Deck 18: Amino Acid Oxidation and the Production of Urea

Question 1

In amino acid catabolism,the first reaction for many amino acids is a(n):

A)decarboxylation requiring thiamine pyrophosphate (TPP).
B)hydroxylation requiring NADPH and O₂.
C)oxidative deamination requiring NAD⁺.
D)reduction requiring pyridoxal phosphate (PLP).
E)transamination requiring pyridoxal phosphate (PLP).

Answer 1

transamination requiring pyridoxal phosphate (PLP).

Question 2

Which of the following statements is false in reference to the mammalian synthesis of urea?

A)Krebs was a major contributor to the elucidation of the pathway involved.
B)The amino acid arginine is the immediate precursor to urea.
C)The carbon atom of urea is derived from mitochondrial HCO₃^-.
D)The precursor to one of the nitrogens of urea is aspartate.
E)The process of urea production is an energy-yielding series of reactions.

Answer 2

The process of urea production is an energy-yielding series of reactions.

Question 3

Transamination from alanine to $\alpha$ -ketoglutarate requires the coenzyme:

A)biotin.
B)NADH.
C)No coenzyme is involved.
D)pyridoxal phosphate (PLP).
E)thiamine pyrophosphate (TPP).

Answer 3

pyridoxal phosphate (PLP).

Question 4

Which of the following is a zymogen that can be converted to an endopeptidase that hydrolyzes peptide bonds adjacent to Lys and Arg residues?

A)Chymotrypsinogen
B)Pepsin
C)Pepsinogen
D)Trypsin
E)Trypsinogen

Question 5

Which of the following reactions involving an amino acid cannot be catalyzed via a PLP-dependent mechanism?

A)Hydrolysis
B)Decarboxylation
C)Racemization
D)Transamination
E)Transimination

Question 6

In the urea cycle,ornithine transcarbamoylase catalyzes:

A)cleavage of urea to ammonia.
B)formation of citrulline from ornithine and another reactant.
C)formation of ornithine from citrulline and another reactant.
D)formation of urea from arginine.
E)transamination of arginine.

Question 7

If a person's urine contains unusually high concentrations of urea,which one of the following diets has he or she probably been eating recently?

A)High carbohydrate,very low protein
B)Very high carbohydrate,no protein,no fat
C)Very very high fat,high carbohydrate,no protein
D)Very high fat,very low protein
E)Very low carbohydrate,very high protein

Question 8

Which of the following is not true of the reaction catalyzed by glutamate dehydrogenase?

A)It is similar to transamination in that it involves the coenzyme pyridoxal phosphate (PLP).
B)NH₄⁺ is produced.
C)The enzyme can use either NAD⁺ or NADP⁺ as a cofactor.
D)The enzyme is glutamate-specific,but the reaction is involved in oxidizing other amino acids.
E)( $\alpha$ -Ketoglutarate is produced from an amino acid.)

Question 9

Under which circumstances are amino acids not metabolized via oxidative degradation?

A)Starvation
B)Plants growing in nutrient-rich soils
C)Normal protein turnover
D)A diet rich in proteins
E)Uncontrolled diabetes

Question 10

The conversion of glutamate to an $\alpha$ -ketoacid and NH₄⁺:

A)does not require any cofactors.
B)is a reductive deamination.
C)is accompanied by ATP hydrolysis catalyzed by the same enzyme.
D)is catalyzed by glutamate dehydrogenase.
E)requires ATP.

Question 11

Which of the following amino acids are essential for humans?

A)Alanine
B)Aspartic acid
C)Asparagine
D)Serine
E)Threonine

Question 12

Which of these directly donates a nitrogen atom for the formation of urea during the urea cycle?

A)Adenine
B)Aspartate
C)Creatine
D)Glutamate
E)Ornithine

Question 13

Which substance is not involved in the production of urea from NH₄⁺ via the urea cycle?

A)Aspartate
B)ATP
C)Carbamoyl phosphate
D)Malate
E)Ornithine

Question 14

Urea synthesis in mammals takes place primarily in tissues of the:

A)brain.
B)kidney.
C)liver.
D)skeletal muscle.
E)small intestine.

Question 15

The coenzyme involved in a transaminase reaction is:

A)biotin phosphate.
B)lipoic acid.
C)nicotinamide adenine dinucleotide phosphate (NADP⁺).
D)pyridoxal phosphate (PLP).
E)thiamine pyrophosphate (TPP).

Question 16

In the digestion of protein that occurs in the small intestine,which enzyme is critical in the activation of zymogens?

A)Enteropeptidase
B)Hexokinase
C)Papain
D)Pepsin
E)Secretin

Question 17

The coenzyme required for all transaminations is derived from:

A)niacin.
B)pyridoxine (vitamin B₆).
C)riboflavin.
D)thiamin.
E)vitamin B₁₂.

Question 18

Conversion of ornithine to citrulline is a step in the synthesis of:

A)aspartate.
B)carnitine.
C)pyruvate.
D)tyrosine.
E)urea.

Question 19

Glutamate is metabolically converted to $\alpha$ -ketoglutarate and NH₄⁺ by a process described as:

A)deamination.
B)hydrolysis.
C)oxidative deamination.
D)reductive deamination.
E)transamination.

Question 20

Which of these is not a protease that acts in the small intestine?

A)Chymotrypsin
B)Elastase
C)Enteropeptidase
D)Secretin
E)Trypsin

Question 21

The human genetic disease phenylketonuria (PKU)can result from:

A)deficiency of protein in the diet.
B)inability to catabolize ketone bodies.
C)inability to convert phenylalanine to tyrosine.
D)inability to synthesize phenylalanine.
E)production of enzymes containing no phenylalanine.

Question 22

Describe (briefly)the role of (a)gastrin, (b)pepsinogen, (c)cholecystokinin,and (d)enteropeptidase in protein digestion.

Question 23

Transamination reactions are catalyzed by a family of enzymes,all of which require __________ as a coenzyme.In the first step of a transamination,the coenzyme in the aldehyde form condenses with the _________ group of an amino acid to form a(n)_________.

Question 24

Which of these amino acids can be directly converted into a citric acid cycle intermediate by transamination?

A)Glutamic acid
B)Serine
C)Threonine
D)Tyrosine
E)Proline

Question 25

Tetrahydrofolate (THF)and its derivatives shuttle between different substrates.

A)electrons
B)H⁺
C)acyl groups
D)one carbon units
E)NH₂ groups

Question 26

Describe the reactions and the role of the glucose-alanine cycle.

Question 27

Describe the three different forms of nitrogen by which different organisms dispose of excess nitrogen obtained in the diet.Give examples of organisms that produce these different forms.

Question 28

Draw the structures of reactants and products in the transamination in which glutamate and pyruvate are the starting materials.What cofactor is required for this reaction?

Question 29

Why does a mammal go to all of the trouble of making urea from ammonia rather than simply excreting ammonia as many bacteria do?

Question 30

Give the name and draw the structure of the $\alpha$ -keto acid resulting when the following amino acids undergo transamination with $\alpha$ -ketoglutarate: (a)glutamate; (b)aspartate; (b)alanine.

Question 31

In the treatment of diabetes,insulin is given intravenously.Why can't this hormone,a small protein,be taken orally?

Question 32

The bacterium responsible for gastric ulcers,Helicobacter pylori,survives the acidic pH of the stomach surprisingly well.It does so in part by synthesizing and excreting large amounts of the enzyme urease,which hydrolyzes urea to bicarbonate and ammonia.Explain how these reaction products lead to a less acidic environment for H.pylori.

Question 33

During starvation,more urea production occurs.Explain this observation.

Question 34

The amino acids serine,alanine,and cysteine can be catabolized to yield:

A)fumarate.
B)pyruvate.
C)succinate.
D)( $\alpha$ -ketoglutarate.)
E)None of the above

Question 35

Amino acid catabolism involves the breakdown of 20 amino acids all of which contain nitrogen but have different carbon skeletons.What overall strategy is used to deal with this problem? Illustrate the strategy with two examples.

Question 36

Serine or cysteine may enter the citric acid cycle as acetyl-CoA after conversion to:

A)oxaloacetate.
B)propionate.
C)pyruvate.
D)succinate.
E)succinyl-CoA.

Question 37

Define zymogen and describe the role of one zymogen in protein digestion.

Question 38

Describe the roles of glutamine synthetase and glutaminase in the metabolism of amino groups in mammals.

Question 39

Show the reaction in which ammonia is formed from glutamate;include any required cofactors and/or intermediates.

Question 40

In the human genetic disease maple syrup urine disease,the metabolic defect involves:

A)a deficiency of the vitamin niacin.
B)oxidative decarboxylation.
C)synthesis of branched chain amino acids.
D)transamination of an amino acid.
E)uptake of branched chain amino acids into liver.

Question 41

Name four amino acids that can be converted directly (in one step)into pyruvate or a citric acid cycle intermediate,and name the intermediate formed from each.

Question 42

Suppose you are responsible for formulating the diet for a 4-year-old boy with phenylketonuria.How do you decide what kind and amount of protein to include in the diet?

Question 43

Explain why one might provide high concentrations of the amino acid Arg to patients who have a deficiency in one of the first three enzymes of the urea cycle.

Question 44

Diagram the degradative pathway from proline to an intermediate of either glycolysis or the citric acid cycle.Show structures of intermediates and indicate where cofactors are involved.

Question 45

Degradation of amino acids yields compounds that are common intermediates in the major metabolic pathways.Explain the distinction between glucogenic and ketogenic amino acids in terms of their metabolic fates.

Question 46

Describe (a)the fundamental nutritional problem faced by individuals with genetic defects in enzymes involved in urea formation and (b)two approaches to treatment of these diseases.

Question 47

Name one amino acid whose oxidation proceeds via the intermediate shown: (a)pyruvate; (b)oxaloacetate; (c) $\alpha$ -ketoglutarate; (d)succinyl-CoA; (e)fumarate.

Question 48

If you received a laboratory report showing the presence of a high concentration of phenylalanine and its metabolites in the urine of a patient,what disease would you suspect? What defect(s)in metabolism account(s)for the accumulation of phenylalanine in such patients?