Amino Acid Metabolism

Question 1

Assertion: Branched chain amino acids (BCAAs) are essential amino acids. Reason: Humans cannot synthesize BCAAs and they must be obtained from the diet.

Answer 1

A

Question 2

Assertion: The first step in BCAA catabolism is transamination using BCAA aminotransferase. Reason: This reaction converts BCAAs into their corresponding α-keto acids.

Answer 2

A

Question 3

Assertion: In Maple Syrup Urine Disease (MSUD) there is a defect in the α‑keto acid dehydrogenase complex. Reason: The defective enzyme causes accumulation of α‑keto acids in blood and urine.

Answer 3

A

Question 4

Assertion: Leucine is classified as a strictly ketogenic amino acid. Reason: Leucine metabolism produces acetyl-CoA and acetoacetyl-CoA.

Answer 4

A

Question 5

Assertion: Isoleucine is both glucogenic and ketogenic. Reason: Its catabolism yields both acetyl-CoA and propionyl-CoA.

Answer 5

A

Question 6

Assertion: Valine catabolism produces propionyl-CoA. Reason: Propionyl-CoA is a glucogenic precursor to succinyl-CoA.

Answer 6

A

Question 7

Assertion: The muscle is the predominant site for BCAA catabolism. Reason: Approximately 65% of BCAA transamination occurs in skeletal muscle.

Answer 7

A

Question 8

Assertion: A deficiency in the α-keto acid dehydrogenase complex causes MSUD. Reason: The resulting accumulation of α-keto acids is toxic to the brain.

Answer 8

A

Question 9

Assertion: Isovaleryl CoA dehydrogenase deficiency leads to isovaleric acidemia. Reason: The enzyme defect causes accumulation of isovaleryl compounds which give a cheesy odor.

Answer 9

A

Question 10

Assertion: Phenylketonuria (PKU) is due to a deficiency of phenylalanine hydroxylase. Reason: The enzyme deficiency leads to accumulation of phenylalanine and phenylpyruvate, causing a musty odor in urine.

Answer 10

A

Question 11

Assertion: In PKU, dietary management involves phenylalanine restriction. Reason: Reducing phenylalanine intake prevents the accumulation of its toxic metabolites.

Answer 11

A

Question 12

Assertion: Tyrosine is synthesized from phenylalanine. Reason: Phenylalanine hydroxylase converts phenylalanine into tyrosine using tetrahydrobiopterin as a cofactor.

Answer 12

A

Question 13

Assertion: Tyrosinemia type I results from fumarylacetoacetase deficiency. Reason: The enzyme deficiency leads to accumulation of fumaryl acetoacetate and causes liver damage.

Answer 13

A

Question 14

Assertion: Tyrosinemia type II is associated with eye and skin lesions. Reason: It is due to a deficiency in tyrosine aminotransferase.

Answer 14

A

Question 15

Assertion: Tyrosinemia type III is caused by a deficiency in 4-hydroxyphenylpyruvate dioxygenase. Reason: The enzyme defect leads to neurological symptoms such as seizures and ataxia.

Answer 15

A

Question 16

Assertion: Homogentisic aciduria (alkaptonuria) is characterized by darkening of the urine on standing. Reason: The accumulation of homogentisic acid oxidizes on exposure to air forming a dark pigment.

Answer 16

A

Question 17

Assertion: Melanin protects the skin against sunlight. Reason: It absorbs UV radiation preventing damage to skin cells.

Answer 17

A

Question 18

Assertion: Albinism results from a deficiency of tyrosinase. Reason: Without tyrosinase, melanocytes cannot produce melanin.

Answer 18

A

Question 19

Assertion: In tyrosine metabolism, the conversion of tyrosine to L-DOPA is catalyzed by tyrosine hydroxylase. Reason: This step requires tetrahydrobiopterin as a cofactor.

Answer 19

A

Question 20

Assertion: Catecholamines are derived from tyrosine. Reason: Tyrosine is converted to L-DOPA, then to dopamine, norepinephrine, and epinephrine.

Answer 20

A

Question 21

Assertion: Tryptophan is a precursor of serotonin. Reason: Tryptophan is hydroxylated and decarboxylated to form serotonin.

Answer 21

A

Question 22

Assertion: Tryptophan metabolism also leads to the formation of niacin. Reason: Nicotinic acid (niacin) is synthesized via several steps from tryptophan.

Answer 22

A

Question 23

Assertion: The catabolic pathways of leucine, isoleucine, and valine share common initial enzymes. Reason: Their first two catabolic steps involve the same transamination and oxidative decarboxylation enzymes.

Answer 23

A

Question 24

Assertion: In MSUD, dietary restriction of BCAAs is a key management strategy. Reason: Reducing BCAA intake lowers the load on the defective α-keto acid dehydrogenase complex.

Answer 24

A

Question 25

Assertion: The accumulation of α‑keto acids in MSUD leads to neurological damage. Reason: The toxic effects of these metabolites impair brain development.

Answer 25

A

Question 26

Assertion: Isoleucine’s dual nature (glucogenic and ketogenic) is due to its catabolic products. Reason: Isoleucine catabolism yields both acetyl-CoA and propionyl-CoA.

Answer 26

A

Question 27

Assertion: In phenylketonuria, the defective enzyme causes reduced tyrosine synthesis. Reason: The deficiency in phenylalanine hydroxylase prevents conversion of phenylalanine to tyrosine.

Answer 27

A

Question 28

Assertion: Tetrahydrobiopterin is an essential cofactor in both phenylalanine hydroxylation and tyrosine hydroxylation. Reason: It facilitates the hydroxylation reactions that convert phenylalanine to tyrosine and tyrosine to L-DOPA.

Answer 28

A

Question 29

Assertion: In isovaleric acidemia, protein ingestion may provoke metabolic decompensation. Reason: The ingestion of leucine increases isovaleryl-CoA levels which cannot be degraded due to enzyme deficiency.

Answer 29

A

Question 30

Assertion: The accumulation of phenylpyruvic acid in PKU is responsible for its musty urine odor. Reason: Phenylpyruvate and related metabolites are volatile compounds that impart a distinct odor.

Answer 30

A

Question 31

Assertion: In tyrosinemia type I, liver failure is a common outcome. Reason: The accumulation of toxic metabolites such as fumarylacetoacetate damages hepatocytes.

Answer 31

A

Question 32

Assertion: Tyrosinemia type II leads to ocular lesions. Reason: The deficiency of tyrosine aminotransferase results in elevated tyrosine levels that affect eye tissues.

Answer 32

A

Question 33

Assertion: In homogentisic aciduria, ochronosis occurs due to pigment deposition in connective tissues. Reason: Homogentisic acid, upon oxidation, polymerizes to form dark pigment deposits.

Answer 33

A

Question 34

Assertion: Catecholamine biosynthesis begins with the hydroxylation of tyrosine. Reason: Tyrosine hydroxylase catalyzes the conversion of tyrosine to L-DOPA which is the first step in the pathway.

Answer 34

A

Question 35

Assertion: The synthesis of serotonin from tryptophan requires decarboxylation. Reason: Aromatic acid decarboxylase converts 5-hydroxytryptophan to serotonin.

Answer 35

A

Question 36

Assertion: Melatonin is synthesized from serotonin in the pineal gland. Reason: The conversion of serotonin to melatonin is regulated by light exposure.

Answer 36

A

Question 37

Assertion: Vitamin B12 deficiency can lead to elevated FIGLU excretion. Reason: Folic acid metabolism is impaired, affecting the conversion of formiminoglutamate.

Answer 37

A

Question 38

Assertion: In histidinaemia, histidine levels in blood and urine are elevated. Reason: A deficiency of histidine ammonia lyase impairs the normal breakdown of histidine.

Answer 38

A

Question 39

Assertion: Defective processing of tyrosine can lead to pigmentary disorders such as albinism. Reason: Tyrosinase deficiency prevents melanin production.

Answer 39

A

Question 40

Assertion: The probability of a child having PKU is influenced by parental carrier status. Reason: PKU is inherited as an autosomal recessive trait.

Answer 40

A

Question 41

Assertion: In BCAA catabolism, the use of a common branched-chain α-keto acid dehydrogenase complex explains the interdependence of leucine, isoleucine, and valine metabolism. Reason: The shared enzyme complex means that a defect in this enzyme can affect the metabolism of all three amino acids simultaneously.

Answer 41

A

Question 42

Assertion: The intermittent form of branched chain ketonuria is less severe than classic MSUD. Reason: A milder deficiency in the α‑keto acid dehydrogenase complex allows partial enzyme activity which delays symptom onset.

Answer 42

A

Question 43

Assertion: In isovaleric acidemia, the accumulation of isovaleryl compounds directly leads to metabolic acidosis. Reason: The buildup of isovaleryl-CoA and its derivatives interferes with normal acid-base homeostasis.

Answer 43

A

Question 44

Assertion: The pathogenesis of PKU involves both the toxic accumulation of phenylalanine and the deficiency of tyrosine-derived neurotransmitters. Reason: In PKU, the high level of phenylalanine competes with tyrosine for transport into the brain, disrupting catecholamine synthesis.

Answer 44

A

Question 45

Assertion: The accumulation of homogentisic acid in alkaptonuria leads to ochronosis. Reason: Oxidation of homogentisic acid produces pigment deposits that accumulate in connective tissues.

Answer 45

A

Question 46

Assertion: Tyrosine hydroxylase activity is the rate-limiting step in catecholamine synthesis. Reason: The conversion of tyrosine to L-DOPA is tightly regulated and dependent on tetrahydrobiopterin availability.

Answer 46

A

Question 47

Assertion: The cofactor tetrahydrobiopterin is necessary for both phenylalanine and tyrosine hydroxylase reactions. Reason: Its deficiency can lead to hyperphenylalanemia as well as reduced catecholamine production.

Answer 47

A

Question 48

Assertion: The accumulation of tyrosine metabolites in tyrosinemia type I directly contributes to renal tubular dysfunction. Reason: The toxic intermediates damage both hepatic and renal tissues, leading to Fanconi’s syndrome.

Answer 48

A

Question 49

Assertion: In tyrosinemia type II, ocular and skin lesions occur due to local deposition of tyrosine. Reason: Excess tyrosine levels have a cytotoxic effect on melanocytes in the eye and skin.

Answer 49

B

Question 50

Assertion: The synthesis of melatonin from serotonin is regulated by environmental light. Reason: Light exposure inhibits the activity of enzymes converting serotonin to melatonin in the pineal gland.

Answer 50

A

Question 51

Assertion: The formation of nicotinamide adenine dinucleotide (NAD) from tryptophan involves multiple enzymatic steps. Reason: Tryptophan is metabolized via the kynurenine pathway to ultimately yield NAD.

Answer 51

A

Question 52

Assertion: Elevated levels of formiminoglutamate (FIGLU) indicate a deficiency in folate or vitamin B12. Reason: FIGLU excretion increases when the conversion of formiminoglutamate to glutamate is impaired.

Answer 52

A

Question 53

Assertion: In albinism, the absence of melanin renders individuals highly sensitive to sunlight. Reason: Without melanin, the skin loses its natural protection against ultraviolet (UV) radiation.

Answer 53

A

Question 54

Assertion: The genetic mutations causing albinism and PKU affect different metabolic outcomes despite both involving tyrosine metabolism. Reason: In PKU, dietary management can mitigate low tyrosine issues, whereas in albinism the enzyme defect prevents melanin synthesis irrespective of substrate availability.

Answer 54

A

Question 55

Assertion: Histidinaemia causes neurological symptoms due to the accumulation of histidine. Reason: Elevated histidine and its metabolites interfere with normal neurotransmission.

Answer 55

B

Question 56

Assertion: The integration of amino acid catabolism with energy production is exemplified by the oxidation of BCAA-derived acyl-CoA derivatives in the TCA cycle. Reason: The CoA derivatives from BCAA catabolism, such as propionyl-CoA, eventually enter the TCA cycle as succinyl-CoA.

Answer 56

A

Question 57

Assertion: In alkaptonuria, arthritis later in life is attributed to the deposition of pigmented polymers in joint tissues. Reason: The polymerization of oxidized homogentisic acid leads to ochronotic pigment deposition which damages connective tissue.

Answer 57

A

Question 58

Assertion: The catecholamine biosynthetic pathway requires a tightly regulated balance between substrate availability and enzyme activity. Reason: Enzymes such as tyrosine hydroxylase are subject to feedback regulation by the end products and cofactor availability.

Answer 58

A