L30- Amino Acid Breakdown

Question 1

Which amino acids are solely ketogenic?

Answer 1

Leucine and lysine.

Question 2

Which amino acids are both ketogenic and glycogenic?

Answer 2

Isoleucine, phenylalanine, threonine, tryptophan, and tyrosine.

Question 3

At which points can the carbon skeleton of glycogenic amino acids enter carbohydrate metabolism?

Answer 3

Pyruvate, alpha-ketoglutarate, succinyl CoA, fumarate and oxaloacetate.

Question 4

Which amino acids can be metabolized to form pyruvate?

Answer 4

Tryptophan, alanine, threonine, glycine, serine and cysteine.

Question 5

Which amino acids can be metabolized to form alpha-ketoglutarate?

Answer 5

Arginine, proline, glutamine, histidine, and glutamate.

Question 6

Which amino acids can be metabolized to form succinyl CoA?

Answer 6

Threonine, methionine, isoleucine, and valine.

Question 7

What is the first step in the catabolism of isoleucine and valine?

Answer 7

The transamination of the amino acids to alpha-ketoacids by an aminotransferase.

Question 8

What is the enzyme in the succinyl CoA pathway that converts alpha-ketoacid intermediates to propionyl CoA?

Answer 8

Branched-chain ketoacid dehydrogenase complex.

Question 9

What are the cofactors for branched-chain ketoacid dehydrogenase?

Answer 9

Thiamine pyrophosphate (TPP), lipoate, FAD, CoA and NAD+.

Question 10

Deficiency of which enzyme results in maple syrup urine disease?

Answer 10

Branched-chain ketoacid dehydrogenase.

Question 11

What three enzymes are involved in converting propionyl CoA to succinyl CoA?

Answer 11

Propionyl carboxylase, methlymalonyl CoA epimerase, and methylmalonyl CoA mutase.

Question 12

What is the most common cause of methylmalonic acidemia?

Answer 12

Vitamin B12 deficiency.

Question 13

A patient with methylmalonic acidemia is unresponsive to vitamin B12 supplementation. What is the most probable cause for methylmalonic acidemia in this patient?

Answer 13

Abnormality in the enzyme methylmalonyl CoA mutase.

Question 14

What is the essential cofactor for the enzyme methylmalonyl CoA mutase?

Answer 14

Adenosylcobalamin (adenosyl B12).

Question 15

What is the cofactor for the enzyme propionyl carboxylase?

Answer 15

Biotin.

Question 16

What is the function of the enzyme methylmalonyl CoA epimerase?

Answer 16

It converts the D-isomer of methymalonyl CoA into the L-isomer, as only the L-isomer is a substrate for methylmalonyl CoA mustase.

Question 17

Methylmalonic acidemia and pernicious anemia are both caused by vitamin B12 deficiency. What is the difference in the defect that causes these two conditions?

Answer 17

Methylmalonic acidemia is most often due to a lack of dietary vitamin B12 or the inability to efficiently convert vitamin B12 to its adenosylcobalamin form. Pernicious anemia is most often due to malabsorption of vitamin B12 in the small intestine, most often because of a lack in intrinsic factor production in the stomach.

Question 18

Adenosylcobalamin is similar in structure to the tetrapyrrole in heme but coordinates _______ instead of iron.

Answer 18

Cobalt.

Question 19

A deficiency in intrinsic factor can lead to which hematologic condition?

Answer 19

Pernicious anemia.

Question 20

Which amino acids can be metabolized to form oxaloacetate?

Answer 20

Asparagine and aspartate.

Question 21

Which amino acids can be metabolized to form fumarate?

Answer 21

Phenylalanine and tyrosine.

Question 22

Which enzyme deaminates asparagine to form aspartate?

Answer 22

Asparaginase.

Question 23

What are the substrates and products of the enzyme glutamate-oxaloacetate transaminase?

Answer 23

Asparate + alpha-ketoglutarate -> oxaloacetate + glutamate.

Question 24

Which enzyme is most commonly mutated in phenylketonuria (PKU)?

Answer 24

Phenylalanine hydroxylase.

Question 25

Which metabolites of phenylalanine accumulate to abnormal levels in the blood of patients with phenylketonuria (PKU)?

Answer 25

Phenylacetate, phenyllactate and phenylpyruvate.

Question 26

What are two causes of phenylketonuria (PKU)?

Answer 26

Phenylalanine hydroxylase deficiency and inability to synthesize or reduce the cofactor tetrahydrobiopterin.

Question 27

What enzyme deficiency causes alcaptonuria?

Answer 27

Homogentisic acid oxidase.

Question 28

Why are some cases of phenylketonuria characterized by light pigmentation in skin and eyes?

Answer 28

Phenylalanine is a precursor for tyrosine. Therefore, PKU may lead to deficiency of tyrosine.

Question 29

What is the most characteristic clinical sign of alcaptonuria?

Answer 29

Dark urine due the air oxidation of homogentisic acid.

Question 30

Which enzyme reduces dihydrobiopterin to tetrahydrobiopterin using NADH?

Answer 30

Dihydrobiopetrin reductase.