Amino Acid Metabolism Practice Questions

Q: Which will activate carbamoyl phosphate synthase I?

N-acetyl glutamate. **N-acetyl glutamate** - **N-acetyl glutamate** is an **allosteric activator** of **carbamoyl phosphate synthase I (CPS I)**, which is the mitochondrial enzyme that catalyzes the first committed step of the **urea cycle**. - Its synthesis is stimulated by high levels of **arginine**, linking nitrogen load to urea production. *Acetyl-CoA* - Acetyl-CoA is a common **substrate** and **product** in various metabolic pathways, but it is not a direct activator of CPS I. - It is a precursor for the synthesis of **N-acetyl glutamate**, but does not activate CPS I directly. *Ornithine* - **Ornithine** is a key intermediate of the **urea cycle**, but it does not directly activate CPS I. - It combines with carbamoyl phosphate (the product of CPS I) in the second step of the urea cycle to form citrulline. *ATP* - **ATP** is a **substrate** used by CPS I to provide energy for the synthesis of carbamoyl phosphate. - While essential for the reaction, ATP itself does not act as an allosteric activator of the enzyme.

Q: Neonatal tyrosinemia is due to deficiency of which enzyme?

Hydroxyphenyl pyruvate hydroxylase. ***Hydroxyphenyl pyruvate hydroxylase*** - **Neonatal (transient) tyrosinemia** is caused by delayed maturation or deficiency of **hydroxyphenylpyruvate hydroxylase** (also called 4-hydroxyphenylpyruvate dioxygenase or HPPD). - This enzyme converts 4-hydroxyphenylpyruvate to homogentisic acid in tyrosine catabolism. - Common in **premature infants** and newborns, leading to elevated tyrosine levels in blood. - The condition is **benign and self-limiting**, usually resolving with **vitamin C supplementation** or as the enzyme matures. - Note: Severe hereditary deficiency of this enzyme causes **tyrosinemia type III**, a distinct and rare disorder. *Fumarylacetoacetate hydroxylase* - Deficiency of **fumarylacetoacetate hydroxylase (FAH)** causes **tyrosinemia type I** (hepatorenal tyrosinemia), NOT neonatal tyrosinemia. - This is a severe hereditary disorder with liver failure, renal tubular dysfunction, and accumulation of toxic metabolites like succinylacetone. - Distinct from the benign transient neonatal form. *Tyrosine transaminase* - Deficiency of **tyrosine transaminase** (tyrosine aminotransferase) causes **tyrosinemia type II** (Richner-Hanhart syndrome). - Presents with corneal ulcers, palmoplantar hyperkeratosis, and sometimes intellectual disability. *Tyrosinase* - Deficiency of **tyrosinase** causes **albinism**, characterized by lack of melanin pigment in skin, hair, and eyes. - Not involved in tyrosine catabolism but in melanin synthesis.

Q: Taurine is biosynthesized from which amino acid?

Cysteine. ***Cysteine*** - **Taurine** is primarily synthesized from the amino acid **cysteine** through a pathway involving **cysteine sulfinic acid** and **hypotaurine**. - This pathway utilizes enzymes like **cysteine dioxygenase** and **cysteine sulfinic acid decarboxylase**. - The biosynthetic pathway: Cysteine → Cysteine sulfinic acid → Hypotaurine → Taurine. *Arginine* - **Arginine** is a precursor for **nitric oxide**, **urea**, and **creatine**, not taurine. - It is involved in various metabolic pathways, including the **urea cycle** and protein synthesis. *Valine* - **Valine** is a **branched-chain amino acid (BCAA)** involved in protein synthesis and energy production. - It is not a direct precursor for taurine biosynthesis. *Leucine* - **Leucine** is also a **branched-chain amino acid (BCAA)** crucial for protein synthesis and muscle metabolism. - It does not participate in the synthesis of taurine.

Q: Methionine can enter the TCA cycle at which level?

Succinyl-CoA. ***Succinyl - CoA*** - Methionine is a **glucogenic amino acid** that is catabolized to propionyl-CoA, which is then converted to **methylmalonyl-CoA** and finally to **succinyl-CoA**. - **Succinyl-CoA** is an intermediate of the **TCA cycle**, allowing methionine-derived carbons to enter the cycle. *Fumarate* - Fumarate is an intermediate of the TCA cycle, but methionine catabolism does not directly produce **fumarate**. - Amino acids like **phenylalanine** and **tyrosine** can be catabolized to fumarate. *Oxaloacetate* - **Oxaloacetate** is a TCA cycle intermediate and can be formed from **pyruvate** (via pyruvate carboxylase) or from certain amino acids like **aspartate** and **asparagine**. - Methionine does not directly convert to oxaloacetate. *Citrate* - **Citrate** is the first intermediate formed in the TCA cycle when **acetyl-CoA** combines with **oxaloacetate**. - Methionine catabolism does not lead to the direct formation of citrate.

Q: Carnitine is synthesised from -

Lysine. ***Lysine*** - **Carnitine** is synthesized in the liver and kidneys from the amino acids **lysine** and methionine. - **Lysine provides the essential carbon backbone** for carnitine synthesis (trimethyllysine is the actual precursor formed from protein-bound lysine residues). - Methionine contributes methyl groups via S-adenosylmethionine (SAM), but lysine is the primary structural precursor. *Arginine* - **Arginine** is a precursor for **nitric oxide**, urea, and creatine, but not a direct precursor for carnitine synthesis. - While arginine is an amino acid, its metabolic pathways are distinct from those involved in carnitine formation. *Histidine* - **Histidine** is a precursor for **histamine** and contributes to protein synthesis, but is not involved in carnitine biosynthesis. - Its metabolic fate differs significantly from the pathway leading to carnitine. *Choline* - **Choline** is a precursor for **acetylcholine** and phospholipids, but not directly for carnitine. - Although both choline and carnitine contain methyl groups, they have different biosynthetic origins.

Q: Which amino acid is used by the liver in the urea cycle?

Aspartate. ***Aspartate*** - **Aspartate** provides the second nitrogen atom to the urea cycle, directly contributing to the formation of **argininosuccinate** through condensation with citrulline. - It is crucial for the efficient removal of **ammonia** in the form of urea. *Glutamine* - **Glutamine** transports ammonia from peripheral tissues to the liver and kidneys, but it is typically deamidated to **glutamate** before its nitrogen can enter the urea cycle. - While it's a major ammonia carrier, it's not directly incorporated into urea as an intact amino acid. *Glutamate* - **Glutamate** can donate its amino group to form **aspartate** (via transamination with oxaloacetate) or release ammonia directly (via glutamate dehydrogenase), both of which then enter the urea cycle. - However, glutamate itself is not directly incorporated into the urea molecule in the same way aspartate is. *Ornithine* - **Ornithine** is an amino acid that participates in the urea cycle as a carrier molecule, being regenerated at the end of each cycle. - While essential for the cycle to function, it is not "used" in the sense of being consumed or providing nitrogen for urea formation - rather it acts as a catalytic intermediate that is recycled.

Q: Which of the following substances is not derived from tyrosine?

Nicotinic acid. ***Nicotinic acid*** - **Nicotinic acid** (niacin, vitamin B3) is synthesized from **tryptophan** in the body, not tyrosine. - It plays a crucial role in metabolism as a precursor for NAD+ and NADH, which are involved in various enzymatic reactions. *Thyroxine* - **Thyroxine** (T4), a thyroid hormone, is derived from the amino acid **tyrosine**. - **Iodine** is incorporated into tyrosine residues on thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT), which then couple to form T4 (and T3). *Melanin* - **Melanin**, the pigment responsible for skin, hair, and eye color, is synthesized from **tyrosine** through a pathway involving the enzyme **tyrosinase**. - This process involves the hydroxylation of tyrosine to 3,4-dihydroxyphenylalanine (DOPA) and subsequent oxidation reactions. *Dopamine* - **Dopamine**, an important neurotransmitter, is synthesized from **tyrosine** in a two-step process in the brain and adrenal medulla. - Tyrosine is first hydroxylated to DOPA by **tyrosine hydroxylase**, and then DOPA is decarboxylated to dopamine by DOPA decarboxylase.

Q: Which enzyme is deficient in Isovaleric acidemia?

Isovaleryl CoA dehydrogenase. ***Isovaleryl CoA dehydrogenase*** - **Isovaleric acidemia** is an **autosomal recessive** metabolic disorder caused by a deficiency in the enzyme **isovaleryl-CoA dehydrogenase** - This enzyme is crucial for the metabolism of **leucine**, a branched-chain amino acid, leading to the accumulation of toxic byproducts like **isovaleryl-CoA** and **isovaleric acid** - Characteristic **sweaty feet odor** due to isovaleric acid accumulation *Phenylalanine hydroxylase* - A deficiency in **phenylalanine hydroxylase** is responsible for **phenylketonuria (PKU)**, a different metabolic disorder involving the metabolism of **phenylalanine** - This enzyme converts **phenylalanine to tyrosine**, and its deficiency leads to the accumulation of phenylalanine and its metabolites, causing neurological damage if untreated *Arginase* - A deficiency in **arginase** causes **argininemia (hyperargininemia)**, which is a disorder of the **urea cycle** - This enzyme converts **arginine into urea and ornithine**, and its deficiency leads to the buildup of arginine and ammonia in the blood, causing neurological symptoms and developmental delay *Methylmalonyl CoA mutase* - A deficiency in **methylmalonyl CoA mutase** causes **methylmalonic acidemia**, another organic acidemia distinct from isovaleric acidemia - This disorder involves **propionate metabolism** and can present with metabolic acidosis, but affects a different metabolic pathway than leucine catabolism

Q: Which enzyme deficiency is responsible for Hyperammonemia type-1?

Carbamoyl phosphate synthetase I (CPS-1) deficiency. ***Carbamoyl phosphate synthetase I (CPS-1) deficiency*** - This enzyme deficiency is classified as **Hyperammonemia type-1**, or **CPS1 deficiency**, and results in the inability to initiate the urea cycle. - **CPS-1** catalyzes the first committed step of the urea cycle, combining ammonia and bicarbonate to form carbamoyl phosphate. *Arginase deficiency* - This deficiency causes **Hyperargininemia**, which is a disorder of the urea cycle distinct from Hyperammonemia type-1. - Arginase is involved in the final step of the urea cycle, converting arginine to urea and ornithine. *Arginosuccinate lyase deficiency* - This deficiency leads to **Argininosuccinic aciduria**, another urea cycle disorder. - **Arginosuccinate lyase** is responsible for breaking down argininosuccinate into arginine and fumarate. *Arginosuccinate synthase deficiency* - This deficiency causes **Citrullinemia type 1**, a metabolic disorder characterized by high levels of citrulline and ammonia. - **Arginosuccinate synthase** catalyzes the condensation of citrulline and aspartate to form argininosuccinate.

Q: Where does oxidative deamination primarily occur in the human body?

Mitochondria of liver cells. ***Mitochondria of liver cells*** - **Oxidative deamination**, particularly of glutamate, is a central process in **amino acid catabolism** and occurs predominantly in the **mitochondria of liver cells**. - This process is crucial for removing the **amino group (NH3)** from amino acids, forming ammonia, which is then detoxified into urea. *Cytoplasm of all cells* - While cells have cytoplasmic metabolic pathways, the primary enzyme for oxidative deamination, **glutamate dehydrogenase**, is located in the mitochondria. - The cytoplasm primarily handles glycolysis and various synthetic pathways, but not the bulk of oxidative deamination. *Mitochondria of all cells* - Although mitochondria are the site of oxidative metabolism in most cells, the **liver** is the main organ responsible for processing exogenous amino acids and their subsequent comprehensive deamination. - Other cells perform some amino acid metabolism, but not the large-scale oxidative deamination seen in the liver. *Cytoplasm of liver cells* - The cytoplasm of liver cells is involved in various metabolic processes, including gluconeogenesis and fatty acid synthesis. - However, the key enzymes for oxidative deamination are specifically compartmentalized within the **mitochondria** of these cells, not the cytoplasm.

Question 1

Which will activate carbamoyl phosphate synthase I?

Accepted Answer

N-acetyl glutamate

Answer

ATP

Answer

Acetyl-CoA

Answer

Ornithine

Question 2

Neonatal tyrosinemia is due to deficiency of which enzyme?

Accepted Answer

Hydroxyphenyl pyruvate hydroxylase

Answer

Tyrosine transaminase

Answer

Fumarylacetoacetate hydroxylase

Answer

Tyrosinase

Question 3

Taurine is biosynthesized from which amino acid?

Accepted Answer

Cysteine

Answer

Valine

Answer

Arginine

Answer

Leucine

Question 4

Methionine can enter the TCA cycle at which level?

Accepted Answer

Succinyl-CoA

Answer

Fumarate

Answer

Oxaloacetate

Answer

Citrate

Question 5

Carnitine is synthesised from -

Accepted Answer

Lysine

Answer

Histidine

Answer

Choline

Answer

Arginine

Question 6

Which amino acid is used by the liver in the urea cycle?

Accepted Answer

Aspartate

Answer

Glutamine

Answer

Glutamate

Answer

Ornithine

Question 7

Which of the following substances is not derived from tyrosine?

Accepted Answer

Nicotinic acid

Answer

Thyroxine

Answer

Melanin

Answer

Dopamine

Question 8

Which enzyme is deficient in Isovaleric acidemia?

Accepted Answer

Isovaleryl CoA dehydrogenase

Answer

Phenylalanine hydroxylase

Answer

Arginase

Answer

Methylmalonyl CoA mutase

Question 9

Which enzyme deficiency is responsible for Hyperammonemia type-1?

Accepted Answer

Carbamoyl phosphate synthetase I (CPS-1) deficiency

Answer

Arginase deficiency

Answer

Arginosuccinate lyase deficiency

Answer

Arginosuccinate synthase deficiency

Question 10

Where does oxidative deamination primarily occur in the human body?

Accepted Answer

Mitochondria of liver cells

Answer

Cytoplasm of all cells

Answer

Mitochondria of all cells

Answer

Cytoplasm of liver cells

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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