Amino Acid Metabolism Practice Questions

Q: Elevated levels of vanillylmandelic acid (VMA) in urine are characteristically found in which of the following conditions?

Pheochromocytoma. ***Pheochromocytoma*** - **Vanillylmandelic acid (VMA)** is a major urinary metabolite of the **catecholamines epinephrine and norepinephrine**. - **Pheochromocytoma** is a tumor of the adrenal medulla that secretes excessive amounts of these catecholamines, leading to significantly elevated VMA levels in urine. *Phenyl ketonuria* - Characterized by the inability to metabolize **phenylalanine** due to a deficiency of the enzyme **phenylalanine hydroxylase**. - Leads to accumulation of **phenylalanine** and its metabolites, such as **phenylpyruvic acid**, not VMA. *Diabetic ketoacidosis* - A severe complication of **diabetes mellitus** resulting from a profound insulin deficiency, leading to high blood glucose and **ketone body** production. - While it alters metabolism, it does not directly lead to elevated VMA levels. *Alkaptonuria* - A rare genetic disorder caused by a deficiency of the enzyme **homogentisate 1,2-dioxygenase**, involved in **tyrosine metabolism**. - Results in the accumulation of **homogentisic acid**, which is excreted in the urine and turns dark on exposure to air, but does not involve VMA.

Q: Ammonia causes depletion of which of the following in TCA cycle?

Alpha-ketoglutarate. ***Alpha-ketoglutarate*** - Ammonia is detoxified in the brain by conversion to **glutamine**, a process that consumes **alpha-ketoglutarate** in the glutamate dehydrogenase reaction (alpha-ketoglutarate + NH3 + NADH glutamate + NAD+). - The depletion of **alpha-ketoglutarate** in the TCA cycle impairs cellular respiration and ATP production, contributing to the neurological dysfunction seen in hyperammonemia. *Malate* - While malate is a component of the TCA cycle, its depletion is not a direct consequence of ammonia detoxification. - Ammonia metabolism primarily impacts the availability of alpha-ketoglutarate through the synthesis of glutamate and glutamine. *Oxaloacetate* - Although **oxaloacetate** is a key intermediate in the TCA cycle, its levels are not directly depleted by ammonia metabolism. - **Oxaloacetate** can be replenished through anaplerotic reactions, even if the TCA cycle is slightly inhibited due to alpha-ketoglutarate depletion. *Fumarate* - **Fumarate** is an intermediate of the TCA cycle and is not directly consumed or depleted by the ammonia detoxification pathway. - Its levels would only indirectly be affected if the overall flux of the TCA cycle is significantly reduced due to depletion of other intermediates.

Q: Glutamine is increased in CSF, blood & urine WITHOUT elevated orotic acid in which defect:

CPS-I. ***CPS-I*** - A deficiency in **Carbamoyl Phosphate Synthetase I (CPS-I)** leads to a severe block in the **urea cycle**, resulting in profound hyperammonemia. - The elevated ammonia is then shunted to produce more **glutamine** (via glutamine synthetase), which serves as a detoxification mechanism but also causes high levels of glutamine in CSF, blood, and urine. *Arginase* - **Arginase deficiency** primarily leads to elevated **arginine** levels and mild to moderate hyperammonemia, but not typically a dramatic increase in glutamine due to the block occurring later in the cycle. - Clinical features include progressive spasticity, growth retardation, and intellectual disability. *Argininosuccinate lyase deficiency* - This deficiency causes accumulation of **argininosuccinate** in body fluids, which is a diagnostic marker, rather than primarily increased glutamine. - It presents with severe hyperammonemia, neurological symptoms, and often hepatomegaly. *Arginosuccinate synthetase* - A deficiency in **argininosuccinate synthetase** (also known as citrullinemia type I) leads to a buildup of **citrulline** and severe hyperammonemia. - While hyperammonemia can indirectly increase glutamine, the primary diagnostic marker is elevated citrulline, and the glutamine increase is not as pronounced or directly symptomatic as in CPS-I deficiency. *OTC* - **Ornithine Transcarbamylase (OTC) deficiency** is the most common urea cycle disorder and leads to severe hyperammonemia, accompanied by elevated **orotic acid** due to carbamoyl phosphate shunting to pyrimidine synthesis. - While hyperammonemia drives glutamine synthesis, the presence of elevated orotic acid is a key differentiator from CPS-I deficiency, which does not have increased orotic acid.

Q: Which amino acid is involved in the synthesis of creatinine, NO, and urea?

Arginine. ***Arginine*** - **Arginine** is a precursor for **Nitric Oxide (NO)** synthesis via **nitric oxide synthase**. - It is also a substrate for **creatinine** synthesis (along with glycine and methionine) and plays a key role in the **urea cycle**. *Glycine* - **Glycine** is a precursor for **creatinine** synthesis, but not directly involved in NO or urea production as the primary amino acid. - It is also a component of glutathione and purine synthesis. *Aspartate* - **Aspartate** is a key intermediate in the **urea cycle**, contributing one nitrogen atom to urea. - It is not directly a precursor for NO or creatinine. *Citrulline* - **Citrulline** is an intermediate in the **urea cycle** and is converted to arginine. - It is not directly involved in the synthesis of creatinine or as the primary precursor for NO.

Q: Serotonin is also known as?

5-hydroxytryptamine (5-HT). ***5-hydroxytryptamine (5-HT)*** - **Serotonin** is the common name for the neurotransmitter **5-hydroxytryptamine (5-HT)**. - This chemical name reflects its structure, derived from tryptophan, indicating a **hydroxyl group** at the 5-position of the **indole ring** and an **ethylamine side chain**. *3-Methoxytyramine* - **3-Methoxytyramine** is a metabolite of **dopamine**, not serotonin. - It is formed by the action of **catechol-O-methyltransferase (COMT)** on dopamine. *Phenethylamine* - **Phenethylamine** is a naturally occurring trace amine that acts as a **neuromodulator** or neurotransmitter. - It is structurally similar to certain recreational drugs but is not serotonin. *N-methyl phenylamine* - **N-methyl phenylamine** or N-methylaniline is an **organic chemical** used in dyes and other industrial applications. - It is not a neurotransmitter and has no relation to serotonin.

Q: Most important amino acid which acts as a methyl group donor?

Methionine. ***Methionine*** - **Methionine** is the precursor to **S-adenosylmethionine (SAM)**, which serves as the primary and most important **methyl group donor** in various biochemical reactions, including DNA methylation and neurotransmitter synthesis. - The methyl group of methionine is transferred to a wide range of acceptors via SAM, playing a crucial role in metabolism and gene regulation. *Tryptophan* - **Tryptophan** is an essential amino acid primarily known as a precursor for the synthesis of **serotonin** and **niacin**. - While it has various metabolic roles, it does not directly act as a significant methyl group donor. *Cysteine* - **Cysteine** is important for maintaining protein structure through **disulfide bonds** and is a precursor for **glutathione** synthesis. - It contains a sulfur-containing thiol group but does not donate methyl groups. *Tyrosine* - **Tyrosine** is a non-essential amino acid that is a precursor for **thyroid hormones**, **catecholamines** (dopamine, norepinephrine, epinephrine), and **melanin**. - It does not function as a methyl group donor in biochemical pathways.

Q: Amino acid required for conversion of norepinephrine to epinephrine:-

Methionine. ***Methionine*** - **Norepinephrine** is converted to **epinephrine** by the enzyme **phenylethanolamine N-methyltransferase (PNMT)**. - This enzyme uses **S-adenosylmethionine (SAM)** as a **methyl donor**, which is derived from methionine. *Lysine* - **Lysine** is an essential amino acid primarily involved in **protein synthesis**, **calcium absorption**, and the production of **carnitine**. - It does not directly participate in the methylation reaction converting norepinephrine to epinephrine. *Tryptophan* - **Tryptophan** is a precursor for **serotonin** and **niacin** synthesis. - It is not involved in the catecholamine synthesis pathway from norepinephrine to epinephrine. *Phenylalanine* - **Phenylalanine** is the initial amino acid in the **catecholamine synthesis pathway**, being converted to **tyrosine**, then to DOPA, dopamine, and norepinephrine. - While it's crucial for the synthesis *up to* norepinephrine, it is not directly involved in the *conversion of norepinephrine to epinephrine*.

Q: A patient complains of knee pain. Routine investigations are unremarkable and still, the patient is unsatisfied. Urine turns black on standing, what is the enzyme involved?

Homogentisate oxidase. ***Homogentisate oxidase*** - **Homogentisate oxidase** deficiency leads to **alkaptonuria**, where homogentisic acid accumulates and is excreted in urine, turning it black upon standing or exposure to air. The knee pain could be due to **ochronosis**, the deposition of oxidized homogentisic acid in cartilage and connective tissues. - The unremarkable routine investigations with persistent knee pain suggest a less common metabolic disorder, which is consistent with **alkaptonuria**'s chronic and insidious presentation. *Methylmalonyl-CoA mutase* - A deficiency in **methylmalonyl-CoA mutase** causes **methylmalonic acidemia**, a metabolic disorder typically presenting in infancy with neurological symptoms, feeding difficulties, and developmental delay. - It does not cause urine to turn black on standing or lead to specific joint pain like that seen in ochronosis. *Xanthine oxidase* - **Xanthine oxidase** is involved in purine metabolism, and its inhibition (e.g., by allopurinol) is used to treat **gout**. - While gout can cause knee pain, it is characterized by elevated uric acid and typically responds to conventional treatments and the urine does not turn black. *Tyrosine hydroxylase* - **Tyrosine hydroxylase** is the rate-limiting enzyme in catecholamine synthesis, converting tyrosine to L-DOPA. - Defects in this enzyme are rare and result in neurological disorders, such as **DOPA-responsive dystonia**, and do not cause urine to blacken or lead to direct joint damage.

Q: NO is synthesized from:

L-arginine. ***L-arginine*** - **Nitric oxide (NO)** is synthesized from the amino acid **L-arginine** by the enzyme **nitric oxide synthase (NOS)**. - This reaction requires **molecular oxygen** and **NADPH** as cofactors, producing **L-citrulline** as a co-product. - **Three NOS isoforms** exist: neuronal (nNOS), inducible (iNOS), and endothelial (eNOS). - **Clinical significance**: NO is crucial for **vascular smooth muscle relaxation**, **neurotransmission**, and **immune defense**. *L-citrulline* - **L-citrulline** is the co-product (not precursor) of the NO synthesis reaction. - While it can be recycled back to **L-arginine** via the **citrulline-arginine cycle**, it does not directly yield NO without first being converted to arginine. - This recycling pathway helps sustain NO production in endothelial cells. *Glycine* - **Glycine** is involved in various metabolic processes, including **heme synthesis**, **purine synthesis**, and as a neurotransmitter. - It plays no direct role in **nitric oxide** biosynthesis. - Not a substrate for any **NOS isoform**. *Lysine* - **Lysine** is an essential amino acid important for **protein synthesis**, **collagen formation**, and **carnitine synthesis**. - It is structurally distinct from arginine and is not recognized by **nitric oxide synthase**. - Not involved in the NO synthesis pathway.

Q: A newborn presents with metabolic acidosis, high ammonia, and orotic acid in urine. Which enzyme deficiency is most likely?

Ornithine transcarbamylase. ***Ornithine transcarbamylase*** - Deficiency of **ornithine transcarbamylase (OTC)**, a key enzyme in the urea cycle, leads to the accumulation of **carbamoyl phosphate**. - **Carbamoyl phosphate** is then shunted to the pyrimidine synthesis pathway, leading to increased production and excretion of **orotic acid** in the urine, along with **hyperammonemia** and **metabolic acidosis**. *Arginase* - **Arginase deficiency** in the urea cycle primarily causes increased **arginine levels** and **hyperammonemia**, but it does not typically lead to the accumulation of orotic acid. - The effects are often more chronic with gradual onset of symptoms, rather than severe neonatal presentation with orotic aciduria. *Glutamine synthetase* - **Glutamine synthetase** catalyzes the synthesis of glutamine from glutamate and ammonia, playing a crucial role in **ammonia detoxification**. - A deficiency would impair ammonia detoxification and lead to **hyperammonemia**, but it would not directly cause **orotic aciduria**. *CPS-1* - **Carbamoyl phosphate synthetase I (CPS-1)** deficiency is the first step of the urea cycle and leads to severe **hyperammonemia** due to the inability to form carbamoyl phosphate. - Unlike OTC deficiency, **CPS-1 deficiency** does not involve the buildup of carbamoyl phosphate; hence, **orotic acid levels** would be low or normal, not high.

Question 1

Elevated levels of vanillylmandelic acid (VMA) in urine are characteristically found in which of the following conditions?

Accepted Answer

Pheochromocytoma

Answer

Phenyl ketonuria

Answer

Diabetic ketoacidosis

Answer

Alkaptonuria

Question 2

Ammonia causes depletion of which of the following in TCA cycle?

Accepted Answer

Alpha-ketoglutarate

Answer

Malate

Answer

Oxaloacetate

Answer

Fumarate

Question 3

Glutamine is increased in CSF, blood & urine WITHOUT elevated orotic acid in which defect:

Accepted Answer

CPS-I

Answer

Arginase

Answer

Argininosuccinate lyase deficiency

Answer

Arginosuccinate synthetase

Answer

OTC

Question 4

Which amino acid is involved in the synthesis of creatinine, NO, and urea?

Accepted Answer

Arginine

Answer

Glycine

Answer

Aspartate

Answer

Citrulline

Question 5

Serotonin is also known as?

Accepted Answer

5-hydroxytryptamine (5-HT)

Answer

3-Methoxytyramine

Answer

Phenethylamine

Answer

N-methyl phenylamine

Question 6

Most important amino acid which acts as a methyl group donor?

Accepted Answer

Methionine

Answer

Tryptophan

Answer

Cysteine

Answer

Tyrosine

Question 7

Amino acid required for conversion of norepinephrine to epinephrine:-

Accepted Answer

Methionine

Answer

Lysine

Answer

Tryptophan

Answer

Phenylalanine

Question 8

A patient complains of knee pain. Routine investigations are unremarkable and still, the patient is unsatisfied. Urine turns black on standing, what is the enzyme involved?

Accepted Answer

Homogentisate oxidase

Answer

Methylmalonyl-CoA mutase

Answer

Xanthine oxidase

Answer

Tyrosine hydroxylase

Question 9

NO is synthesized from:

Accepted Answer

L-arginine

Answer

L-citrulline

Answer

Glycine

Answer

Lysine

Question 10

A newborn presents with metabolic acidosis, high ammonia, and orotic acid in urine. Which enzyme deficiency is most likely?

Accepted Answer

Ornithine transcarbamylase

Answer

Arginase

Answer

Glutamine synthetase

Answer

CPS-1

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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