leucine
B. Lysine
C. Methionine (Correct Answer)
D. Tryptophan

Explanation: **Explanation:** Amino acids are classified based on their metabolic end-products into three categories: **Glucogenic** (converted to glucose), **Ketogenic** (converted to acetyl-CoA or acetoacetate), or **Both**. **1. Why Methionine is the Correct Answer:** Methionine is a **purely glucogenic** amino acid. Its metabolism leads to the formation of **Succinyl-CoA**, a key intermediate of the TCA cycle, which can then be used for gluconeogenesis. It does not produce ketone bodies or acetyl-CoA. **2. Analysis of Incorrect Options:** * **Leucine (A) & Lysine (B):** These are the only two **purely ketogenic** amino acids. They are metabolized directly into acetyl-CoA or acetoacetate and cannot be used to synthesize glucose. * **Tryptophan (D):** This is **both glucogenic and ketogenic**. Its breakdown yields pyruvate (glucogenic) and acetoacetyl-CoA (ketogenic). Other amino acids in this dual category include Isoleucine, Phenylalanine, and Tyrosine (Mnemonic: **PITTT** – Phenylalanine, Isoleucine, Tyrosine, Tryptophan, Threonine). **High-Yield Clinical Pearls for NEET-PG:** * **Purely Ketogenic:** Leucine and Lysine (The "L" amino acids). * **Both Glucogenic & Ketogenic:** Phenylalanine, Isoleucine, Tyrosine, Tryptophan, Threonine. * **Purely Glucogenic:** All remaining 13 amino acids (including Methionine, Valine, and Histidine). * **Clinical Correlation:** In cases of **Maple Syrup Urine Disease (MSUD)**, the metabolism of branched-chain amino acids (Leucine, Isoleucine, Valine) is impaired. Among these, Leucine is the most potent ketogenic amino acid and contributes significantly to neurological symptoms.

Question 428: Which of the following is NOT an essential amino acid?

A. Leucine
B. Lysine
C. Methionine
D. Proline (Correct Answer)

Explanation: ### Explanation **Concept Overview** Amino acids are categorized based on the body's ability to synthesize them. **Essential amino acids (EAA)** cannot be synthesized de novo by the human body and must be obtained through the diet. There are 10 essential amino acids, often remembered by the mnemonic **PVT TIM HALL** (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine). **Why Proline is the Correct Answer** **Proline** is a **non-essential amino acid**. It is synthesized endogenously from glutamate. Structurally, proline is unique as it is an **imino acid** (containing a secondary amino group), which plays a critical role in the formation of the collagen triple helix by causing "kinks" in the polypeptide chain. **Analysis of Incorrect Options** * **Leucine:** A branched-chain amino acid (BCAA) and a strictly ketogenic essential amino acid. It is vital for protein synthesis and muscle repair. * **Lysine:** A strictly ketogenic essential amino acid. Deficiency can lead to impaired growth and reproductive issues. * **Methionine:** A sulfur-containing essential amino acid. It serves as the precursor for S-adenosylmethionine (SAM), the body's primary methyl donor, and is the first amino acid incorporated into every protein chain (encoded by the start codon AUG). **NEET-PG High-Yield Pearls** * **Purely Ketogenic:** Leucine and Lysine (The only two). * **Both Glucogenic & Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, Isoleucine (Mnemonic: **PITTT**). * **Semi-essential:** Arginine and Histidine (Essential during periods of rapid growth/pregnancy). * **Collagen Marker:** Hydroxyproline and Hydroxylysine are post-translational modifications; their presence in urine is a marker of bone resorption.

Question 429: Inherited hyperammonemia is a result of deficiency of which enzyme of the Krebs-Henseleit urea cycle?

A. Malate dehydrogenase
B. Isocitrate dehydrogenase
C. N-acetyl glutamate synthetase (Correct Answer)
D. Succinate dehydrogenase

Explanation: ### Explanation **Correct Option: C. N-acetyl glutamate synthetase (NAGS)** The **Krebs-Henseleit cycle (Urea Cycle)** is the primary mechanism for detoxifying ammonia into urea. The first and rate-limiting enzyme of this cycle is **Carbamoyl Phosphate Synthetase I (CPS-I)**. However, CPS-I is inactive without its essential allosteric activator, **N-acetylglutamate (NAG)**. NAG is synthesized from acetyl-CoA and glutamate by the enzyme **N-acetylglutamate synthetase (NAGS)**. A deficiency in NAGS leads to a failure to activate CPS-I, resulting in a complete block of the urea cycle. This causes a toxic buildup of ammonia in the blood (**Hyperammonemia**), leading to encephalopathy and cerebral edema. **Analysis of Incorrect Options:** * **A, B, and D (Malate, Isocitrate, and Succinate Dehydrogenase):** These are enzymes of the **TCA Cycle (Krebs Cycle)**, not the Urea Cycle. While the TCA cycle provides the ATP and aspartate required for the urea cycle, deficiencies in these specific enzymes do not typically present with isolated primary hyperammonemia. **Clinical Pearls for NEET-PG:** * **Most Common Urea Cycle Disorder:** Ornithine Transcarbamoylase (OTC) deficiency (X-linked recessive). * **NAGS Deficiency Mimicry:** NAGS deficiency clinically mimics CPS-I deficiency. Both present with hyperammonemia and **low/absent orotic acid** levels. * **Treatment:** NAGS deficiency is unique because it can be treated with **Carglumic acid** (an analog of N-acetylglutamate), which directly activates CPS-I. * **Mnemonic:** "NAGS is the key that unlocks CPS-I."

Question 430: A patient presents with arthritis, and arthroscopy reveals blackening of the joints. Which enzyme deficiency is most likely responsible for this condition?

A. Fumarase
B. Tyrosinase
C. Branched-chain alpha-ketoacid dehydrogenase
D. Homogentisate oxidase (Correct Answer)

Explanation: ***Homogentisate oxidase***- Deficiency of this enzyme causes **Alkaptonuria**, resulting in the accumulation of **homogentisic acid**, which oxidizes into dark polymeric pigment called **ochronotic pigment**.- This pigment deposits in cartilage and connective tissues (**Ochronosis**), leading to severe destructive arthritis, especially in large joints, and causing the characteristic **blackening of joint tissues** observed on arthroscopy.*Branched-chain alpha-ketoacid dehydrogenase*- Deficiency of this enzyme causes **Maple Syrup Urine Disease (MSUD)**, leading to the accumulation of **branched-chain amino acids** (leucine, isoleucine, valine).- MSUD is characterized by severe neurologic dysfunction and a distinctive maple syrup odor in the urine, not chronic arthritis or joint pigmentation.*Tyrosinase*- This enzyme is crucial for the synthesis of **melanin** from tyrosine.- Deficiency results in conditions like **Albinism**, characterized by hypopigmentation of the skin, hair, and eyes, which is distinct from ochronotic arthritis.*Fumarase*- Deficiency of this enzyme (Fumarate hydratase) impairs the **Krebs cycle** (TCA cycle).- Clinical features include severe intellectual disability, microcephaly, and metabolic acidosis, without the development of ochronosis or black joints.

Practice by Chapter

Protein Digestion and Absorption

Practice Questions

Transamination and Deamination

Practice Questions

Urea Cycle

Practice Questions

Disorders of Urea Cycle

Practice Questions

Metabolism of Individual Amino Acids

Practice Questions

Inborn Errors of Amino Acid Metabolism

Practice Questions

Phenylketonuria and Alkaptonuria

Practice Questions

Homocystinuria and Methionine Metabolism

Practice Questions

Synthesis of Biologically Important Compounds from Amino Acids

Practice Questions

Nitrogen Balance

Practice Questions

Ammonia Metabolism and Toxicity

Practice Questions

One-Carbon Transfer Reactions

Practice Questions

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