Amino Acid Metabolism Practice Questions

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: 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: A 4-year-old boy from a first-degree consanguineous couple presents with darkening of the urine to an almost black color when left standing. He has a normal sibling and no other medical problems. His growth and development are normal. Which of the following metabolites is most likely to be elevated in this patient?

Homogentisate. ***Correct: Homogentisate*** - The clinical presentation of urine darkening to an almost black color upon standing, especially in a child from a consanguineous union, is classic for **alkaptonuria**. - **Alkaptonuria** is an autosomal recessive disorder caused by a deficiency of homogentisate 1,2-dioxygenase, leading to the accumulation of **homogentisate** in the body. - The homogentisic acid polymerizes upon exposure to air, causing the characteristic dark discoloration of urine. - Patients are typically asymptomatic in childhood; **ochronosis** (pigment deposition in connective tissues) and arthritis develop in adulthood. *Incorrect: Methylmalonate* - Elevated **methylmalonate** is characteristic of **methylmalonic aciduria**, a disorder of branched-chain amino acid metabolism or vitamin B12 deficiency. - This condition typically presents with metabolic acidosis, developmental delays, and failure to thrive, which are not described in this patient. *Incorrect: Phenylpyruvate* - Elevated **phenylpyruvate** is a hallmark of **phenylketonuria (PKU)**, an inborn error of metabolism affecting phenylalanine breakdown. - Untreated PKU leads to severe neurological impairment and intellectual disability, which are absent in this child. *Incorrect: α-Ketoisovalerate* - Elevated **α-Ketoisovalerate** is one of the branched-chain keto acids found in **maple syrup urine disease (MSUD)**. - MSUD presents in infancy with poor feeding, lethargy, and a characteristic maple syrup odor in the urine, not darkening urine.

Q: Conversion of Norepinephrine to epinephrine is mainly by?

S-adenosyl methionine. **S-adenosyl methionine (SAM)** - SAM acts as the **methyl donor** in the enzymatic conversion of **norepinephrine to epinephrine** by phenylethanolamine N-methyltransferase (PNMT). - This **methylation reaction** adds a methyl group to the nitrogen atom of norepinephrine, forming epinephrine. *Arginine* - Arginine is a precursor for **nitric oxide (NO)** synthesis, an important signaling molecule, and is also involved in the **urea cycle**. - It is not directly involved in the methylation of norepinephrine to epinephrine. *Phenylalanine* - Phenylalanine is an **essential amino acid** and a precursor for the synthesis of **tyrosine**, which is subsequently converted to **catecholamines** like dopamine, norepinephrine, and epinephrine. - However, it does not directly facilitate the final conversion step from norepinephrine to epinephrine. *Dehydrogenase* - Dehydrogenases are enzymes that catalyze **redox reactions** by removing hydrogen atoms from a substrate. - These enzymes are crucial in many metabolic pathways, but they are not involved in the **methylation reaction** that converts norepinephrine to epinephrine.

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.

Q: Tyrosinemia Type I is caused due to deficiency of which enzyme?

Fumarylacetoacetate hydrolase. ***Fumarylacetoacetate hydrolase*** - **Tyrosinemia Type I**, also known as **hereditary tyrosinemia type 1 (HT1)**, is an **autosomal recessive** metabolic disorder caused by a deficiency of the enzyme **fumarylacetoacetate hydrolase (FAH)**. - This enzyme is crucial for the final step in the **tyrosine degradation pathway**, leading to the accumulation of toxic metabolites like fumarylacetoacetate and succinylacetone. *Tyrosine aminotransferase* - Deficiency of **tyrosine aminotransferase** causes **Tyrosinemia Type II**, a distinct disorder from Type I. - Type II tyrosinemia primarily affects the eyes and skin, presenting with **corneal ulcers** and painful **hyperkeratotic plaques**. *Homogentisate 1,2-dioxygenase* - Deficiency of **homogentisate 1,2-dioxygenase** leads to **alkaptonuria (black urine disease)**, a rare metabolic disorder. - This condition involves the accumulation of **homogentisic acid**, which causes dark urine, **ochronosis** (bluish-black pigmentation of connective tissues), and severe arthropathy. *4-hydroxyphenylpyruvate dioxygenase* - Deficiency of **4-hydroxyphenylpyruvate dioxygenase** results in **Tyrosinemia Type III**, another rare form of tyrosinemia. - This type is typically milder, often presenting with **neurological symptoms** such as intellectual disability and seizures, but without the severe liver and kidney damage seen in Type I.

Q: What is the scientific name for the neurotransmitter serotonin?

5-hydroxytryptamine (5-HT). ***5-hydroxytryptamine (5-HT)*** - **Serotonin** is the common name for the neurotransmitter **5-hydroxytryptamine**, often abbreviated as **5-HT**. - It plays a crucial role in regulating mood, appetite, sleep, and other physiological and behavioral functions. *N-methylphenylamine* - **N-methylphenylamine** is not the chemical name for serotonin. - This compound is a **synthetic amine** and does not serve as a well-known neurotransmitter in the human body. *3-Methoxytyramine* - **3-Methoxytyramine** is a **dopamine metabolite** and a trace amine, not serotonin. - It is typically formed from dopamine through the action of **catechol-O-methyltransferase (COMT)**. *Phenethylamine* - **Phenethylamine** is a **trace amine** that can act as a neuromodulator, but it is not serotonin. - It is known for its stimulant effects and is found in some foods like chocolate.

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

Citrulline. ***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.

Q: Hyperammonaemia inhibits the TCA cycle by depleting which of the following?

α-ketoglutarate. ***a keto glutarate*** - **Hyperammonemia** leads to the depletion of **α-ketoglutarate** through its amination to form **glutamate** by glutamate dehydrogenase and subsequently glutamine by glutamine synthetase. - The removal of **α-ketoglutarate** from the TCA cycle impairs its ability to produce energy and essential intermediates, contributing to neurological dysfunction in hyperammonemia. *succinate* - **Succinate** is an intermediate in the TCA cycle, but its depletion is not the primary mechanism by which hyperammonemia inhibits the cycle. - The direct consumption of **α-ketoglutarate** for ammonia detoxification is the more direct and significant impact. *malate* - **Malate** is another intermediate in the TCA cycle but is downstream from **α-ketoglutarate**. - Its depletion is a consequence of overall TCA cycle inhibition, not the initial cause mediated by hyperammonemia. *fumarate* - **Fumarate** is also a TCA cycle intermediate and is produced after succinate. - Its levels would be affected by the overall inhibition of the cycle, but it is not the direct target or substrate for ammonia detoxification that depletes the cycle.

Q: What is the enzyme that is deficient in Phenylketonuria?

Phenylalanine hydroxylase. ***Phenylalanine hydroxylase*** - **Phenylalanine hydroxylase** is the enzyme responsible for converting the amino acid **phenylalanine** into **tyrosine**. - A deficiency in this enzyme leads to the accumulation of **phenylalanine** in the body, causing **phenylketonuria (PKU)**. *Phenylalanine (amino acid)* - **Phenylalanine** is an essential **amino acid** that accumulates in PKU due to the enzyme deficiency, but it is not the deficient enzyme itself. - High levels of **phenylalanine** are toxic to the brain, leading to the clinical manifestations of PKU. *Phenylene* - **Phenylene** is a **bivalent organic radical** derived from benzene, and is not an enzyme or directly related to the metabolic pathway of phenylalanine. - This term is chemically distinct and irrelevant to the enzymatic defect in PKU. *None of the options* - This option is incorrect because **phenylalanine hydroxylase** is indeed the deficient enzyme in PKU, making one of the provided choices correct.

Question 1

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

Accepted Answer

Tryptophan

Answer

Glutamine

Answer

Glycine

Answer

Phenylalanine

Question 2

Rate limiting step in urea cycle is catalyzed by ?

Accepted Answer

Carbamoyl-phosphate synthase I (CPS1)

Answer

Arginase

Answer

Argininosuccinase

Answer

Ornithine transcarbamylase

Question 3

A 4-year-old boy from a first-degree consanguineous couple presents with darkening of the urine to an almost black color when left standing. He has a normal sibling and no other medical problems. His growth and development are normal. Which of the following metabolites is most likely to be elevated in this patient?

Accepted Answer

Homogentisate

Answer

Methylmalonate

Answer

Phenylpyruvate

Answer

α-Ketoisovalerate

Question 4

Conversion of Norepinephrine to epinephrine is mainly by?

Accepted Answer

S-adenosyl methionine

Answer

Arginine

Answer

Phenylalanine

Answer

Dehydrogenase

Question 5

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

Question 6

Tyrosinemia Type I is caused due to deficiency of which enzyme?

Accepted Answer

Fumarylacetoacetate hydrolase

Answer

Homogentisate 1,2-dioxygenase

Answer

Tyrosine aminotransferase

Answer

4-hydroxyphenylpyruvate dioxygenase

Question 7

What is the scientific name for the neurotransmitter serotonin?

Accepted Answer

5-hydroxytryptamine (5-HT)

Answer

N-methylphenylamine

Answer

3-Methoxytyramine

Answer

Phenethylamine

Question 8

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

Accepted Answer

Citrulline

Answer

Arginine

Answer

Ornithine

Answer

Lysine

Question 9

Hyperammonaemia inhibits the TCA cycle by depleting which of the following?

Accepted Answer

α-ketoglutarate

Answer

succinate

Answer

malate

Answer

fumarate

Question 10

What is the enzyme that is deficient in Phenylketonuria?

Accepted Answer

Phenylalanine hydroxylase

Answer

Phenylene

Answer

Phenylalanine (amino acid)

Answer

None of the options

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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