Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 91: Which amino acid forms an integral part of collagen synthesis?

A. Lysine (Correct Answer)
B. Leucine
C. Arginine
D. Glutamic acid

Explanation: **Explanation:** **Why Lysine is the Correct Answer:** Collagen is the most abundant protein in the human body, characterized by a unique triple-helical structure. **Lysine** (along with Proline) plays a fundamental role in its synthesis and structural integrity. 1. **Hydroxylation:** Specific lysine residues are hydroxylated to **hydroxylysine** by the enzyme *lysyl hydroxylase* (requiring Vitamin C as a cofactor). These hydroxyl groups are essential for the subsequent O-glycosylation of collagen. 2. **Cross-linking:** Lysine and hydroxylysine residues undergo oxidative deamination by the copper-dependent enzyme **lysyl oxidase** to form reactive aldehydes (allysine). These aldehydes form covalent cross-links between collagen fibrils, providing the fiber with necessary tensile strength. **Why the Other Options are Incorrect:** * **B. Leucine:** An essential branched-chain amino acid (BCAA) primarily involved in muscle protein synthesis and ketogenic metabolism, but it does not have a specialized structural role in collagen. * **C. Arginine:** While involved in the Urea cycle and Nitric Oxide synthesis, it is not a primary structural determinant of the collagen triple helix. * **D. Glutamic Acid:** An excitatory neurotransmitter and a precursor for GABA. While it is found in many proteins, it does not undergo the specific post-translational modifications required for collagen stability. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin C Deficiency (Scurvy):** Leads to defective hydroxylation of lysine/proline, resulting in weak collagen and symptoms like bleeding gums and poor wound healing. * **Menkes Disease:** A defect in copper absorption leads to decreased **lysyl oxidase** activity, causing "kinky" hair and connective tissue defects. * **Ehlers-Danlos Syndrome (Type VI):** Specifically caused by a deficiency in the enzyme **lysyl hydroxylase**. * **Amino Acid Sequence:** Collagen follows a repeating **Gly-X-Y** pattern, where Glycine is every third residue, X is usually Proline, and Y is usually Hydroxyproline or Hydroxylysine.

Question 92: All of the following vitamins are involved in sulfur-containing amino acid metabolism except?

A. Vitamin B3 (Niacin) (Correct Answer)
B. Vitamin B6 (Pyridoxine)
C. Vitamin B9 (Folate)
D. Vitamin B12 (Cobalamin)

Explanation: ### Explanation The metabolism of sulfur-containing amino acids (Methionine and Cysteine) relies heavily on specific B-complex vitamins acting as cofactors. **Vitamin B3 (Niacin)** is the correct answer because it is not directly involved in the primary metabolic pathways of these amino acids (transmethylation and transsulfuration). #### Why Vitamin B3 is the Exception: Niacin (as NAD+/NADP+) is primarily involved in redox reactions and the TCA cycle. While it plays a role in general energy metabolism, it does not serve as a specific cofactor for the enzymes converting methionine to homocysteine or homocysteine to cysteine. #### Role of Other Vitamins: * **Vitamin B6 (Pyridoxine):** Essential for the **Transsulfuration pathway**. It is a cofactor for *Cystathionine β-synthase* (CBS) and *Cystathionase*, which convert Homocysteine to Cysteine. * **Vitamin B9 (Folate) & Vitamin B12 (Cobalamin):** Both are critical for the **Remethylation pathway**. Vitamin B12 is a cofactor for *Methionine Synthase*, while B9 (as N5-methyl THF) provides the methyl group to convert Homocysteine back into Methionine. #### High-Yield Clinical Pearls for NEET-PG: 1. **Hyperhomocysteinemia:** Deficiencies in B6, B9, or B12 lead to elevated homocysteine levels, a significant risk factor for coronary artery disease and thrombosis. 2. **Homocystinuria:** The most common cause is a deficiency of the B6-dependent enzyme **Cystathionine β-synthase**. 3. **Methylfolate Trap:** A B12 deficiency "traps" folate in the N5-methyl THF form, leading to functional folate deficiency and megaloblastic anemia. 4. **Cysteine** becomes an **essential amino acid** if there is a block in the transsulfuration pathway.

Question 93: Thyroxine (T4) is derived from which amino acid?

A. Threonine
B. Tyrosine (Correct Answer)
C. Thiamine
D. Tryptophan

Explanation: ### Explanation **Correct Option: B. Tyrosine** Thyroxine (T4) and Triiodothyronine (T3) are synthesized in the follicular cells of the thyroid gland from the amino acid **Tyrosine**. The process involves the iodination of tyrosine residues present on a large glycoprotein called **Thyroglobulin**. Specifically, two molecules of Diiodotyrosine (DIT) undergo a coupling reaction to form T4. Tyrosine is a versatile precursor; besides thyroid hormones, it is also the parent compound for **Catecholamines** (Dopamine, Epinephrine, Norepinephrine) and **Melanin**. **Analysis of Incorrect Options:** * **A. Threonine:** This is an essential hydroxy-amino acid. It is primarily involved in the synthesis of mucins and O-linked glycosylation of proteins, but it does not serve as a precursor for hormones. * **C. Thiamine:** This is not an amino acid; it is **Vitamin B1**. It acts as a coenzyme (TPP) for decarboxylation reactions (e.g., Pyruvate dehydrogenase). * **D. Tryptophan:** This is the precursor for **Serotonin, Melatonin, and Niacin (Vitamin B3)**. While it is a common "distractor" for tyrosine-derived products, it is not involved in thyroid hormone synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The oxidation of iodide and its attachment to tyrosine is catalyzed by **Thyroid Peroxidase (TPO)**. * **Wolff-Chaikoff Effect:** An autoregulatory phenomenon where high levels of circulating iodide inhibit TPO, temporarily decreasing thyroid hormone synthesis. * **Phenylketonuria (PKU):** In PKU, the conversion of Phenylalanine to Tyrosine is blocked. Consequently, Tyrosine becomes an **essential amino acid** for these patients.

Question 94: What is the composition of creatine?

A. Glycine, arginine, methionine (Correct Answer)
B. Glycine and histidine
C. Glycine, glutamate, aspartate
D. Histidine and methionine

Explanation: **Explanation:** Creatine is a nitrogenous organic acid synthesized primarily in the liver and kidneys. It plays a vital role in energy metabolism by recycling ATP in muscle and brain tissue. **Why Option A is Correct:** The synthesis of creatine requires three specific amino acids: 1. **Glycine:** Provides the backbone. 2. **Arginine:** Donates an amidino group to glycine to form guanidinoacetate (catalyzed by AGAT). 3. **Methionine:** Acts as the methyl donor in the form of **S-adenosylmethionine (SAM)** to convert guanidinoacetate into creatine (catalyzed by GAMT). **Why Other Options are Incorrect:** * **Option B:** Histidine is not involved in creatine synthesis; it is a precursor for histamine and carnosine. * **Option C:** Glutamate and aspartate are involved in the Urea cycle and nucleotide synthesis, but not directly in the formation of the creatine molecule. * **Option D:** While methionine is required, histidine is not. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Synthesis:** It begins in the **Kidney** (formation of guanidinoacetate) and is completed in the **Liver** (methylation). * **Storage:** 95% of creatine is stored in skeletal muscle as **Creatine Phosphate**, a high-energy reservoir. * **Excretion:** Creatine non-enzymatically cyclizes to **Creatinine**, which is excreted in urine. The rate of excretion is proportional to the individual's total muscle mass. * **Enzyme Marker:** Creatine Kinase (CK/CPK) is the enzyme that phosphorylates creatine; its isoenzymes (MB, MM, BB) are crucial diagnostic markers for MI and muscle diseases.

Question 95: Glycine is present in all except?

A. Creatine
B. Purine
C. Glutamine (Correct Answer)
D. Glutathione

Explanation: ### Explanation The question tests your knowledge of the metabolic precursors and derivatives of **Glycine**, the simplest amino acid. **Why Glutamine is the correct answer:** **Glutamine** is a distinct non-essential amino acid synthesized from **Glutamate** and ammonia via the enzyme *Glutamine synthetase*. It is not derived from Glycine. While both are involved in nitrogen metabolism, Glycine does not contribute to the carbon skeleton or the amino group of Glutamine. **Analysis of incorrect options (Derivatives of Glycine):** * **Creatine:** Glycine is the first precursor in creatine synthesis. It reacts with Arginine to form Guanidinoacetate (GAA) in the kidney, which is later methylated in the liver to form Creatine. * **Purine:** Glycine is a major contributor to the purine ring (Adenine and Guanine). Specifically, it provides the **C4, C5, and N7** atoms of the purine skeleton. * **Glutathione:** This is a tripeptide (γ-glutamyl-cysteinyl-**glycine**). Glycine is the third amino acid added during its synthesis, playing a crucial role in antioxidant defense. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Glycine Products:** "Glycine makes **H**appy **P**eople **G**et **C**reative" (**H**eme, **P**urines, **G**lutathione, **C**reatine). * **Heme Synthesis:** Glycine + Succinyl CoA are the starting materials for Heme synthesis (catalyzed by ALA synthase). * **Conjugation:** Glycine is used for the conjugation of bile acids (Glycocholic acid) and the detoxification of Benzoic acid to form **Hippuric acid**. * **Inhibitory Neurotransmitter:** Glycine acts as an inhibitory neurotransmitter in the spinal cord and brainstem.

Question 96: Transamination of aspartate directly leads to which intermediate in the citric acid cycle?

A. Citrate
B. Oxaloacetate (Correct Answer)
C. alpha-ketoglutarate
D. Fumarate

Explanation: ### Explanation **1. Why Oxaloacetate is Correct:** Transamination is the process where an amino group is transferred from an amino acid to a keto acid, catalyzed by **aminotransferases (transaminases)**. This reaction requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. * **Aspartate** is a 4-carbon amino acid. * When aspartate undergoes transamination (typically transferring its amino group to $\alpha$-ketoglutarate), it is converted into its corresponding keto acid, **Oxaloacetate (OAA)**. * The enzyme involved is **Aspartate Aminotransferase (AST)**, also known as Serum Glutamate Oxaloacetate Transaminase (SGOT). **2. Why Other Options are Incorrect:** * **A. Citrate:** This is formed by the condensation of Acetyl-CoA and Oxaloacetate. It is not a direct product of amino acid transamination. * **C. $\alpha$-ketoglutarate:** This is the keto acid corresponding to **Glutamate**. While $\alpha$-ketoglutarate is often the *acceptor* of the amino group in transamination, it is not the product derived from the carbon skeleton of aspartate. * **D. Fumarate:** This is an intermediate in the TCA cycle and the Urea cycle. While aspartate enters the Urea cycle and eventually contributes to fumarate formation, this occurs via the **Argininosuccinate** pathway, not directly through transamination. **3. High-Yield Clinical Pearls for NEET-PG:** * **AST (SGOT):** Found in the liver, heart, and skeletal muscle. It is a sensitive marker for hepatocellular injury and was historically used as a cardiac marker. * **The "Pairs" to Remember:** * Alanine $\leftrightarrow$ Pyruvate (via ALT) * Aspartate $\leftrightarrow$ Oxaloacetate (via AST) * Glutamate $\leftrightarrow$ $\alpha$-ketoglutarate * **Cofactor:** Always remember **PLP (B6)** for any transamination, deamination, or decarboxylation of amino acids.

Question 97: Albinism is due to the deficiency of which enzyme?

A. Tyrosinase (Correct Answer)
B. Homogentisate oxidase
C. Fumaryl acetoacetate hydrolase
D. Chylomicrons

Explanation: **Explanation:** **1. Why Tyrosinase is the Correct Answer:** Albinism (specifically Oculocutaneous Albinism Type 1) is caused by a congenital deficiency of the enzyme **Tyrosinase**. In the melanocytes, Tyrosinase is the rate-limiting enzyme responsible for converting **L-Tyrosine to DOPA** and subsequently **DOPA to Dopaquinone**. This pathway is essential for the synthesis of **melanin**, the pigment responsible for the color of skin, hair, and eyes. A lack of this enzyme leads to a total or partial absence of melanin. **2. Analysis of Incorrect Options:** * **Homogentisate oxidase:** Deficiency of this enzyme leads to **Alkaptonuria**, characterized by the accumulation of homogentisic acid, resulting in dark urine (on standing) and ochronosis (pigmentation of connective tissue). * **Fumaryl acetoacetate hydrolase:** Deficiency of this enzyme causes **Tyrosinemia Type I** (Hepatorenal Tyrosinemia), which presents with liver failure, rickets, and a "cabbage-like" odor. * **Chylomicrons:** These are lipoprotein particles responsible for transporting dietary lipids from the intestines to other locations in the body; they are unrelated to amino acid metabolism or pigmentation. **3. NEET-PG High-Yield Pearls:** * **Inheritance:** Most forms of Albinism are **Autosomal Recessive**. * **Clinical Presentation:** Patients have milky white skin, white hair, and photophobia. They are at a significantly increased risk for **Squamous Cell Carcinoma** of the skin due to lack of UV protection. * **Biochemical Link:** Remember that Tyrosine is the precursor for Melanin, Catecholamines (Dopamine, Epinephrine, Norepinephrine), and Thyroid hormones (T3, T4). * **Distinction:** In Albinism, the **number of melanocytes is normal**, but melanin production is absent. In Vitiligo, there is an autoimmune destruction of melanocytes.

Question 98: Transamination of pyruvate and glutamic acid leads to the formation of which compound?

A. Oxaloacetate
B. alpha-ketoglutarate (Correct Answer)
C. Aspartate
D. Malate

Explanation: ### Explanation **Concept Overview:** Transamination is the process where an amino group ($\text{-NH}_2$) is transferred from an amino acid to a keto acid, resulting in the formation of a new amino acid and a new keto acid. This reaction is catalyzed by **aminotransferases (transaminases)** and requires **Pyridoxal Phosphate (Vitamin B6)** as a mandatory cofactor. **Why the Correct Answer is Right:** In this specific reaction, **Glutamic acid** (the amino donor) transfers its amino group to **Pyruvate** (the keto acid acceptor). 1. When Glutamic acid loses its amino group, it is converted into its corresponding keto acid: **$\alpha$-ketoglutarate**. 2. When Pyruvate accepts the amino group, it is converted into its corresponding amino acid: **Alanine**. This reaction is catalyzed by **Alanine Aminotransferase (ALT)**, also known as Serum Glutamic Pyruvic Transaminase (SGPT). **Analysis of Incorrect Options:** * **A. Oxaloacetate:** This is the keto acid produced when **Aspartate** undergoes transamination. * **C. Aspartate:** This is the amino acid formed when Oxaloacetate accepts an amino group (catalyzed by AST). * **D. Malate:** This is an intermediate in the TCA cycle formed by the hydration of fumarate or the reduction of oxaloacetate; it is not a direct product of transamination. **NEET-PG High-Yield Pearls:** * **Cofactor:** All transaminases require **Pyridoxal Phosphate (PLP)**, a derivative of Vitamin B6. * **Exceptions:** Lysine, Threonine, Proline, and Hydroxyproline **do not** undergo transamination. * **Clinical Marker:** ALT (SGPT) is more specific for **liver cell damage** than AST (SGOT), as AST is also found in cardiac and skeletal muscle. * **Equilibrium:** Transamination reactions are freely reversible and play a crucial role in both amino acid synthesis and catabolism.

Question 99: Enzymes for urea synthesis are present in which cellular compartment?

A. Cytoplasm
B. Mitochondrial matrix
C. Both cytoplasm and mitochondrial matrix (Correct Answer)
D. None of the above

Explanation: ### Explanation The Urea Cycle (Ornithine cycle) is the primary mechanism for detoxifying ammonia into urea. It is unique because it is a **compartmentalized metabolic pathway**, meaning its enzymes are distributed between two different parts of the cell. **1. Why Option C is Correct:** The synthesis of urea requires five key enzymatic steps. The first two steps occur within the **mitochondrial matrix**, while the remaining three steps occur in the **cytosol (cytoplasm)**. * **Mitochondria:** Carbamoyl Phosphate Synthetase I (CPS-I) and Ornithine Transcarbamoylase (OTC) convert ammonia and bicarbonate into Citrulline. * **Cytoplasm:** Citrulline is transported out of the mitochondria, where Argininosuccinate synthetase, Argininosuccinate lyase, and Arginase complete the cycle to produce Urea and regenerate Ornithine. **2. Why Other Options are Incorrect:** * **Option A:** While the majority of the steps (3 out of 5) occur here, the cycle cannot initiate without the mitochondrial enzymes. * **Option B:** Only the initial "priming" steps occur here. If the cycle were restricted to the mitochondria, the final cleavage of Arginine to Urea could not take place. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Step:** CPS-I is the rate-limiting enzyme of the urea cycle and requires **N-acetylglutamate (NAG)** as an essential allosteric activator. * **Site of Urea Cycle:** It occurs exclusively in the **Liver**. * **Mnemonic for Dual-Compartment Pathways:** "**H**ot **U****M**ans" — **H**eme synthesis, **U**rea cycle, and **G**luconeogenesis occur in both the mitochondria and cytosol. * **Most Common Deficiency:** Ornithine Transcarbamoylase (OTC) deficiency is the most common urea cycle disorder and is the only one that is **X-linked recessive** (others are autosomal recessive).

Question 100: The reactions of the urea cycle occur in which one of the following locations?

A. The cytosol
B. Lysosomes
C. The mitochondrial matrix
D. The mitochondrial matrix and the cytosol (Correct Answer)

Explanation: The urea cycle (Ornithine cycle) is a critical metabolic pathway that converts toxic ammonia into non-toxic urea for excretion. It is unique because it is **compartmentalized**, requiring enzymes located in both the mitochondria and the cytoplasm. ### **Detailed Explanation** The cycle consists of five main steps, split between two cellular locations: 1. **Mitochondrial Matrix:** The first two reactions occur here. * Ammonia and bicarbonate are converted to **Carbamoyl Phosphate** by *Carbamoyl Phosphate Synthetase I (CPS-I)*—the rate-limiting step. * Carbamoyl phosphate combines with Ornithine to form **Citrulline** via *Ornithine Transcarbamoylase (OTC)*. 2. **Cytosol:** Citrulline is transported out of the mitochondria. The remaining three reactions (formation of Argininosuccinate, cleavage into Arginine and Fumarate, and finally the release of Urea) occur in the cytosol. ### **Analysis of Options** * **A & C (The cytosol / The mitochondrial matrix):** These are incorrect because neither compartment contains the full complement of enzymes required to complete the cycle. * **B (Lysosomes):** Lysosomes are involved in macromolecule degradation (proteolysis), not the metabolic processing of nitrogenous waste. ### **High-Yield NEET-PG Pearls** * **Rate-Limiting Enzyme:** CPS-I (requires **N-acetylglutamate/NAG** as an essential allosteric activator). * **Mnemonic for Location:** *"**M**other **C**ooking"*: **M**itochondria (first 2 steps) and **C**ytosol (remaining steps). * **Link to TCA Cycle:** Fumarate produced in the cytosol (by Argininosuccinate lyase) links the Urea cycle to the TCA cycle (the "Krebs Bicycle"). * **Most Common Deficiency:** **OTC deficiency** (the only X-linked urea cycle disorder; all others are Autosomal Recessive).

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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