Amino Acid Metabolism Practice Questions

Q: All of the following are true about glutathione, except:

It converts hemoglobin to methemoglobin. ***It converts hemoglobin to methemoglobin*** - Glutathione is a **reducing agent** that helps protect hemoglobin from oxidation, thus **preventing** the formation of methemoglobin. - **Methemoglobin** occurs when the iron in hemoglobin is oxidized from the ferrous (Fe2+) to the ferric (Fe3+) state, which is a process glutathione actively counters. *It is co-factor of various enzymes* - Glutathione serves as a crucial **co-factor** for several enzymes, including **glutathione peroxidase**, which plays a vital role in antioxidant defense. - It participates in various **detoxification reactions** and catalyzes the reduction of harmful reactive oxygen species. *It is a tripeptide* - Glutathione is indeed a **tripeptide** composed of three amino acids: **glutamate**, **cysteine**, and **glycine**. - Its unique structure enables its diverse biological functions, including its prominent role as an antioxidant. *It conjugates xenobiotics* - Glutathione plays a critical role in **detoxifying xenobiotics** (foreign compounds) by conjugating with them, making them more water-soluble and easier to excrete. - This process is mediated by **glutathione S-transferases**, which attach glutathione to various toxic compounds.

Q: Ammonia in brain is trapped by

Glutamine. ***Glutamine*** - Ammonia in the brain is primarily detoxified by its conversion to **glutamine** through the enzyme **glutamine synthetase**. - This reaction combines **ammonia** with **glutamate**, effectively trapping the toxic ammonia in a non-toxic form that can be transported out of the brain. *Alanine* - **Alanine** plays a role in ammonia transport within the **glucose-alanine cycle** between muscle and liver, but it is not the primary mechanism for trapping ammonia in the brain. - While it can be formed from pyruvate and glutamate, its formation is not the main brain ammonia detoxification pathway. *Aspartate* - **Aspartate** is involved in the urea cycle and as a neurotransmitter, but it does not directly trap free ammonia in the brain. - It participates in transamination reactions with alpha-ketoglutarate, forming oxaloacetate and glutamate, but this isn't the main ammonia trapping mechanism. *Ornithine* - **Ornithine** is a key intermediate in the **urea cycle**, which primarily occurs in the liver for the detoxification of ammonia. - It is not directly involved in trapping ammonia within the brain tissue itself.

Q: In Cystinuria, all of the following amino acids are excreted in urine, except:-

Leucine. ***Leucine*** - Cystinuria is a disorder characterized by impaired transport of **dibasic amino acids** and **cystine**, not neutral amino acids like leucine. - Therefore, **leucine** would be properly reabsorbed and not significantly excreted in the urine. *Cystine* - **Cystine** is one of the four amino acids whose reabsorption is impaired in cystinuria, leading to its excessive excretion in urine and potential **kidney stone** formation. - The defect is in the **renal tubular transport system** for dibasic amino acids and cystine. *Ornithine* - **Ornithine** is a **dibasic amino acid** and its renal reabsorption is defective in cystinuria. - Like cystine, ornithine is excessively excreted in the urine due to the shared transport system. *Arginine* - **Arginine** is also a **dibasic amino acid** whose renal reabsorption is impaired in cystinuria. - Its presence in the urine is increased, along with cystine, ornithine, and lysine, forming the classic pattern of amino acid excretion in this condition.

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*.

Question 1

All of the following are true about glutathione, except:

Accepted Answer

It converts hemoglobin to methemoglobin

Answer

It is co-factor of various enzymes

Answer

It is a tripeptide

Answer

It conjugates xenobiotics

Question 2

Ammonia in brain is trapped by

Accepted Answer

Glutamine

Answer

Alanine

Answer

Aspartate

Answer

Ornithine

Question 3

In Cystinuria, all of the following amino acids are excreted in urine, except:-

Accepted Answer

Leucine

Answer

Cystine

Answer

Ornithine

Answer

Arginine

Question 4

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 5

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

Accepted Answer

Alpha-ketoglutarate

Answer

Malate

Answer

Oxaloacetate

Answer

Fumarate

Question 6

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 7

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

Accepted Answer

Arginine

Answer

Glycine

Answer

Aspartate

Answer

Citrulline

Question 8

Serotonin is also known as?

Accepted Answer

5-hydroxytryptamine (5-HT)

Answer

3-Methoxytyramine

Answer

Phenethylamine

Answer

N-methyl phenylamine

Question 9

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

Accepted Answer

Methionine

Answer

Tryptophan

Answer

Cysteine

Answer

Tyrosine

Question 10

Amino acid required for conversion of norepinephrine to epinephrine:-

Accepted Answer

Methionine

Answer

Lysine

Answer

Tryptophan

Answer

Phenylalanine

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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