Amino Acid Metabolism Practice Questions

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**.

Q: Acetylcholine is the neurotransmitter in the cholinergic neurons. What is the precursor that acetylcholine is derived from?

Choline. ***Choline*** - **Acetylcholine** is synthesized from **choline** and acetyl-CoA by the enzyme **choline acetyltransferase** in the cytoplasm of cholinergic neurons. - Choline is an essential nutrient obtained from the diet or synthesized endogenously, and it is actively transported into the neuron. *Dicholine* - **Dicholine** is not a naturally occurring biological precursor for acetylcholine synthesis. - This term does not refer to a standard biochemical compound involved in neurotransmitter synthesis. *Tyrosine* - **Tyrosine** is the precursor for **catecholamine neurotransmitters** such as dopamine, norepinephrine, and epinephrine, not acetylcholine. - The synthesis pathway for catecholamines involves enzymes like **tyrosine hydroxylase**, distinct from the acetylcholine pathway. *Cholic acid* - **Cholic acid** is a primary **bile acid** produced by the liver, involved in fat digestion and absorption in the small intestine. - It has no role in the synthesis of acetylcholine or other neurotransmitters.

Q: Because free ammonia in the blood is toxic to the body, it is transported in which of the following non-toxic forms?

Alanine and glutamine. ***Alanine and glutamine*** - **Glutamine** is the primary transporter of ammonia from most peripheral tissues to the liver and kidneys, where it can be safely released and processed. - **Alanine** transports ammonia from muscles to the liver via the glucose-alanine cycle, allowing for hepatic urea synthesis and glucose production. *Glutamine and urea* - While **glutamine** is a major ammonia transporter, **urea** is the *end product* of ammonia detoxification and is primarily synthesized in the liver for excretion. - Urea itself is not a transport form of ammonia *within* the blood before its synthesis; rather, it is the waste product. *Phenylalanine and methionine* - **Phenylalanine** and **methionine** are essential amino acids involved in protein synthesis and various metabolic pathways. - They do not serve as significant carriers for the transport of toxic free ammonia in the bloodstream. *Histidine and urea* - **Histidine** is an amino acid involved in protein synthesis and histamine production, but not primarily in ammonia transport. - As mentioned, **urea** is the final excretory form of detoxified ammonia, not a transport form within the blood.

Q: A 35-year-old man presents to the physician with arthritic pain in both knees along with back pain. He states that the pain has been present for months. The patient is afebrile, and his slightly swollen knee joints are neither hot nor tender to palpation; however the pain does restrict his motion. The cartilage of his ears appears slightly darker than normal. No tophi are present. A urine specimen is taken for analysis of uric acid content and turns black in the laboratory while standing. What is the most likely diagnosis at this point?

Alkaptonuria. ### Alkaptonuria - The key diagnostic features are **arthritic pain in the knees and back**, **darkening of ear cartilage (ochronosis)** [1], and **urine turning black on standing** due to the oxidation of homogentisic acid. - This is an **autosomal recessive disorder** caused by a deficiency of **homogentisate 1,2-dioxygenase**, leading to the accumulation of homogentisic acid. ### Ankylosing spondylitis - Characterized by **chronic inflammatory back pain and stiffness** that improves with exercise, often associated with sacroiliitis. - While back pain is present, the absence of findings like **HLA-B27 positivity** (not mentioned) and the specific urine and cartilage findings make it less likely. ### Maple syrup urine disease - This is a metabolic disorder affecting the metabolism of **branched-chain amino acids** (leucine, isoleucine, valine), leading to a characteristic **sweet-smelling urine**. - It typically presents in infancy with severe neurological symptoms and feeding difficulties, not with arthritic pain and dark urine. ### Phenylketonuria - Caused by a deficiency of **phenylalanine hydroxylase**, leading to the accumulation of phenylalanine. - Presents with severe intellectual disability, seizures, and a musty odor in urine if untreated, without the specific arthritic or ochronotic findings seen here.

Q: A 20-year-old male presents with weight loss, heat intolerance, bilateral exophthalmos, a lid lag, sweating, and tachycardia. These symptoms are due to an increased production and secretion of a hormone that is derived from which one of the following?

Tyrosine. ***Tyrosine*** - The symptoms described (weight loss, heat intolerance, exophthalmos, lid lag, sweating, tachycardia) are characteristic of **hyperthyroidism**, specifically **Graves' disease**. - Thyroid hormones (**T3 and T4**) are synthesized from the amino acid **tyrosine** through iodination of tyrosine residues on thyroglobulin in the thyroid gland. - This makes tyrosine the direct precursor for thyroid hormone synthesis. *Tryptophan* - **Tryptophan** is a precursor for the synthesis of **serotonin** and **melatonin**, neurotransmitters involved in mood regulation and sleep-wake cycles, not thyroid function. - Deficiency or altered metabolism of tryptophan is associated with conditions like **depression** or **carcinoid syndrome**, which have different clinical presentations. *Dopamine* - **Dopamine** is a catecholamine neurotransmitter, not a precursor for thyroid hormones. - While dopamine is synthesized from tyrosine (via the pathway: tyrosine → L-DOPA → dopamine), this is a completely separate biochemical pathway from thyroid hormone synthesis. - Symptoms of altered dopamine levels are associated with conditions like **Parkinson's disease** (dopamine deficiency) or **schizophrenia** (dopamine dysregulation), which do not match this patient's hyperthyroid presentation. *Cholesterol* - **Cholesterol** is the precursor for **steroid hormones** (glucocorticoids, mineralocorticoids, androgens, estrogens) and vitamin D, but not thyroid hormones. - Conditions related to altered cholesterol-derived hormone metabolism include various **endocrine disorders** involving adrenal or gonadal dysfunction, which present with different clinical features.

Q: Ammonia is detoxified in brain to :

Glutamine. ***Glutamine*** - In the brain, **ammonia** is primarily detoxified through its conversion into **glutamine** by the enzyme **glutamine synthetase**. - This process is crucial for preventing **neurotoxicity** as ammonia can disrupt neuronal function and energy metabolism. *Urea* - **Urea** is the primary end product of **ammonia detoxification** in the **liver** through the **urea cycle**. - While urea can cross the blood-brain barrier, it is not the main mechanism for local ammonia detoxification within brain cells. *GABA* - **GABA (gamma-aminobutyric acid)** is an **inhibitory neurotransmitter** formed from **glutamate**. - It plays a vital role in neuronal signaling but is not directly involved in the detoxification of ammonia in the brain. *Uric acid* - **Uric acid** is the end product of **purine metabolism** and acts as an antioxidant. - It is not directly involved in the detoxification pathway of ammonia in the brain or any other organ.

Q: Amino acid derived hormones are all except:

FSH.. ***FSH (Follicle-Stimulating Hormone)*** - **FSH is a glycoprotein/peptide hormone**, NOT an amino acid-derived hormone - While all proteins are made from amino acid chains, **amino acid-derived hormones** specifically refers to hormones derived from **single modified amino acids** (tyrosine or tryptophan) - FSH belongs to the **peptide/protein hormone** class, which is a distinct category - **This is the correct answer** to this "all EXCEPT" question *Adrenal medullary hormone* - Refers to **epinephrine** and **norepinephrine** (catecholamines) - These are derived from the single amino acid **tyrosine** through enzymatic modification - Classic examples of **amino acid-derived hormones** *Thyroid hormone* - **Thyroxine (T4)** and **triiodothyronine (T3)** are derived from **tyrosine** - Formed by coupling of iodinated tyrosine residues on thyroglobulin - Clear examples of **amino acid-derived hormones** *None* - This would imply all listed options are amino acid-derived hormones - This is incorrect since FSH is a peptide hormone, not an amino acid-derived hormone

Q: BOAA toxin responsible for Neurolathyrism contains which amino acid?

Alanine. ***Alanine*** - BOAA (beta-N-oxalylamino-L-alanine), the neurotoxin responsible for neurolathyrism, is a non-protein amino acid that is a **derivative of alanine**. - Its chemical structure is characterized by an **oxalyl group attached to the amino group of alanine**. *Methionine* - Methionine is an essential amino acid with a **sulfur-containing side chain**, not found in BOAA. - It is involved in various metabolic processes like **methylation**, but not directly related to neurolathyrism. *Lysine* - Lysine is a basic essential amino acid with a **long side chain ending in an amino group**. - It plays a role in **protein synthesis** and **collagen formation**, but its structure is distinct from BOAA. *Tryptophan* - Tryptophan is an essential amino acid known for its **indole ring structure**, serving as a precursor for serotonin and niacin. - Its complex cyclic structure is **not related to the simpler oxalyl-alanine structure** of BOAA.

Question 1

Nitric oxide is synthesised from:

Accepted Answer

Arginine

Answer

Cysteine

Answer

Cystine

Answer

Methionine

Question 2

Positive nitrogen balance is seen in -

Accepted Answer

Growing children

Answer

Scurvy

Answer

Osteomalacia

Answer

Acute illness

Question 3

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

Question 4

Acetylcholine is the neurotransmitter in the cholinergic neurons. What is the precursor that acetylcholine is derived from?

Accepted Answer

Choline

Answer

Dicholine

Answer

Tyrosine

Answer

Cholic acid

Question 5

Because free ammonia in the blood is toxic to the body, it is transported in which of the following non-toxic forms?

Accepted Answer

Alanine and glutamine

Answer

Glutamine and urea

Answer

Phenylalanine and methionine

Answer

Histidine and urea

Question 6

A 35-year-old man presents to the physician with arthritic pain in both knees along with back pain. He states that the pain has been present for months. The patient is afebrile, and his slightly swollen knee joints are neither hot nor tender to palpation; however the pain does restrict his motion. The cartilage of his ears appears slightly darker than normal. No tophi are present. A urine specimen is taken for analysis of uric acid content and turns black in the laboratory while standing. What is the most likely diagnosis at this point?

Accepted Answer

Alkaptonuria

Answer

Ankylosing spondylitis

Answer

Maple syrup urine disease

Answer

Phenylketonuria

Question 7

A 20-year-old male presents with weight loss, heat intolerance, bilateral exophthalmos, a lid lag, sweating, and tachycardia. These symptoms are due to an increased production and secretion of a hormone that is derived from which one of the following?

Accepted Answer

Tyrosine

Answer

Tryptophan

Answer

Dopamine

Answer

Cholesterol

Question 8

Ammonia is detoxified in brain to :

Accepted Answer

Glutamine

Answer

Urea

Answer

GABA

Answer

Uric acid

Question 9

Amino acid derived hormones are all except:

Accepted Answer

FSH.

Answer

None.

Answer

Adrenal medullary hormone.

Answer

Thyroid hormone.

Question 10

BOAA toxin responsible for Neurolathyrism contains which amino acid?

Accepted Answer

Alanine

Answer

Methionine

Answer

Lysine

Answer

Tryptophan

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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