Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 521: Pulses are known to be low in which of the following amino acids?

A. Lysine
B. Methionine
C. Cysteine (Correct Answer)
D. Arginine

Explanation: ***Cysteine*** - Pulses (legumes) are typically deficient in the sulfur-containing amino acids, **methionine** and **cysteine**. - This makes them an incomplete protein source if consumed exclusively, as these amino acids are essential for human nutrition. *Lysine* - Lysine is often a **limiting amino acid** in cereals, but pulses are generally **rich in lysine**. - This **complementarity** is why combining cereals and pulses creates a complete protein profile. *Methionine* - Methionine is a **sulfur-containing amino acid** that, along with cysteine, is deficient in pulses. - This is why pulses are often combined with cereals (which contain adequate methionine) to provide complete protein nutrition. *Arginine* - Arginine is an amino acid that is often found in good quantities in pulses. - It is not typically considered a **limiting amino acid** in legumes.

Question 522: Which amino acid contains an extra NH2 (amino) group in its structure?

A. Aspartate
B. Lysine (Correct Answer)
C. Glutamate
D. Histidine

Explanation: ***Lysine*** - Lysine is a basic amino acid that contains an **additional amino group (-NH2)** at the epsilon (ε) position of its side chain, in addition to the standard α-amino group. - Its structure: H2N-CH(COOH)-(CH2)4-NH2 clearly shows **two amino groups**. - This ε-amino group is positively charged at physiological pH, making lysine a **dibasic amino acid**. - The presence of this extra amino group makes lysine essential for **protein cross-linking** (e.g., in collagen) and various post-translational modifications. *Histidine* - Histidine contains an **imidazole ring** in its side chain with two nitrogen atoms, but this is NOT an amino group (-NH2). - While the imidazole ring makes histidine basic and can accept protons, it structurally does not contain an "extra NH2 group." - The imidazole side chain acts as a **buffer at physiological pH** due to its pKa (~6.0). *Aspartate* - Aspartate contains an **extra carboxyl group (-COOH)**, not an amino group. - This makes it an **acidic amino acid** with a negatively charged side chain at physiological pH. *Glutamate* - Glutamate also contains an **extra carboxyl group (-COOH)**, not an amino group. - Like aspartate, it is an **acidic amino acid** with a longer side chain by one methylene group.

Question 523: Which amino acid is a precursor for the synthesis of nicotinic acid?

A. Glutamine
B. Tryptophan (Correct Answer)
C. Glycine
D. Phenylalanine

Explanation: ***Tryptophan*** - **Tryptophan** is an essential amino acid that serves as a precursor for various vital compounds, including **nicotinic acid** (niacin), serotonin, and melatonin, making it crucial for metabolic pathways. - The conversion of tryptophan to nicotinic acid involves a complex pathway (kynurenine pathway) that results in the synthesis of **NAD+** and **NADP+**, which are coenzymes vital for redox reactions in the body. - Approximately 60 mg of tryptophan can be converted to 1 mg of niacin equivalent. *Glutamine* - **Glutamine** is the most abundant amino acid in the body and plays an important role in immune function, gut health, and nitrogen transport. - While essential for many bodily functions, glutamine is not directly involved in the synthesis pathway of nicotinic acid. *Glycine* - **Glycine** is the simplest amino acid and plays important roles in protein synthesis, collagen formation, and as a neurotransmitter. - It serves as a precursor for heme, purines, and creatine, but is not involved in nicotinic acid synthesis. *Phenylalanine* - **Phenylalanine** is an essential amino acid that is a precursor for tyrosine, which in turn is a precursor for thyroid hormones, catecholamines (dopamine, norepinephrine, epinephrine), and melanin. - While it has significant metabolic roles, phenylalanine does not participate in the synthesis of nicotinic acid.

Question 524: Cabbage-like odour is seen in ?

A. Alkaptonuria
B. Phenylketonuria
C. Hartnup disease
D. Tyrosinemia (Correct Answer)

Explanation: ***Tyrosinemia*** - **Tyrosinemia type 1 (hereditary tyrosinemia)** leads to the accumulation of toxic metabolites due to a deficiency in **fumarylacetoacetate hydrolase**. - These metabolites, particularly **succinylacetone**, are responsible for the characteristic **cabbage-like (or rancid butter) odor** in urine and sweat. *Alkaptonuria* - This condition is characterized by a deficiency in **homogentisate-1,2-dioxygenase**, leading to the accumulation of **homogentisic acid**. - The urine turns **dark (black)** when exposed to air, but there is no specific odor associated with cabbage. *Phenylketonuria* - PKU results from a deficiency in **phenylalanine hydroxylase**, causing a buildup of **phenylalanine** and its metabolites. - The classic odor in PKU is described as **musty or mousy**, not cabbage-like. *Hartnup disease* - This is a disorder of **amino acid transport** across the intestinal and renal tubules, specifically for neutral amino acids like **tryptophan**. - It does not involve a distinct body or urine odor like the "cabbage-like" smell.

Question 525: Tyrosine enters gluconeogenesis by forming which substrate

A. Fumarate (Correct Answer)
B. Succinyl CoA
C. Alpha-ketoglutarate
D. Citrate

Explanation: ***Fumarate*** - Tyrosine is both **glucogenic and ketogenic**, producing two products during catabolism. - The glucogenic portion of tyrosine degradation yields **fumarate**, which enters the **citric acid cycle** and can be converted to oxaloacetate, the key substrate for gluconeogenesis. - Tyrosine → Homogentisate → Maleylacetoacetate → **Fumarate + Acetoacetate** (the latter being ketogenic). - Fumarate is subsequently converted to **malate → oxaloacetate → phosphoenolpyruvate**, initiating gluconeogenesis. *Pyruvate* - While pyruvate is an important gluconeogenic substrate, tyrosine does **not directly form pyruvate**. - Amino acids like **alanine, serine, and cysteine** are converted to pyruvate, not tyrosine. - Tyrosine's gluconeogenic pathway proceeds through **fumarate → oxaloacetate**, bypassing pyruvate formation. *Succinyl CoA* - Succinyl CoA is formed from the catabolism of **valine, isoleucine, methionine, and threonine**. - It is a **citric acid cycle intermediate** but not the product of tyrosine degradation. *Alpha-ketoglutarate* - **Glutamate, glutamine, proline, arginine, and histidine** are catabolized to alpha-ketoglutarate. - This is another **citric acid cycle intermediate** but is not formed from tyrosine metabolism. *Citrate* - Citrate is formed from **oxaloacetate and acetyl-CoA** in the citric acid cycle. - It is not a direct product of amino acid catabolism and does not serve as an entry point for gluconeogenesis.

Question 526: Cystathionine lyase requires which cofactor ?

A. Thiamine
B. Riboflavin
C. Niacin
D. Vitamin B6 (Correct Answer)

Explanation: ***Vitamin B6*** - **Cystathionine lyase** is a pyridoxal phosphate (PLP)-dependent enzyme, meaning it requires **vitamin B6** (in its active form, PLP) as a cofactor. - This enzyme is crucial in the **transsulfuration pathway**, converting cystathionine to cysteine and α-ketobutyrate. *Thiamine* - **Thiamine (vitamin B1)** is a cofactor for enzymes involved in carbohydrate metabolism, such as **pyruvate dehydrogenase** and **alpha-ketoglutarate dehydrogenase**. - It does not directly participate in the activity of cystathionine lyase. *Riboflavin* - **Riboflavin (vitamin B2)** is a precursor for **FAD** and **FMN**, which are vital cofactors for various redox reactions in metabolism. - It is not directly involved as a cofactor for cystathionine lyase. *Niacin* - **Niacin (vitamin B3)** is a precursor for NAD+ and NADP+, which are critical coenzymes in many **redox reactions**. - These coenzymes are not required for the catalytic activity of cystathionine lyase.

Question 527: Urea is synthesized in all except:

A. Spleen
B. Liver
C. Kidney
D. Brain (Correct Answer)

Explanation: ***Brain*** - The brain **does not possess the complete set of enzymes** required for the urea cycle, particularly lacking carbamoyl phosphate synthetase I (CPS I). - Due to the **blood-brain barrier**, the central nervous system maintains a tightly controlled metabolic environment and handles ammonia through alternative pathways (glutamine synthesis). - The brain **does not synthesize urea** and is the most commonly cited answer for organs lacking this capability. *Liver* - The **liver is the primary and main site of urea synthesis** in mammals, essential for detoxifying ammonia produced from amino acid catabolism. - All five enzymes of the **urea cycle** are predominantly expressed in hepatocytes (periportal region). *Kidney* - The kidney possesses some urea cycle enzymes and has **limited capacity for urea synthesis** in certain cell types. - However, its primary role in nitrogen metabolism is **urea excretion** and gluconeogenesis from amino acids, not significant urea production. - The kidney is NOT a major site of urea synthesis. *Spleen* - The **spleen is primarily involved in immune responses** and erythrocyte recycling (hemolysis and iron metabolism). - It **completely lacks the enzymes of the urea cycle** necessary for converting ammonia to urea.

Question 528: Citrullinemia is due to deficiency of?

A. Argininosuccinate synthase (Correct Answer)
B. Arginase
C. Ornithine transcarbamylase
D. Argininosuccinate lyase

Explanation: ***Argininosuccinate synthase*** - **Citrullinemia** is primarily caused by a deficiency in **argininosuccinate synthase (ASS)**, an enzyme crucial for the conversion of **citrulline** and **aspartate** to **argininosuccinate** in the urea cycle. - This deficiency leads to an accumulation of **citrulline** in the blood and urine, resulting in hyperammonemia and neurological symptoms. *Arginase* - A deficiency in **arginase** (also known as argininemia) leads to the accumulation of **arginine**, but not citrulline. - This enzyme is responsible for the final step of the urea cycle, converting **arginine** to **ornithine** and **urea**. *Ornithine transcarbamylase* - A deficiency in **ornithine transcarbamylase (OTC)** is another common urea cycle disorder, but it results in a buildup of **ammonia** and **carbamoyl phosphate**, not citrulline. - OTC catalyzes the conversion of **carbamoyl phosphate** and **ornithine** to **citrulline**. *Argininosuccinate lyase* - A deficiency in **argininosuccinate lyase (ASL)** causes **argininosuccinic aciduria**, characterized by the accumulation of **argininosuccinate** in the blood and urine. - This enzyme is responsible for converting **argininosuccinate** to **arginine** and **fumarate**.

Question 529: Rate limiting step in urea cycle is catalyzed by ?

A. Arginase
B. Argininosuccinase
C. Ornithine transcarbamylase
D. Carbamoyl-phosphate synthase I (CPS1) (Correct Answer)

Explanation: ***Carbamoyl-phosphate synthase I (CPS1)*** - **Carbamoyl-phosphate synthase I (CPS1)** is the enzyme that catalyzes the first committed step of the urea cycle, - This reaction involves the synthesis of **carbamoyl phosphate** from ammonia, carbon dioxide, and two molecules of ATP in the mitochondria. *Arginase* - **Arginase** is the enzyme that catalyzes the final step of the urea cycle, converting **arginine** to urea and ornithine. - This enzyme is responsible for generating urea for excretion but does not regulate the initiation of the cycle. *Argininosuccinase* - **Argininosuccinase (argininosuccinate lyase)** catalyzes the breakdown of argininosuccinate into fumarate and arginine. - This is an intermediate step in the urea cycle and not the rate-limiting step. *Ornithine transcarbamylase* - **Ornithine transcarbamylase (OTC)** catalyzes the reaction where **ornithine** reacts with carbamoyl phosphate to form citrulline. - While an essential mitochondrial enzyme in the urea cycle, it is not the rate-limiting step; CPS1 activity primarily controls the flux through the pathway.

Question 530: In Cystinuria, which of the following amino acids is not affected by the reabsorption defect?

A. Arginine
B. Ornithine
C. Citrulline (Correct Answer)
D. Lysine

Explanation: ***Citrulline*** - **Citrulline** is the correct answer because it is **not a dibasic amino acid** and does not share the same renal tubular transporter as the affected amino acids. - In cystinuria, the defect involves the **rBAT-b0,+AT transporter system**, which specifically transports **cystine** and **dibasic amino acids** (ornithine, arginine, lysine) - remembered by the mnemonic **COAL**. - Citrulline uses a **different transport mechanism** and therefore its reabsorption remains **normal** in cystinuria. *Lysine* - **Lysine** is one of the four amino acids affected in cystinuria (part of the COAL group). - Being a **dibasic amino acid**, its reabsorption is **impaired**, leading to increased urinary excretion. - This is **incorrect** as the question asks for the amino acid NOT affected. *Arginine* - **Arginine** is a **dibasic amino acid** whose reabsorption is significantly reduced in cystinuria. - It is part of the COAL group and shows **elevated urinary concentration** in affected individuals. - This is **incorrect** as arginine IS affected by the reabsorption defect. *Ornithine* - **Ornithine** is another **dibasic amino acid** included in the COAL group. - Its renal tubular reabsorption is **defective** in cystinuria, resulting in increased urinary excretion. - This is **incorrect** as ornithine IS affected by the transport defect.

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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