Amino Acid Metabolism Practice Questions

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

Cysteine. ***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.

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

Lysine. ***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.

Q: Tyrosine enters gluconeogenesis by forming which substrate

Fumarate. ***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.

Q: Cabbage-like odour is seen in ?

Tyrosinemia. ***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.

Q: Citrullinemia is due to deficiency of?

Argininosuccinate synthase. ***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**.

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

Tryptophan. ***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.

Q: Cystathionine lyase requires which cofactor ?

Vitamin B6. ***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.

Q: Rate limiting step in urea cycle is catalyzed by ?

Carbamoyl-phosphate synthase I (CPS1). ***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.

Q: Urea is synthesized in all except:

Brain. ***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.

Q: An increase in glutamine levels in the cerebrospinal fluid (CSF), blood, and urine is indicative of a deficiency in which enzyme?

Carbamoyl Phosphate Synthetase I (CPS-I). ***Carbamoyl Phosphate Synthetase I (CPS-I)*** - A deficiency in **Carbamoyl Phosphate Synthetase I (CPS-I)**, the first enzyme in the urea cycle, blocks the conversion of ammonia and bicarbonate into carbamoyl phosphate. - This leads to severe **hyperammonemia**, which is detoxified by glutamine synthetase, causing marked elevation of **glutamine in CSF, blood, and urine**. - **Key distinguishing feature**: CPS-I deficiency causes isolated hyperammonemia with elevated glutamine but **NO orotic aciduria**, unlike OTC deficiency. - This is the most proximal urea cycle defect, presenting with the purest pattern of glutamine elevation without other metabolite abnormalities. *Ornithine transcarbamylase (OTC)* - **OTC deficiency** is the most common urea cycle disorder and also causes severe hyperammonemia with elevated glutamine. - However, it leads to accumulation of carbamoyl phosphate, which diverts into the pyrimidine synthesis pathway, resulting in **elevated urinary orotic acid**. - **Key differentiator**: The presence of orotic aciduria distinguishes OTC from CPS-I deficiency—both have elevated glutamine, but only OTC has elevated orotic acid. *Argininosuccinate synthetase* - A deficiency causes **citrullinemia**, characterized by markedly elevated **citrulline** in blood and urine. - While hyperammonemia and secondary glutamine elevation occur, the **hallmark finding is elevated citrulline**, which makes this diagnosis distinct from isolated glutamine elevation. *Arginase* - Arginase deficiency leads to accumulation of **arginine** in blood and urine, causing **hyperargininemia**. - This deficiency presents with progressive spasticity, growth retardation, and intellectual disability, with **elevated arginine** being the distinguishing metabolite rather than isolated glutamine elevation.

Question 1

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

Accepted Answer

Cysteine

Answer

Lysine

Answer

Methionine

Answer

Arginine

Question 2

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

Accepted Answer

Lysine

Answer

Aspartate

Answer

Glutamate

Answer

Histidine

Question 3

Tyrosine enters gluconeogenesis by forming which substrate

Accepted Answer

Fumarate

Answer

Succinyl CoA

Answer

Alpha-ketoglutarate

Answer

Citrate

Question 4

Cabbage-like odour is seen in ?

Accepted Answer

Tyrosinemia

Answer

Alkaptonuria

Answer

Phenylketonuria

Answer

Hartnup disease

Question 5

Citrullinemia is due to deficiency of?

Accepted Answer

Argininosuccinate synthase

Answer

Arginase

Answer

Ornithine transcarbamylase

Answer

Argininosuccinate lyase

Question 6

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

Accepted Answer

Tryptophan

Answer

Glutamine

Answer

Glycine

Answer

Phenylalanine

Question 7

Cystathionine lyase requires which cofactor ?

Accepted Answer

Vitamin B6

Answer

Thiamine

Answer

Riboflavin

Answer

Niacin

Question 8

Rate limiting step in urea cycle is catalyzed by ?

Accepted Answer

Carbamoyl-phosphate synthase I (CPS1)

Answer

Arginase

Answer

Argininosuccinase

Answer

Ornithine transcarbamylase

Question 9

Urea is synthesized in all except:

Accepted Answer

Brain

Answer

Spleen

Answer

Liver

Answer

Kidney

Question 10

An increase in glutamine levels in the cerebrospinal fluid (CSF), blood, and urine is indicative of a deficiency in which enzyme?

Accepted Answer

Carbamoyl Phosphate Synthetase I (CPS-I)

Answer

Arginase

Answer

Ornithine transcarbamylase (OTC)

Answer

Argininosuccinate synthetase

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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