Amino Acid Metabolism -

Question 1

Assertion: In homocystinuria there is a deficiency of cystathionine-β synthase. Reason: This deficiency leads to the accumulation of homocysteine in the blood.

Answer 1

A

Question 2

Assertion: In the catabolism of methionine, S-adenosyl methionine is the principal methyl group donor. Reason: It is produced as an early metabolic intermediate from methionine.

Answer 2

A

Question 3

Assertion: Biotin is involved in carboxylation reactions. Reason: Biotin acts as a coenzyme for carboxylase enzymes.

Answer 3

A

Question 4

Assertion: Tetrahydrofolate is synthesized by the intestinal flora. Reason: It is a vital coenzyme involved in one-carbon metabolism.

Answer 4

A

Question 5

Assertion: Cysteine degradation can produce pyruvate. Reason: Pyruvate is a key intermediate in energy metabolism.

Answer 5

A

Question 6

Assertion: Taurine can conjugate with bile acids. Reason: This conjugation helps enhance the clearance of cholesterol by the liver.

Answer 6

A

Question 7

Assertion: A major metabolic fate of methionine is its conversion to S-adenosyl methionine. Reason: S-adenosyl methionine serves as the main donor of methyl groups in biosynthetic reactions.

Answer 7

A

Question 8

Assertion: A deficiency of sulfite oxidase leads to neurological abnormalities. Reason: The accumulation of sulfite is toxic to neurons.

Answer 8

A

Question 9

Assertion: Cystathionuria involves the accumulation of cystathionine. Reason: It results from a deficiency of cystathionine γ-lyase.

Answer 9

A

Question 10

Assertion: Excess cysteine can suppress the synthesis of cystathionine-β synthase. Reason: Cysteine acts as an allosteric inhibitor of the enzyme.

Answer 10

A

Question 11

Assertion: Homocysteine can be converted to homocysteinethiolactone. Reason: This conversion occurs as an alternate route when homocysteine accumulates.

Answer 11

A

Question 12

Assertion: Taurine is involved in osmoregulation. Reason: It helps regulate intracellular calcium and fluid balance.

Answer 12

A

Question 13

Assertion: Methionine can be metabolized via the trans-sulfuration pathway. Reason: This pathway leads to the synthesis of cystathionine and eventually cysteine.

Answer 13

A

Question 14

Assertion: S-adenosyl homocysteine is formed after S-adenosyl methionine donates a methyl group. Reason: It is an intermediate in the catabolism of methionine.

Answer 14

A

Question 15

Assertion: Cysteine is a sulfur-containing amino acid. Reason: It contains a sulfhydryl (-SH) group in its structure.

Answer 15

A

Question 16

Assertion: Tetrahydrofolate carries activated one‑carbon units. Reason: Its structure allows binding of carbon groups in different oxidation states.

Answer 16

A

Question 17

Assertion: The accumulation of homocysteine interferes with collagen cross-linking. Reason: Homocysteine reacts with lysyl semialdehydes needed for collagen formation.

Answer 17

A

Question 18

Assertion: Betaine is used in the management of homocystinuria. Reason: It provides an alternative methyl donor to convert homocysteine back to methionine.

Answer 18

A

Question 19

Assertion: The conversion of cystathionine to cysteine requires pyridoxal phosphate (PLP). Reason: PLP serves as an essential coenzyme for cystathionine γ‑lyase.

Answer 19

A

Question 20

Assertion: Increased dietary methionine exacerbates homocystinuria. Reason: Excess methionine leads to increased production of homocysteine.

Answer 20

A

Question 21

Assertion: A deficiency of cystathionine-β synthase impairs collagen cross‑linking. Reason: The resulting accumulation of homocysteine interferes with the normal cross‑linking process.

Answer 21

A

Question 22

Assertion: Uncooked egg white can cause biotin deficiency. Reason: Avidin in egg white binds biotin, preventing its absorption.

Answer 22

A

Question 23

Assertion: Cysteine can be converted to sulfite and subsequently to sulfate. Reason: This conversion is catalyzed by mitochondrial sulfite oxidase.

Answer 23

A

Question 24

Assertion: Sulfate is metabolized to form 3’-phosphoadenosine 5’-phosphosulfate (PAPS). Reason: PAPS serves as the universal sulfate group donor in biosynthetic reactions.

Answer 24

A

Question 25

Assertion: The trans‑sulfuration pathway converts homocysteine to cysteine via cystathionine. Reason: This pathway enables the transfer of the sulfhydryl group from homocysteine to serine.

Answer 25

A

Question 26

Assertion: One‑carbon groups in metabolism can exist in multiple oxidation states. Reason: They range from carbon dioxide (fully oxidized) to methyl groups (fully reduced).

Answer 26

A

Question 27

Assertion: The conversion of methionine to S‑adenosyl methionine is essential for methyl group transfers. Reason: S‑adenosyl methionine is produced in the trans‑sulfuration pathway.

Answer 27

C

Question 28

Assertion: Homocystinuria leads to accumulation of both homocysteine and methionine. Reason: A deficiency of cystathionine-β synthase prevents the conversion of homocysteine and causes feedback inhibition of methionine breakdown.

Answer 28

B

Question 29

Assertion: Taurine formation from cysteine occurs in a single enzymatic step. Reason: Cysteine is directly converted into taurine via a hypotaurine intermediate.

Answer 29

D

Question 30

Assertion: A deficiency of sulfite oxidase results in neurological deterioration. Reason: The lack of this enzyme causes the accumulation of toxic sulfite compounds.

Answer 30

A

Question 31

Assertion: Cystathionuria is a common metabolic defect. Reason: It is due to a deficiency of cystathionine γ‑lyase.

Answer 31

D

Question 32

Assertion: Biotin is solely obtained through the diet. Reason: Humans cannot synthesize biotin endogenously.

Answer 32

A

Question 33

Assertion: Tetrahydrofolate can accept only one carbon unit at a time. Reason: Its structure limits it to binding a single oxidation state of carbon.

Answer 33

D

Question 34

Assertion: Cysteine degradation to pyruvate is a major pathway when energy is required. Reason: Pyruvate subsequently enters the TCA cycle to generate ATP.

Answer 34

A

Question 35

Assertion: S‑adenosyl homocysteine is a dead‑end metabolite. Reason: It cannot participate further in methylation reactions.

Answer 35

D

Question 36

Assertion: Elevated levels of homocysteine can lead to atheroma formation. Reason: Homocysteine reacts with lysyl semialdehydes, interfering with collagen cross‑linking and contributing to LDL modification.

Answer 36

A

Question 37

Assertion: Feeding patients betaine is beneficial in managing homocystinuria. Reason: Betaine provides an alternative methyl donor that helps convert homocysteine to methionine.

Answer 37

A

Question 38

Assertion: The reaction converting cystathionine to cysteine requires vitamin B6 in the form of PLP. Reason: Pyridoxal phosphate is an essential coenzyme for cystathionine γ‑lyase.

Answer 38

A

Question 39

Assertion: Increased dietary methionine worsens homocystinuria. Reason: Excess methionine leads to higher production of homocysteine.

Answer 39

A

Question 40

Assertion: A deficiency of cystathionine-β synthase impairs collagen cross‑linking. Reason: The resulting homocysteine accumulation disrupts lysyl semialdehyde-mediated cross‑link formation.

Answer 40

A

Question 41

Assertion: Tetrahydrofolate is considered a vitamin. Reason: It can be synthesized de novo by human cells under all conditions.

Answer 41

D

Question 42

Assertion: Taurine is synthesized from cysteine via a hypotaurine intermediate. Reason: The conversion requires oxygen-dependent reactions.

Answer 42

A

Question 43

Assertion: The trans‑sulfuration pathway links methionine metabolism with cysteine synthesis. Reason: It converts homocysteine into cystathionine, which is then converted into cysteine.

Answer 43

A

Question 44

Assertion: The oxidative decarboxylation of α‑ketobutyric acid to propionyl CoA is unique in cysteine catabolism. Reason: It links amino acid catabolism with energy production through the TCA cycle.

Answer 44

A

Question 45

Assertion: Uncooked egg white can lead to biotin deficiency. Reason: Avidin in egg white binds biotin strongly, preventing its absorption.

Answer 45

A

Question 46

Assertion: Accumulation of cystathionine is a biochemical marker of cystathionuria. Reason: This accumulation results from a deficiency in the enzyme that degrades cystathionine.

Answer 46

A

Question 47

Assertion: Elevated homocysteine levels impair connective tissue integrity. Reason: Homocysteine interferes with collagen cross‑linking by reacting with lysyl groups.

Answer 47

A

Question 48

Assertion: One‑carbon metabolism involves several cofactors. Reason: Cofactors such as biotin, tetrahydrofolate, and S‑adenosyl methionine are integral for one‑carbon transfers.

Answer 48

A

Question 49

Assertion: Methionine is catabolized via two major metabolic pathways. Reason: One pathway leads to methyl donation while the other involves trans‑sulfuration.

Answer 49

A

Question 50

Assertion: The conversion of homocysteine to cystathionine is an irreversible step. Reason: This reaction, catalyzed by PLP‑dependent cystathionine‑β synthase, commits homocysteine to the trans‑sulfuration pathway.

Answer 50

A

Question 51

Assertion: A deficiency of cystathionine γ‑lyase always results in pronounced clinical symptoms. Reason: The enzyme’s role in cysteine degradation is critical for metabolic homeostasis.

Answer 51

D

Question 52

Assertion: Both homocystinuria and cystathionuria involve the accumulation of non‑protein metabolites. Reason: They arise from deficiencies in sequential enzymes of the trans‑sulfuration pathway.

Answer 52

A

Question 53

Assertion: Inadequate levels of tetrahydrofolate can indirectly affect methionine catabolism. Reason: Tetrahydrofolate is required for the remethylation of homocysteine to methionine.

Answer 53

A

Question 54

Assertion: Taurine’s involvement in brain development is a direct consequence of its role in osmoregulation. Reason: Taurine regulates intracellular calcium, which is necessary for neuronal function.

Answer 54

B

Question 55

Assertion: Provision of extra cysteine in the diet is an effective treatment for homocystinuria. Reason: Increased cysteine levels exert negative feedback that reduces homocysteine accumulation.

Answer 55

B

Question 56

Assertion: Methionine’s dual metabolic fates are interrelated. Reason: The production of S‑adenosyl methionine precedes the formation of homocysteine, linking methylation and trans‑sulfuration pathways.

Answer 56

A

Question 57

Assertion: The accumulation of homocysteine in the blood contributes to connective tissue defects. Reason: Homocysteine reacts with lysyl semialdehydes, impairing collagen cross‑linking.

Answer 57

A

Question 58

Assertion: Inhibition of cystathionine‑β synthase by cysteine is an example of feedback inhibition. Reason: Elevated levels of an end product typically downregulate the activity of the enzyme that produces it.

Answer 58

A

Question 59

Assertion: The oxidative decarboxylation of α‑ketobutyric acid to propionyl CoA is a unique step in cysteine catabolism. Reason: This reaction links amino acid degradation to energy production pathways.

Answer 59

A

Question 60

Assertion: A deficiency of sulfite oxidase leads to an accumulation of thiosulfate and sulfite that causes neurological decline. Reason: The buildup of these compounds disrupts mitochondrial function and neural metabolism.

Answer 60

A

Question 61

Assertion: The structure of tetrahydrofolate, composed of a substituted pteridine, para‑aminobenzoate, and glutamate, is directly related to its role in one‑carbon transfer reactions. Reason: These structural components allow it to stabilize various oxidation states of carbon.

Answer 61

A

Question 62

Assertion: The allosteric inhibition of cystathionine γ‑lyase by cysteine is a regulatory mechanism in cysteine metabolism. Reason: End‑product inhibition is a classical mechanism for regulating enzyme activity in metabolic pathways.

Answer 62

A

Question 63

Assertion: Cystathionuria is less clinically significant than homocystinuria. Reason: Despite the enzyme deficiency, patients with cystathionuria are mostly asymptomatic.

Answer 63

A

Question 64

Assertion: S‑adenosyl homocysteine plays a regulatory role in methylation reactions. Reason: Its accumulation can inhibit methyltransferases, thereby affecting cellular methylation.

Answer 64

A

Question 65

Assertion: The catabolism of sulfur‑containing amino acids is tightly regulated by substrate availability and product inhibition. Reason: Enzymes such as cystathionine‑β synthase and γ‑lyase are modulated by the concentration of cysteine.

Answer 65

A

Question 66

Assertion: The conversion of cysteine to taurine involves the intermediate formation of hypotaurine. Reason: Hypotaurine is rapidly oxidized to taurine, making its detection in vivo challenging.

Answer 66

A

Question 67

Assertion: One‑carbon metabolism is integral to nucleotide synthesis and methylation reactions. Reason: Cofactors like tetrahydrofolate donate one‑carbon units essential for these biosynthetic processes.

Answer 67

A

Question 68

Assertion: Dietary factors can influence the flux through the trans‑sulfuration pathway. Reason: The intake of methionine and cysteine alters substrate concentrations, thereby modulating enzyme activities.

Answer 68

A

Question 69

Assertion: The enzymatic activity of cystathionine‑β synthase depends solely on the availability of vitamin B6 in the form of PLP. Reason: PLP is the coenzyme required for its catalytic action.

Answer 69

D

Question 70

Assertion: The trans‑sulfuration pathway provides intermediates for glutathione synthesis. Reason: Cysteine, produced from this pathway, is a precursor for glutathione.

Answer 70

A

Question 71

Assertion: The presence of avidin in egg whites can lead to biotin deficiency. Reason: Avidin binds biotin strongly, preventing its absorption in the gastrointestinal tract.

Answer 71

A

Question 72

Assertion: An increase in homocysteine levels is solely responsible for the clinical manifestations of homocystinuria. Reason: Elevated homocysteine interferes with multiple metabolic processes, including collagen cross‑linking.

Answer 72

D

Question 73

Assertion: The regulation of one‑carbon metabolism relies on the interconversion of various one‑carbon units. Reason: This flexibility allows cells to adapt to different biosynthetic demands.

Answer 73

A

Question 74

Assertion: The accumulation of methionine in homocystinuria is due to feedback inhibition of its metabolic conversion. Reason: The block in the trans‑sulfuration pathway prevents the breakdown of methionine, leading to its accumulation.

Answer 74

B