L29- Nitrogen Metabolism

Question 1

More than 95% of the amino acids in plasma are replaced every ___ minutes.

Answer 1

10

Question 2

Increased levels of which of the following compound(s) reflects an increase in protein intake: urea, ammonia, uric acid, and/or creatinine?

Answer 2

Urea.

Question 3

Name two physiological examples in which it would be advantageous for an individual to be in positive nitrogen balance.

Answer 3

Childhood and pregnancy.

Question 4

In which form is excess nitrogen excreted?

Answer 4

Urea.

Question 5

What are the three general steps in which amino nitrogens from amino acids are excreted?

Answer 5

1. Transfer of the amino group to a common carrier such as glutamine 2. Release of the transferred amino group as ammonia 3. Incorporation of ammonia into urea.

Question 6

Which amino acids cannot be modified via transamination reactions?

Answer 6

Lysine and threonine.

Question 7

List the three most commonly used alpha-ketoacids for transaminase reactions.

Answer 7

Pyruvate, alpha-ketoglutarate and oxaloacetate.

Question 8

Which coenzyme is required for all aminotransferase enzymes?

Answer 8

Pyridoxal phosphate (PLP).

Question 9

From which vitamin is pyridoxal phosphate derived?

Answer 9

Vitamin B6.

Question 10

In the absence of an amino acid, the coenzyme pyridoxal phosphate is covalently bound to a _______ (amino acid) residue at the active site of the aminotransferase.

Answer 10

Lysine.

Question 11

What is the role of glutamate dehydrogenase with regards to the production of ammonia?

Answer 11

It catalyzes the release of the alpha-amino group from glutamate by oxidative deamination.

Question 12

In which tissue or organ, and in which cellular compartment, is the enzyme glutamate dehydrogenase found?

Answer 12

Liver mitochondria.

Question 13

True or False. Glutamate dehydrogenase can use either NAD+ or NADP+ as a coenzyme.

Answer 13

True.

Question 14

What are the allosteric activators of the deamination reaction catalyzed by glutamate dehydrogenase?

Answer 14

GDP and ADP.

Question 15

What are the allosteric inhibitors of the deamination reaction catalyzed by glutamate dehydrogenase?

Answer 15

GTP and ATP.

Question 16

Glutamate dehydrogenase can use either NAD+ or NADPH as a coenzyme. When is each coenzyme preferentially used?

Answer 16

NAD+ is generally used for deamination of glutamate, producing alpha-ketoglutarate and ammonia. NADPH is used for the reverse reaction, trapping ammonia as glutamate.

Question 17

Which enzyme catalyzes the irreversible addition of ammonia to glutamate, trapping ammonia as glutamine?

Answer 17

Glutamine synthetase.

Question 18

What is the reaction catalyzed by asparagine synthetase?

Answer 18

This enzyme catalyzes the transfer of the amide nitrogen of glutamine to aspartate using ATP, forming asparagine, glutamate, AMP and PPi.

Question 19

Which enzyme catalyzes the irreversible release of ammonia from glutamine?

Answer 19

Glutaminase.

Question 20

What is the physiologic function of the glutaminase isoform found in the liver?

Answer 20

It couples ammonia production to urea synthesis for excretion of nitrogen.

Question 21

What is the physiologic function of the glutaminase isoform found in the kidneys?

Answer 21

It produces NH3, which gets protonated to NH4+ in the kidney tubules. This permits the conservation of other cations such as Na+ and K+, and therefore helps to regulate blood pH and combat acidosis.

Question 22

What is the source of the two nitrogens found in urea?

Answer 22

One enters the urea cycle as free ammonia while the other enters the urea cycle as the amino group on aspartate.

Question 23

How many ATP molecules are required for the production of one urea molecule through the urea cycle?

Answer 23

3 ATP’s are used (but 4 high-energy bonds are cleaved).

Question 24

In which tissue or organ does the urea cycle occur?

Answer 24

Liver.

Question 25

What are the precursors of the cofactor N-acetylglutamate?

Answer 25

Glutamate and acetyl-CoA.

Question 26

Which amino acid stimulates the activity of N-acetylglutamate synthase?

Answer 26

Arginine.

Question 27

In which cellular compartment is the enzyme carbamoyl phosphate synthetase I (CPSI) found?

Answer 27

Mitochondria.

Question 28

How many molecules of ATP are required for the synthesis of carbamoyl phosphate by carbamoyl phosphate synthetase I?

Answer 28

2 ATP’s.

Question 29

What are the precursors of carbamoyl phosphate?

Answer 29

Carbon dioxide, ammonia and ATP.

Question 30

Which urea cycle intermediates travel in and out of the mitochondria?

Answer 30

Ornithine and citrulline.

Question 31

Which urea cycle enzyme is encoded on the X chromosome?

Answer 31

Ornithine transcarbamoylase.

Question 32

True or False. The levels of urea cycle enzymes in the liver are controlled primarily at the transcription level by glucagon, insulin and glucocorticosteroids.

Answer 32

True.

Question 33

What is the key regulatory step of the urea cycle?

Answer 33

Formation of carbamoyl phosphate by carbamoyl phosphate synthetase I (CPSI).

Question 34

What is the required cofactor for activity of carbamoyl phosphate synthetase I (CPSI)?

Answer 34

N-acetylglutamate (NAG).

Question 35

In which cellular compartment is the cofactor N-acetylglutamate (NAG) formed?

Answer 35

In the mitocondria only.

Question 36

What is the primary positive regulator of carbamoyl phosphate synthetase I (CPSI)?

Answer 36

N-acetylglutamate (NAG).

Question 37

Which urea cycle enzyme releases urea as a product?

Answer 37

Arginase.

Question 38

What is the reaction catalyzed by ornithine transcarbamoylase?

Answer 38

It condenses ornithine and carbamoyl phosphate to form citrulline and Pi.

Question 39

Which two urea cycle enzymes catalyze the incorporation of a nitrogen molecule into the cycle?

Answer 39

Carbamoyl phosphate synthetase I (CPSI) and argininosuccinate synthetase.

Question 40

Which urea cycle enzyme produces both arginine and fumarate?

Answer 40

Argininosuccinase.

Question 41

How many ATP equivalents are required for the production of one urea molecule in the urea cycle?

Answer 41

4 ATP equivalents are required.