Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 581: Which of the following is derived from tyrosine?

A. Melatonin
B. Serotonin
C. Melanin (Correct Answer)
D. Niacin

Explanation: ***Melanin*** - **Melanin** is synthesized from **tyrosine** through a process involving the enzyme **tyrosinase**. - It is a pigment responsible for skin, hair, and eye color, and plays a role in protecting against UV radiation. *Melatonin* - **Melatonin** is a hormone primarily produced in the pineal gland and is derived from **tryptophan**, not tyrosine. - It regulates sleep-wake cycles and has antioxidant properties. *Serotonin* - **Serotonin** (5-hydroxytryptamine) is a neurotransmitter derived from the amino acid **tryptophan**. - It plays a crucial role in mood regulation, sleep, appetite, and digestion. *Niacin* - **Niacin** (vitamin B3) can be synthesized from the amino acid **tryptophan**, not tyrosine. - It is essential for diverse metabolic functions, including energy production and DNA repair.

Question 582: Which of the following is a symptom associated with metabolites of tryptophan?

A. Diarrhea
B. Vasodilation
C. All of these (Correct Answer)
D. Flushing

Explanation: ***All of these*** - Tryptophan metabolites, particularly **serotonin**, are associated with all three symptoms listed, commonly seen in **carcinoid syndrome**. - **Diarrhea** occurs due to serotonin's effect on increasing **gut motility** and intestinal secretions. - **Vasodilation** results from serotonin acting as a powerful vasodilator when released from enterochromaffin cells. - **Flushing** is a clinical manifestation of the vasodilatory effects of serotonin, causing visible skin redness. - These symptoms collectively form the classic presentation of excess serotonin production from tryptophan metabolism. *Individual options (Diarrhea, Vasodilation, Flushing)* - While each of these is a valid symptom caused by tryptophan metabolites, selecting only one would be incomplete. - The question asks for symptoms (plural context implied), making "All of these" the most comprehensive answer. - All three symptoms are medically accurate manifestations of elevated serotonin levels.

Question 583: Which of the following is a non-protein amino acid?

A. Histidine
B. Ornithine (Correct Answer)
C. Tyrosine
D. Aspartate

Explanation: ***Ornithine*** - **Ornithine** is a non-protein amino acid that plays a crucial role in the **urea cycle**, helping to detoxify ammonia in the body - It is not incorporated into proteins during translation but is an important **metabolic intermediate** - Other examples of non-protein amino acids include **citrulline** (also in urea cycle), **homocysteine**, and **GABA** *Histidine* - **Histidine** is an **essential proteinogenic amino acid** that is directly incorporated into proteins during translation - It cannot be synthesized by the human body and must be obtained from the diet - It is a precursor to **histamine**, a crucial molecule involved in immune responses and gastric acid secretion *Tyrosine* - **Tyrosine** is a **proteinogenic amino acid** synthesized from phenylalanine, making it conditionally essential - It is incorporated into proteins and serves as a precursor for **thyroid hormones**, **catecholamines** (dopamine, norepinephrine, epinephrine), and **melanin** *Aspartate* - **Aspartate** is a standard **proteinogenic amino acid** that is directly incorporated into proteins during translation - It functions as a **neurotransmitter** and participates in metabolic pathways including the synthesis of purines and pyrimidines

Question 584: Which of the following amino acids is extracted predominantly by muscle, having been spared by the liver in the postprandial state?

A. Valine (Correct Answer)
B. Glutamine
C. Glutamate
D. Alanine

Explanation: ***Valine*** - **Valine** is a **branched-chain amino acid (BCAA)** that is primarily metabolized by muscle tissue rather than the liver. - In the **postprandial state**, the liver allows BCAAs to bypass its metabolism, making them readily available for uptake and utilization by muscles for protein synthesis. *Glutamine* - **Glutamine** is an amino acid primarily involved in **nitrogen transport**, ammonia detoxification, and as an energy source for rapidly dividing cells like immune cells and enterocytes. - While muscles can synthesize and release glutamine, it's not predominantly *extracted* by muscles from circulation in the same manner as BCAAs for protein synthesis. *Glutamate* - **Glutamate** is a non-essential amino acid involved in various metabolic pathways, including neurotransmission and protein synthesis. - The liver is highly efficient in metabolizing glutamate, and it is not typically spared for predominant extraction by muscle in the postprandial state. *Alanine* - **Alanine** is a product of muscle protein degradation and is transported to the liver in the **glucose-alanine cycle**. - In the liver, alanine is converted to glucose, making it a source of energy for other tissues, rather than being predominantly extracted by muscle from circulation.

Question 585: Branched-chain ketoaciduria is due to a deficiency of which enzyme?

A. Branched-chain α-ketoacid dehydrogenase (Correct Answer)
B. Methylmalonyl-CoA mutase
C. Fumarylacetoacetate hydrolase
D. α-ketoacid dehydrogenase

Explanation: ***Branched-chain α-ketoacid dehydrogenase*** - **Branched-chain ketoaciduria**, also known as **Maple Syrup Urine Disease (MSUD)**, is caused by a deficiency in the **branched-chain α-ketoacid dehydrogenase (BCKDH)** enzyme complex. - This enzyme complex is crucial for the oxidative decarboxylation of **branched-chain α-keto acids**, which are metabolites of the essential amino acids **leucine**, **isoleucine**, and **valine**. *α-ketoacid dehydrogenase* - This is a general term for enzymes that catalyze the oxidative decarboxylation of α-keto acids. - While **BCKDH** is a type of α-ketoacid dehydrogenase, stating **"α-ketoacid dehydrogenase"** alone is too broad and not specific enough to the metabolic pathway affected in MSUD. *Methylmalonyl-CoA mutase* - A deficiency in **methylmalonyl-CoA mutase** causes **methylmalonic acidemia**, a distinct inherited metabolic disorder. - This enzyme is involved in the metabolism of **valine**, **isoleucine**, **methionine**, and **threonine**, converting methylmalonyl-CoA to succinyl-CoA. *Fumarylacetoacetate hydrolase* - A deficiency in **fumarylacetoacetate hydrolase** is responsible for **tyrosinemia type 1 (hepatorenal tyrosinemia)**. - This enzyme is involved in the final step of **tyrosine degradation**, leading to the accumulation of toxic metabolites like fumarylacetoacetate and succinylacetone.

Question 586: Essential amino acids are named so because they:

A. They are not important for life
B. Not all food sources contain them
C. Because they are not produced in the body (Correct Answer)
D. Because they are required in large quantities

Explanation: ***Because they are not produced in the body*** - **Essential amino acids** are those that the body cannot synthesize on its own or cannot synthesize in sufficient quantities. - Therefore, they **must be obtained through diet** to meet the body's needs for protein synthesis and other metabolic functions. *They are not important for life* - This statement is incorrect; essential amino acids are **crucial for life** and various bodily functions. - They are the building blocks of **proteins**, which are vital for enzyme production, hormone synthesis, tissue repair, and many other biological processes. *Not all food sources contain them* - While it's true that not all food sources contain a complete profile of essential amino acids, this is **not the reason they are named "essential."** - Some plant-based foods may be lacking in one or more essential amino acids, requiring a varied diet to ensure adequate intake. *Because they are required in large quantities* - The quantity required is not the defining characteristic of an **essential amino acid**. - While some amino acids might be needed in larger amounts than others, their "essential" status refers to the **body's inability to synthesize them**, not their dietary quantity.

Question 587: Immediate precursor of creatine is:

A. Carbamoyl phosphate
B. Arginosuccinate
C. Citrulline
D. Guanidinoacetate (Correct Answer)

Explanation: ***Guanidinoacetate*** - **Guanidinoacetate** is directly methylated by **S-adenosylmethionine (SAM)** to form **creatine** in the second step of creatine biosynthesis. - This methylation reaction is catalyzed by the enzyme **guanidinoacetate methyltransferase (GAMT)**. *Carbamoyl phosphate* - **Carbamoyl phosphate** is a precursor in the **urea cycle** and pyrimidine synthesis, not directly for creatine. - It reacts with ornithine to form citrulline in the first step of the urea cycle. *Arginosuccinate* - **Arginosuccinate** is an intermediate in the **urea cycle**, formed from citrulline and aspartate. - It is cleaved to form fumarate and arginine, not directly leading to creatine. *Citrulline* - **Citrulline** is an intermediate in the **urea cycle**, formed from ornithine and carbamoyl phosphate. - It is a precursor to **arginine**, which can then contribute to the first step of creatine synthesis (arginine and glycine forming guanidinoacetate).

Question 588: Which condition is associated with the phenomenon of ochronosis, characterized by a bluish-black discoloration of connective tissues?

A. Alkaptonuria (Correct Answer)
B. Tyrosinemia
C. Phenylketonuria
D. Homocystinuria

Explanation: ***Alkaptonuria*** - **Ochronosis** is a hallmark feature of **alkaptonuria**, resulting from the accumulation of **homogentisic acid** in connective tissues. - The disease is caused by a deficiency of **homogentisate 1,2-dioxygenase**, an enzyme involved in phenylalanine and tyrosine metabolism. *Tyrosinemia* - Tyrosinemia involves various defects in the metabolism of **tyrosine**, leading to different clinical presentations (types I, II, and III). - While it involves tyrosine metabolism, it does not lead to the accumulation of homogentisic acid or the characteristic **bluish-black discoloration** seen in ochronosis. *Phenylketonuria* - **Phenylketonuria (PKU)** is characterized by a deficiency of **phenylalanine hydroxylase**, leading to a buildup of **phenylalanine**. - This condition primarily causes neurological problems and intellectual disability if untreated, but not **ochronosis**. *Homocystinuria* - **Homocystinuria** is a disorder of **methionine metabolism**, characterized by elevated levels of **homocysteine**. - It primarily affects the eyes (lens dislocation), skeletal system, central nervous system, and vascular system, but does not cause widespread connective tissue discoloration or **ochronosis**.

Question 589: Enzyme defect in acute intermittent porphyria is

A. Hydroxymethylbilane synthase (Correct Answer)
B. Aminolevulinic acid dehydratase
C. Uroporphyrinogen decarboxylase
D. Uroporphyrinogen III synthase

Explanation: ***Hydroxymethylbilane synthase*** - **Acute intermittent porphyria (AIP)** results from a deficiency of **hydroxymethylbilane synthase** (also known as porphobilinogen deaminase or PBG deaminase), which is the third enzyme in the heme synthesis pathway. - This enzyme defect leads to an accumulation of neurotoxic heme precursors, **aminolevulinic acid (ALA)** and **porphobilinogen (PBG)**. *Aminolevulinic acid dehydratase* - A deficiency in **aminolevulinic acid dehydratase (ALA dehydratase)** causes **ALA dehydratase deficiency porphyria (ADP)**, a very rare form of porphyria. - This is distinct from AIP, which involves a defect further down the pathway, leading to different precursor accumulation patterns. *Uroporphyrinogen III synthase* - A defect in **uroporphyrinogen III synthase** causes **congenital erythropoietic porphyria (CEP)**, also known as Günther's disease. - This condition is characterized by severe photosensitivity, hemolytic anemia, and red urine, which are not features of AIP. *Uroporphyrinogen decarboxylase* - A deficiency in **uroporphyrinogen decarboxylase (UROD)** is the cause of **porphyria cutanea tarda (PCT)**. - PCT is the most common porphyria and primarily presents with blistering skin lesions and increased iron levels, not acute neurovisceral attacks as seen in AIP.

Question 590: Which of these amino acids does not enter the Krebs cycle by forming acetyl-CoA from pyruvate?

A. Glycine
B. Tyrosine (Correct Answer)
C. Alanine
D. Hydroxyproline

Explanation: ***Tyrosine*** - **Tyrosine** is both ketogenic and glucogenic, degraded to **fumarate** (a Krebs cycle intermediate) and **acetoacetate** (converted to acetyl-CoA). - Importantly, tyrosine does **not form acetyl-CoA via pyruvate** - it bypasses pyruvate entirely in its degradation pathway. - Its catabolism involves homogentisate and maleylacetoacetate, eventually yielding fumarate and acetoacetate directly. *Glycine* - **Glycine** is converted to **serine**, which is then deaminated to **pyruvate**. - This pyruvate undergoes oxidative decarboxylation by pyruvate dehydrogenase to form **acetyl-CoA**. - Thus, glycine enters the Krebs cycle via the pyruvate → acetyl-CoA route. *Alanine* - **Alanine** is directly converted to **pyruvate** via transamination (alanine aminotransferase). - This pyruvate is then converted to **acetyl-CoA**, which enters the Krebs cycle. - This is one of the most direct amino acid → pyruvate → acetyl-CoA pathways. *Hydroxyproline* - **Hydroxyproline** is primarily degraded to **glyoxylate**, which can be converted to glycine. - A portion is metabolized to **α-ketoglutarate**, which enters the Krebs cycle **directly** as an intermediate. - While some metabolic routes may generate pyruvate indirectly (via glycine → serine → pyruvate), the predominant pathway does involve pyruvate formation, making it functionally similar to glycine in this context.

Practice by Chapter

Protein Digestion and Absorption

Practice Questions

Transamination and Deamination

Practice Questions

Urea Cycle

Practice Questions

Disorders of Urea Cycle

Practice Questions

Metabolism of Individual Amino Acids

Practice Questions

Inborn Errors of Amino Acid Metabolism

Practice Questions

Phenylketonuria and Alkaptonuria

Practice Questions

Homocystinuria and Methionine Metabolism

Practice Questions

Synthesis of Biologically Important Compounds from Amino Acids

Practice Questions

Nitrogen Balance

Practice Questions

Ammonia Metabolism and Toxicity

Practice Questions

One-Carbon Transfer Reactions

Practice Questions

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