Amino Acid Metabolism Practice Questions

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.

Q: Which amino acid is involved in nitric oxide synthesis?

Arginine. ***Arginine*** - **Arginine** is the direct precursor for the synthesis of **nitric oxide (NO)** via the enzyme nitric oxide synthase (NOS). - The conversion of arginine to citrulline liberates nitric oxide, a crucial signaling molecule involved in various physiological processes. *Glutamine* - **Glutamine** is an abundant amino acid involved in protein synthesis, acid-base balance, and as a precursor for glutathione and neurotransmitters. - It does not directly serve as a substrate for nitric oxide synthesis. *Proline* - **Proline** is essential for collagen synthesis and wound healing and plays a role in cellular stress responses. - It is not directly involved in the biochemical pathway leading to nitric oxide production. *Lysine* - **Lysine** is an essential amino acid important for protein synthesis, calcium absorption, and the production of hormones and enzymes. - It is not a precursor for nitric oxide synthesis.

Q: Which of these is a key component of the urea cycle in the liver?

Citrulline. ***Citrulline*** - **Citrulline** is an amino acid that plays a crucial role as an intermediate in the **urea cycle**, being formed from ornithine and carbamoyl phosphate in the mitochondria. - It is transported out of the mitochondria into the cytoplasm, where it is further converted to argininosuccinate. - This makes it a **key component** of ammonia detoxification. *Glycogen* - **Glycogen** is a polysaccharide that serves as the primary **storage form of glucose** in animals and fungi. - Its metabolism is involved in energy regulation but is **not a component of the urea cycle**. *Fructose* - **Fructose** is a simple sugar, a **monosaccharide**, found in many fruits and honey. - It is primarily metabolized in the liver via fructolysis, but it is **not directly involved in the urea cycle**. *Lactate* - **Lactate** is produced from pyruvate during anaerobic glycolysis and is involved in the **Cori cycle** for gluconeogenesis. - While it is metabolized in the liver, it is **not a component of the urea cycle**.

Q: An infant presented with vomiting, malnutrition, blue eyes, blonde hair & fair skin. On investigation, Guthrie test was positive. All are true regarding this disease EXCEPT:

Hypopigmentation due to tryptophan deficiency. ***Hypopigmentation due to tryptophan deficiency*** - The characteristic **hypopigmentation** (fair skin, blonde hair, blue eyes) in **phenylketonuria (PKU)** is due to **tyrosine deficiency**, not tryptophan deficiency. - **Phenylalanine hydroxylase (PAH)** deficiency leads to accumulation of phenylalanine, which cannot be converted to **tyrosine**. - **Tyrosine** is the precursor for **melanin synthesis** via the enzyme **tyrosinase**, so tyrosine deficiency results in decreased melanin production and hypopigmentation. *Phenyl acetate positive in urine* - In **phenylketonuria (PKU)**, **phenylalanine** accumulates and is shunted to alternative metabolic pathways, leading to the production and excretion of **phenylacetate, phenylpyruvate, and phenyllactate** in the urine. - The presence of these metabolites gives the urine a characteristic **mousey or musty odor**. *Mental retardation is present* - If **phenylketonuria (PKU)** is left untreated, the accumulation of **phenylalanine** is neurotoxic and leads to severe, **irreversible intellectual disability** and **developmental delay**. - Early detection through newborn screening (the **Guthrie test** detects elevated blood phenylalanine) and dietary phenylalanine restriction are crucial to prevent this outcome. *Due to PAH enzyme defect* - **Phenylketonuria (PKU)** is primarily caused by a deficiency in the enzyme **phenylalanine hydroxylase (PAH)**, which is responsible for converting phenylalanine to tyrosine. - This **autosomal recessive genetic disorder** leads to the accumulation of phenylalanine in the blood and tissues, causing the clinical manifestations.

Q: Which of the following is not an amino acid-derived neurotransmitter?

Creatine. ***Creatine*** - **Creatine** is an organic compound that helps supply energy to cells, primarily muscle cells, but it does **not** function as a neurotransmitter. - It is synthesized from **amino acids** like arginine, glycine, and methionine, but it is not itself an amino acid-derived neurotransmitter. *Dopamine* - **Dopamine** is a **catecholamine neurotransmitter** derived from the amino acid **tyrosine**. - It plays crucial roles in **motivation**, reward, motor control, and various other brain functions. *GABA* - **GABA** (gamma-aminobutyric acid) is the **primary inhibitory neurotransmitter** in the central nervous system, derived from the amino acid **glutamate**. - It works to reduce neuronal excitability throughout the nervous system. *Serotonin* - **Serotonin** (5-hydroxytryptamine or 5-HT) is a **monoamine neurotransmitter** derived from the amino acid **tryptophan**. - It regulates mood, appetite, sleep, and numerous other physiological processes.

Q: Which enzyme deficiency leads to hyperhomocysteinemia?

Methionine synthase. ***Methionine synthase*** - **Methionine synthase** (also known as homocysteine methyltransferase) is critical for converting **homocysteine** back to **methionine** using **methyltetrahydrofolate (MTHF)** as a methyl donor. - A deficiency in this enzyme, or its cofactors like **vitamin B12** or **folate**, leads to the accumulation of homocysteine in the blood, resulting in **hyperhomocysteinemia**. *Phenylalanine hydroxylase* - Deficiency in **phenylalanine hydroxylase** causes **phenylketonuria (PKU)**, characterized by the accumulation of **phenylalanine** and its metabolites, not homocysteine. - This enzyme is responsible for converting **phenylalanine** to **tyrosine**. *Glucose-6-phosphate dehydrogenase* - Deficiency in **glucose-6-phosphate dehydrogenase (G6PD)** leads to **hemolytic anemia** in response to oxidative stress, as it impairs the production of **NADPH** in the **pentose phosphate pathway**. - It is not directly involved in homocysteine metabolism. *Galactose-1-phosphate uridyltransferase* - Deficiency in **galactose-1-phosphate uridyltransferase** is the cause of **classic galactosemia**, leading to the accumulation of **galactose-1-phosphate** and toxicity. - This enzyme is crucial for the metabolism of **galactose**, not homocysteine.

Q: Which enzyme deficiency causes Maple Syrup Urine Disease?

Branched-chain ketoacid dehydrogenase. ***Branched-chain ketoacid dehydrogenase*** - **Maple Syrup Urine Disease (MSUD)** is caused by deficiency in the branched-chain α-keto acid dehydrogenase complex (BCKDC) - This enzyme is crucial for metabolism of the **branched-chain amino acids (BCAAs)**: leucine, isoleucine, and valine - Accumulation of BCAAs and their keto acids gives urine a characteristic **maple syrup odor** - Presents with **neurological symptoms, ketoacidosis, and intellectual disability** if untreated *Glucose-6-phosphatase* - Deficiency causes Glycogen Storage Disease Type I (von Gierke disease), not MSUD - This enzyme is essential for the final step of gluconeogenesis and glycogenolysis - Leads to hypoglycemia and lactic acidosis *Phenylalanine hydroxylase* - Deficiency causes phenylketonuria (PKU), not MSUD - This enzyme converts phenylalanine to tyrosine - Absence leads to toxic accumulation of phenylalanine causing intellectual disability and musty odor *Pyruvate dehydrogenase* - Deficiency affects conversion of pyruvate to acetyl-CoA, not branched-chain amino acid metabolism - Leads to buildup of pyruvate shunted to lactate - Causes lactic acidosis and neurological symptoms

Q: What is the primary source of ammonia in the body?

Amino acid catabolism. ***Amino acid catabolism*** - The **deamination** of amino acids, particularly in the liver, is the primary metabolic process that removes the amino group and releases **ammonia (NH3)**. - This ammonia is then converted to **urea** via the urea cycle for excretion. *Carbohydrate metabolism* - This process primarily yields **ATP**, carbon dioxide, and water from sugars. - While some intermediates may contain nitrogen, it does not directly produce significant amounts of **ammonia**. *Lipid metabolism* - The breakdown of fats mainly produces **fatty acids** and glycerol, which are then metabolized for energy. - **Ammonia** is not a direct or significant end-product of lipid breakdown. *Purine degradation* - The degradation of **purine nucleotides** (adenine and guanine) produces **uric acid** as the main nitrogenous waste product in humans. - While ammonia can be formed from adenosine deamination, it contributes a relatively small amount compared to **amino acid catabolism**.

Question 1

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 2

NO is synthesized from:

Accepted Answer

L-arginine

Answer

L-citrulline

Answer

Glycine

Answer

Lysine

Question 3

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

Question 4

Which amino acid is involved in nitric oxide synthesis?

Accepted Answer

Arginine

Answer

Glutamine

Answer

Proline

Answer

Lysine

Question 5

Which of these is a key component of the urea cycle in the liver?

Accepted Answer

Citrulline

Answer

Glycogen

Answer

Fructose

Answer

Lactate

Question 6

An infant presented with vomiting, malnutrition, blue eyes, blonde hair & fair skin. On investigation, Guthrie test was positive. All are true regarding this disease EXCEPT:

Accepted Answer

Hypopigmentation due to tryptophan deficiency

Answer

Phenyl acetate positive in urine

Answer

Mental retardation is present

Answer

Due to PAH enzyme defect

Question 7

Which of the following is not an amino acid-derived neurotransmitter?

Accepted Answer

Creatine

Answer

Dopamine

Answer

GABA

Answer

Serotonin

Question 8

Which enzyme deficiency leads to hyperhomocysteinemia?

Accepted Answer

Methionine synthase

Answer

Phenylalanine hydroxylase

Answer

Glucose-6-phosphate dehydrogenase

Answer

Galactose-1-phosphate uridyltransferase

Question 9

Which enzyme deficiency causes Maple Syrup Urine Disease?

Accepted Answer

Branched-chain ketoacid dehydrogenase

Answer

Glucose-6-phosphatase

Answer

Phenylalanine hydroxylase

Answer

Pyruvate dehydrogenase

Question 10

What is the primary source of ammonia in the body?

Accepted Answer

Amino acid catabolism

Answer

Carbohydrate metabolism

Answer

Lipid metabolism

Answer

Purine degradation

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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