Amino Acid Metabolism Practice Questions

Q: In a patient with maple syrup urine disease, all of the following amino acids should be restricted in diet except?

Methionine. ***Methionine*** - **Maple syrup urine disease (MSUD)** is a disorder affecting the metabolism of **branched-chain amino acids (BCAAs)**: leucine, isoleucine, and valine. - Therefore, methionine, which is not a BCAA, typically does not need to be restricted and is, in fact, an **essential amino acid** crucial for protein synthesis. *Isoleucine* - **Isoleucine** is a branched-chain amino acid (BCAA) whose metabolism is impaired in MSUD due to a deficiency in **branched-chain alpha-keto acid dehydrogenase complex**. - Accumulation of isoleucine and its corresponding alpha-keto acid is toxic and must be **restricted in the diet**. *Leucine* - **Leucine** is another branched-chain amino acid (BCAA) that cannot be properly metabolized in MSUD. - High levels of leucine and its metabolites are particularly **neurotoxic** and contribute to the characteristic neurological symptoms, necessitating strict dietary restriction. *Valine* - **Valine** is the third branched-chain amino acid (BCAA) whose breakdown is defective in MSUD. - Dietary restriction of valine is essential to prevent its accumulation, which can lead to metabolic crises and **developmental delays**.

Q: Which of the following options is false in a patient with advanced liver disease?

Increased oxidative decarboxylation of alpha-ketoglutarate. ***Increased oxidative decarboxylation of alpha-ketoglutarate*** - In advanced liver disease, **hepatic encephalopathy** is a common complication due to the liver's inability to detoxify **ammonia**. - Alpha-ketoglutarate is crucial for detoxifying ammonia into glutamate and then glutamine, processes that involve reductive amination and **ATP consumption**, not increased oxidative decarboxylation. *Decreased ATP synthesis* - Advanced liver disease impairs various metabolic functions, including those of the **mitochondria**, leading to decreased ATP production. - This reduction in **energy reserves** affects numerous cellular processes and organ functions. *Hyperammonemia* - The liver is responsible for detoxifying **ammonia** into urea via the **urea cycle**. - In advanced liver disease, the impaired function of the liver leads to a buildup of ammonia in the blood, resulting in **hyperammonemia**. *Increased synthesis of glutamine and glutamate from alpha-ketoglutarate* - In the brain, when the liver fails to detoxify ammonia, **astrocytes** attempt to convert excess ammonia into **glutamine** using **alpha-ketoglutarate**. - This process is a compensatory mechanism to reduce ammonia toxicity, but it depletes alpha-ketoglutarate, which is a key intermediate in the **Krebs cycle**, leading to **cerebral energy deficits**.

Q: Ammonia formed in the brain is converted into

Glutamine. ***Glutamine*** - **Ammonia** is detoxified in the brain by combining with **glutamate** to form **glutamine** via the enzyme **glutamine synthetase**. - This conversion is crucial because **glutamine** is non-toxic and can be safely transported out of the brain to the liver for further processing. *Glycine* - **Glycine** is an amino acid that can function as a neurotransmitter, but it is not the primary product of ammonia detoxification in the brain. - While it can be synthesized in the brain, it does not serve as the molecule to which toxic ammonia is directly converted for transport. *Urea* - **Urea** is the primary end-product of ammonia detoxification in the **liver** through the **urea cycle**. - The brain lacks the complete set of enzymes required for the **urea cycle**, so it cannot convert ammonia into urea. *Cysteine* - **Cysteine** is a sulfur-containing amino acid involved in protein synthesis and antioxidant defense, but it is not directly involved in the detoxification pathway of ammonia in the brain. - Its synthesis and metabolism are distinct from the process of ammonia sequestration.

Q: Atherosclerosis is associated with:

Homocysteine. ***Homocysteine*** - Elevated plasma levels of **homocysteine**, known as hyperhomocysteinemia, are an independent **risk factor for atherosclerosis** and cardiovascular disease. - While the precise mechanism is complex, homocysteine is thought to promote endothelial damage, oxidative stress, and lipid peroxidation, contributing to **plaque formation**. *Histidine* - **Histidine** is an essential amino acid involved in various metabolic processes, including the synthesis of **histamine** and carnosine. - It is not directly associated with the pathogenesis or risk factors for atherosclerosis. *Lysine* - **Lysine** is an essential amino acid important for **protein synthesis** and many bodily functions. - There is no established direct link between lysine levels and the development or progression of atherosclerosis. *Leucine* - **Leucine** is a branched-chain amino acid (BCAA) crucial for **muscle protein synthesis** and energy metabolism. - While some studies hint at associations between BCAAs and metabolic health, leucine itself is not a known direct risk factor or etiologic agent for atherosclerosis.

Q: Melanin is derived from which amino acid?

Tyrosine. ***Tyrosine*** - **Melanin synthesis** (melanogenesis) begins with the hydroxylation of **tyrosine** to DOPA (dihydroxyphenylalanine), catalyzed by the enzyme **tyrosinase**. - Subsequent steps involve the oxidation of DOPA and its derivatives to form various types of melanin, such as **eumelanin** (black/brown) and **pheomelanin** (red/yellow). *Phenylalanine* - **Phenylalanine** is an essential amino acid that can be converted to tyrosine in the body by the enzyme **phenylalanine hydroxylase**. - While it's a precursor to tyrosine, it's not directly converted to melanin; rather, it must first be hydroxylated to tyrosine. *Alanine* - **Alanine** is a non-essential amino acid involved in glucose metabolism and protein synthesis. - It is not a precursor for melanin synthesis. *Tryptophan* - **Tryptophan** is an essential amino acid that is a precursor for **serotonin**, **melatonin**, and **niacin** (Vitamin B3). - It is not involved in the biosynthesis of melanin.

Q: All are involved in non-toxic transport of ammonia except:

Glutaminase. ***Glutaminase*** - **Glutaminase** is an enzyme that **removes ammonia from glutamine**, producing glutamate and free ammonia. - While glutamine is a non-toxic carrier of ammonia, glutaminase releases the toxic ammonia, therefore it is not involved in the *non-toxic transport* itself. *Glutamine synthetase* - **Glutamine synthetase** catalyzes the ATP-dependent synthesis of **glutamine from glutamate and ammonia**. - This process effectively **traps free ammonia** into the non-toxic amino acid **glutamine**, making it a key component of non-toxic transport, especially in the brain. *SGPT* - **SGPT** (serum glutamic-pyruvic transaminase), also known as **Alanine transaminase (ALT)**, is involved in the transfer of an amino group from alanine to α-ketoglutarate, forming glutamate and pyruvate. - It plays a role in the **alanine cycle**, which is a significant mechanism for transporting ammonia from muscle to the liver, thereby contributing to non-toxic ammonia transport. *Alanine cycle* - The **alanine cycle** (or glucose-alanine cycle) is a pathway that **transports ammonia from muscle to the liver** in the form of alanine. - In muscle, pyruvate is transaminated to alanine using an amino group from glutamate, and alanine then travels to the liver for gluconeogenesis and urea cycle processing of the ammonia.

Q: Glutathione does all of the following except?

Decreases the stability of erythrocyte membranes. ***Decreases the stability of erythrocyte membranes*** - Glutathione actually **increases** the stability of erythrocyte membranes by protecting them from oxidative damage, particularly through its role in reducing **hydrogen peroxide** and organic hydroperoxides. - A deficiency in glutathione or the enzymes that reduce it (like **G6PD**) leads to increased oxidative stress, which **destabilizes** erythrocyte membranes and can cause **hemolysis**. *Scavenge peroxides* - Glutathione, in its reduced form (GSH), serves as a crucial **antioxidant** by **scavenging peroxides** through the action of **glutathione peroxidase**. - This reaction converts harmful **hydrogen peroxide** into water, protecting cellular components from oxidative damage. *Form conjugates with some drugs to increase water solubility.* - Glutathione plays a vital role in phase II detoxification reactions, where it forms **conjugates** with various xenobiotics and drugs. - This **conjugation** typically increases the **water solubility** of these compounds, facilitating their excretion from the body. *Participates in the transport of amino acids across some cell membranes* - Glutathione is involved in the **gamma-glutamyl cycle**, which is a mechanism for the **transport of amino acids** into cells, particularly in the kidney. - In this cycle, glutathione donates its **gamma-glutamyl group** to an amino acid, forming a gamma-glutamyl amino acid that is then transported across the membrane.

Q: Nitric oxide is synthesised from:

Arginine. ***Arginine*** - **Nitric oxide synthase (NOS)** enzymes catalyze the five-electron oxidation of **L-arginine** to L-citrulline and **nitric oxide (NO)**. - This reaction requires molecular oxygen (O2) and reduced nicotinamide adenine dinucleotide phosphate (**NADPH**) as co-substrates. *Cysteine* - **Cysteine** is a sulfur-containing amino acid involved in protein structure and antioxidant defense. - It is a precursor for **glutathione** and plays a role in **sulfur metabolism**, but not directly in NO synthesis. *Cystine* - **Cystine** is a dimer formed from two cysteine molecules linked by a disulfide bond. - It does not serve as a substrate for nitric oxide synthesis. *Methionine* - **Methionine** is an essential amino acid important for protein synthesis and as a precursor for **S-adenosylmethionine (SAM)**, a methyl donor. - It is not involved in the direct synthesis of nitric oxide.

Q: Positive nitrogen balance is seen in -

Growing children. ***Growing children*** - **Positive nitrogen balance** occurs when nitrogen intake (from protein) exceeds nitrogen excretion, indicating net protein synthesis and tissue anabolism. - This state is essential for **growth and development** as children build new tissues like muscle, bone, and organs, requiring more protein synthesis than breakdown. *Scurvy* - Scurvy is a disease caused by **vitamin C deficiency**, which is crucial for collagen synthesis. - It leads to impaired wound healing, weakened connective tissue, and is not directly associated with positive nitrogen balance. *Osteomalacia* - Osteomalacia is characterized by **softening of bones** due to impaired mineralization, often caused by **vitamin D deficiency**. - It relates to bone metabolism rather than overall protein synthesis or nitrogen balance. *Acute illness* - Acute illness, especially severe illness or trauma, typically leads to a **negative nitrogen balance**. - This occurs due to increased **catabolism** (protein breakdown) to provide energy and amino acids for the acute stress response and inflammation.

Q: Regarding glutathione all of the following are true EXCEPT

It inactivates some enzymes. ***It inactivates some enzymes*** - Glutathione is a crucial **antioxidant** that plays a vital role in protecting enzymes and other cellular components from oxidative damage by **reducing disulfide bonds**, thereby preserving or restoring enzyme activity rather than inactivating them. - Its primary function is to **detoxify harmful compounds** and maintain the cellular redox state, which requires enzymes to be active. *It helps in membrane transport* - Glutathione is involved in the active transport of certain substances across cell membranes, particularly in the **GSH-dependent transport systems** in the kidney and liver. - It forms conjugates with toxins, which are then transported out of the cell or body, a process often referred to as **efflux pumps**. *It helps in conjugation reactions* - Glutathione is a key substrate for **glutathione S-transferases (GSTs)**, enzymes that catalyze the conjugation of glutathione with various electrophilic compounds. - This process is vital for the **detoxification** and elimination of xenobiotics and endogenous toxic metabolites. *It helps in absorption of certain amino acids* - The **gamma-glutamyl cycle** (also known as the Meister cycle) in the kidneys utilizes glutathione for the transport of amino acids across cell membranes. - In this cycle, glutathione is broken down to release an amino acid, which is then transported into the cell, thus facilitating **amino acid uptake**.

Question 1

In a patient with maple syrup urine disease, all of the following amino acids should be restricted in diet except?

Accepted Answer

Methionine

Answer

Isoleucine

Answer

Leucine

Answer

Valine

Question 2

Which of the following options is false in a patient with advanced liver disease?

Accepted Answer

Increased oxidative decarboxylation of alpha-ketoglutarate

Answer

Decreased ATP synthesis

Answer

Hyperammonemia

Answer

Increased synthesis of glutamine and glutamate from alpha-ketoglutarate

Question 3

Ammonia formed in the brain is converted into

Accepted Answer

Glutamine

Answer

Glycine

Answer

Urea

Answer

Cysteine

Question 4

Atherosclerosis is associated with:

Accepted Answer

Homocysteine

Answer

Histidine

Answer

Lysine

Answer

Leucine

Question 5

Melanin is derived from which amino acid?

Accepted Answer

Tyrosine

Answer

Phenylalanine

Answer

Alanine

Answer

Tryptophan

Question 6

All are involved in non-toxic transport of ammonia except:

Accepted Answer

Glutaminase

Answer

Glutamine synthetase

Answer

SGPT

Answer

Alanine cycle

Question 7

Glutathione does all of the following except?

Accepted Answer

Decreases the stability of erythrocyte membranes

Answer

Scavenge peroxides

Answer

Form conjugates with some drugs to increase water solubility

Answer

Participates in the transport of amino acids across some cell membranes

Question 8

Nitric oxide is synthesised from:

Accepted Answer

Arginine

Answer

Cysteine

Answer

Cystine

Answer

Methionine

Question 9

Positive nitrogen balance is seen in -

Accepted Answer

Growing children

Answer

Scurvy

Answer

Osteomalacia

Answer

Acute illness

Question 10

Regarding glutathione all of the following are true EXCEPT

Accepted Answer

It inactivates some enzymes

Answer

It helps in membrane transport

Answer

It helps in conjugation reactions

Answer

It helps in absorption of certain amino acids

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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