Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 621: In the first step of heme synthesis, what is required?

A. Fe
B. Folate
C. Glycine (Correct Answer)
D. Histidine

Explanation: ***Glycine*** - The first and **rate-limiting step** in heme synthesis involves the condensation of **succinyl CoA** and **glycine**. - This reaction is catalyzed by **ALA synthase (aminolevulinate synthase)**, forming **δ-aminolevulinate (ALA)**. *Fe* - **Iron (Fe)** is incorporated into the protoporphyrin ring in the **final step** of heme synthesis, catalyzed by **ferrochelatase**. - Its presence is crucial for the functional heme molecule, but it is not a requirement for the initial condensation reaction. *Folate* - **Folate (vitamin B9)** is essential for the synthesis of nucleotides and various methylation reactions, but it is **not directly involved** in the heme synthesis pathway. - Deficiency can lead to **macrocytic anemia**, but not due to a direct defect in heme synthesis. *Histidine* - **Histidine** is an amino acid that plays a role in various protein structures and enzyme functions, but it is **not a precursor** for heme synthesis. - It is not directly consumed or required in the initial step of the heme biosynthesis pathway.

Question 622: Threonine, while being metabolized, is converted to which amino acid?

A. Proline
B. Alanine
C. Glycine (Correct Answer)
D. Threonine metabolism does not produce any amino acid.

Explanation: ***Glycine*** - During the catabolism of **threonine**, an important intermediate is 2-amino-3-ketobutyrate, which is then cleaved to form **acetyl-CoA** and **glycine**. - This conversion is part of the threonine catabolic pathway, where threonine can be broken down into various compounds, including this amino acid. *Proline* - **Proline** is an amino acid synthesized from **glutamate** and is not a direct product of threonine metabolism. - The metabolic pathways of threonine and proline are distinct and do not typically interconvert directly. *Alanine* - **Alanine** is primarily synthesized from **pyruvate** through transamination reactions. - While threonine can be glucogenic, its direct conversion to alanine is not a major metabolic pathway. *Threonine metabolism does not produce any amino acid.* - This statement is incorrect as **threonine catabolism** specifically yields **glycine** as a product. - Threonine is both glucogenic and ketogenic, and its breakdown pathways lead to the formation of other amino acids or their derivatives.

Question 623: At which level does methionine enter the tricarboxylic acid cycle?

A. Succinyl co A (Correct Answer)
B. Acetyl co A
C. Oxaloacetate
D. Pyruvate

Explanation: ***Succinyl-CoA*** - Methionine is a **glucogenic amino acid** that enters the TCA cycle as **succinyl-CoA**. - The degradation pathway involves: Methionine → S-adenosylmethionine (SAM) → Homocysteine → Cystathionine → Cysteine → **Propionyl-CoA → Methylmalonyl-CoA → Succinyl-CoA**. - This conversion requires **vitamin B12** as a cofactor for methylmalonyl-CoA mutase enzyme. - **Succinyl-CoA** directly enters the TCA cycle as a four-carbon intermediate. *Acetyl-CoA* - Acetyl-CoA is the entry point for many amino acids (leucine, isoleucine, lysine, phenylalanine, tyrosine, tryptophan) into the TCA cycle. - However, methionine does **not** produce acetyl-CoA as its primary catabolic product for TCA cycle entry. - Methionine's carbon skeleton is preserved through the transsulfuration pathway to ultimately form succinyl-CoA. *Oxaloacetate* - Oxaloacetate is formed from amino acids like aspartate and asparagine. - Methionine catabolism does not directly yield oxaloacetate. - Instead, methionine follows the propionyl-CoA pathway to succinyl-CoA. *Pyruvate* - Pyruvate is formed from glycolysis and certain amino acids (alanine, serine, glycine, cysteine, threonine). - Methionine degradation does not produce pyruvate as an intermediate. - The methionine pathway specifically leads to succinyl-CoA through the propionyl-CoA route.

Question 624: Guthrie test can be used for the diagnosis of what?

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

Explanation: **Phenylketonuria** - The **Guthrie test** is a bacterial inhibition assay used for **newborn screening** to detect elevated levels of **phenylalanine** in blood, indicative of PKU. - This test is crucial for early diagnosis, allowing timely dietary intervention to prevent severe neurological complications. *Tyrosinemia* - Tyrosinemia is typically diagnosed through **tandem mass spectrometry** which detects elevated levels of tyrosine and succinylacetone. - The Guthrie test is not designed to screen for tyrosinemia, as it primarily screens for phenylalanine. *Galactosemia* - Galactosemia is diagnosed by measuring **galactose-1-phosphate uridyltransferase (GALT) enzyme activity** or detecting elevated **galactose** levels in blood, often via tandem mass spectrometry. - The Guthrie test does not detect the metabolic abnormalities associated with galactosemia. *Alkaptonuria* - Alkaptonuria is characterized by the accumulation of **homogentisic acid** in the body, typically diagnosed by detecting this acid in the urine. - The Guthrie test is not used to identify metabolic byproducts like homogentisic acid.

Question 625: Serine is a direct precursor in the synthesis of which amino acid?

A. Histidine
B. Glycine (Correct Answer)
C. Arginine
D. Cysteine

Explanation: ***Glycine*** - Serine is converted **directly** to glycine by the enzyme **serine hydroxymethyltransferase (SHMT)**. - This is a **one-step reaction** where serine loses its hydroxymethyl group (β-carbon) to form glycine. - The removed one-carbon unit is transferred to **tetrahydrofolate (THF)**, forming N5,N10-methylene-THF. - This represents a true **direct precursor** relationship in amino acid metabolism. *Cysteine* - While serine provides the **carbon skeleton** for cysteine synthesis, this is **not a direct conversion**. - The pathway involves multiple steps: Serine + Homocysteine → Cystathionine → Cysteine. - This is an **indirect** precursor relationship requiring intermediates. *Histidine* - Histidine is synthesized from **phosphoribosyl pyrophosphate (PRPP)** and **ATP**. - This is a complex de novo pathway unrelated to serine metabolism. *Arginine* - Arginine is synthesized from **ornithine** via the **urea cycle** and from **glutamate**. - Serine is not involved in arginine biosynthesis.

Question 626: Decarboxylation of valine, leucine, and isoleucine is defective in which of the following conditions?

A. Phenylketonuria
B. Alkaptonuria
C. Maple Syrup Urine Disease (Correct Answer)
D. Homocystinuria

Explanation: ***Maple Syrup Urine Disease*** - This condition is caused by a deficiency in the **branched-chain alpha-keto acid dehydrogenase complex**, which is responsible for the **decarboxylation** of **valine, leucine, and isoleucine**. - The accumulation of these **branched-chain amino acids** and their corresponding **alpha-keto acids** leads to the characteristic maple syrup odor in urine and neurological symptoms. *Phenylketonuria* - This genetic disorder results from a deficiency of the enzyme **phenylalanine hydroxylase**, which converts **phenylalanine** to tyrosine. - It leads to the accumulation of **phenylalanine** and its metabolites, causing intellectual disability if untreated. *Alkaptonuria* - This rare metabolic disorder is caused by a deficiency in the enzyme **homogentisate 1,2-dioxygenase**, which is involved in the catabolism of **tyrosine**. - It results in the accumulation of **homogentisic acid**, leading to dark urine, ochronosis (bluish-black pigmentation of cartilage and connective tissue), and arthritis. *Homocystinuria* - This condition is primarily caused by a deficiency in **cystathionine beta-synthase**, an enzyme involved in the metabolism of **methionine**. - It leads to the accumulation of **homocysteine** in the blood and urine, causing skeletal abnormalities, intellectual disability, and increased risk of thromboembolism.

Question 627: Serine is converted to which amino acid during metabolism?

A. Proline
B. Alanine
C. Glycine (Correct Answer)
D. None of the options

Explanation: ***Glycine*** - Serine is converted to **glycine** through a reaction catalyzed by **serine hydroxymethyltransferase**. - This reaction involves the transfer of a hydroxymethyl group from serine to **tetrahydrofolate**, forming **5,10-methylenetetrahydrofolate** and glycine. *Proline* - Proline is synthesized from **glutamate**, not directly from serine. - The pathway involves enzymes such as **glutamate 5-kinase** and **pyrroline-5-carboxylate reductase**. *Alanine* - Alanine is typically formed from **pyruvate** through **transamination reactions**, primarily by alanine transaminase. - Serine cannot be directly converted to alanine in a single metabolic step. *None of the options* - This option is incorrect because serine is indeed converted to glycine, a critical step in **one-carbon metabolism**.

Question 628: Ochronosis is associated with which metabolic disorder?

A. Alkaptonuria (Correct Answer)
B. Phenylketonuria
C. Maple syrup urine disease
D. Homocystinuria

Explanation: ***Alkaptonuria*** - Ochronosis is a condition characterized by **dark pigmentation of connective tissues**, which is a direct manifestation of chronic **alkaptonuria**. - **Alkaptonuria** is an autosomal recessive disorder caused by a defect in the enzyme **homogentisate 1,2-dioxygenase**, leading to the accumulation of **homogentisic acid**. *Phenylketonuria* - This disorder involves a defect in the metabolism of **phenylalanine**, leading to its accumulation and causing **intellectual disability** and seizures if untreated. - It does not cause ochronosis or dark pigmentation of tissues. *Homocystinuria* - This is an inherited disorder of metabolism affecting the amino acid **methionine**, leading to the accumulation of **homocysteine** in the blood and urine. - Clinical features include **skeletal abnormalities**, eye problems (dislocated lens), blood clots, and developmental delay, but not ochronosis. *Maple syrup urine disease* - This is a rare genetic disorder characterized by a defect in the metabolism of **branched-chain amino acids** (leucine, isoleucine, and valine). - Its hallmark is a distinctive **sweet odor** in the urine, and it can cause neurological damage and intellectual disability, but not ochronosis.

Question 629: Which of the following is a characteristic of non-essential amino acids?

A. Not used in protein synthesis
B. Must be obtained from the diet
C. Can be synthesized by the body (Correct Answer)
D. Only used during starvation

Explanation: ***Can be synthesized by the body*** - **Non-essential amino acids** are those that the human body can produce from other compounds, meaning dietary intake is not strictly necessary. - This synthesis typically occurs through various metabolic pathways, often starting from precursors like intermediates of glycolysis or the **Krebs cycle**. *Must be obtained from the diet* - This characteristic describes **essential amino acids**, which the body cannot synthesize and must therefore be consumed through food. - Absence of **essential amino acids** in the diet can lead to protein deficiency and various health problems. *Not used in protein synthesis* - All amino acids, whether essential or non-essential, are fundamental **building blocks of proteins**. - They are incorporated into polypeptide chains during **translation** on ribosomes. *Only used during starvation* - This statement is incorrect; **non-essential amino acids** are routinely synthesized and utilized for protein synthesis and other metabolic functions under normal physiological conditions. - While amino acid metabolism can shift during starvation, non-essential amino acids are not exclusively reserved for such states.

Question 630: Which amino acid found in proteins is a component of the urea cycle and is a precursor of nitric oxide?

A. Arginine (Correct Answer)
B. Histidine
C. Glutamine
D. Citrulline

Explanation: ***Arginine*** - **Arginine** is a key intermediate in the **urea cycle**, where it is cleaved by arginase to form **urea** and **ornithine**. - It also serves as the direct precursor for the synthesis of **nitric oxide (NO)** via the enzyme nitric oxide synthase (NOS). - **Arginine is one of the 20 standard proteinogenic amino acids** found in proteins. *Histidine* - **Histidine** is a precursor to **histamine**, a mediator of inflammatory and allergic responses. - It is not directly involved in the urea cycle or the synthesis of nitric oxide. *Glutamine* - **Glutamine** is the most abundant amino acid in the body and plays crucial roles in nitrogen transport, immunity, and as a fuel for rapidly dividing cells. - While it's involved in nitrogen metabolism, it is not a direct component of the urea cycle in the same way as arginine, or a precursor for nitric oxide. *Citrulline* - **Citrulline** is an intermediate in the **urea cycle**, immediately preceding arginine synthesis. - **Critical distinction**: Citrulline is a **non-proteinogenic amino acid** - it is NOT incorporated into proteins during translation, which is what the question specifically asks for. - While it is converted to arginine in the urea cycle, citrulline itself is not a direct precursor of nitric oxide.

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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