Deck 23: Protein Turnover and Amino Acid Catabolism

Question 1

What molecule is formed from excess ammonium ions by ureotelic organisms?

A) uric acid
B) urea
C) ammonia
D) xanthine
E) hypoxanthine

Answer 1

B

Question 2

Nitrogen is transported from muscle to liver in the form of ______________.

Answer 2

alanine or glutamine

Question 3

Which of the following is the primary proteolytic enzyme of the stomach?

A) trypsin
B) chymotrypsin
C) elastase
D) carboxypeptidase A
E) pepsin

Answer 3

E

Question 4

In the transaminase reaction, pyridoxal phosphate is converted into __________________ when the first amino acid is converted into an $\alpha$ -keto acid.

Question 5

The urea cycle is linked to gluconeogenesis through the TCA cycle intermediates fumarate and ________________.

Question 6

In the first step of the urea cycle, CO₂ and NH₄⁺ are converted into __________________.

Question 7

Which of the following is a product of the transamination of aspartate and $\alpha$ -ketoglutarate?

A) glutamine
B) asparagine
C) pyruvate
D) oxaloacetate
E) None of the answers is correct.

Question 8

What protein is used as a tag to identify proteins that are targeted for destruction?

A) proteasome
B) ubiquitin
C) destructin
D) degradin
E) None of the answers is correct.

Question 9

The hydrolysis of arginine by arginase produces ornithine and ______________.

Question 10

What product of the first step of methionine degradation is an important methyl group donor?

A) 5-methyltetrahydrofolate
B) methylcobalamin
C) S-adenosylmethionine
D) S-adenosylhomocysteine
E) None of the answers is correct.

Question 11

Ubiquitin-tagged proteins are digested by the __________________.

Question 12

What class of enzymes cleaves most aromatic rings in biological systems?

A) dioxygenases
B) monooxygenases
C) biopterin-linked lyases
D) aromatic oxidases
E) None of the answers is correct.

Question 13

In the degradation of amino acids other than Thr or Ser, the amino nitrogens can eventually become the amino group of ___________________.

Question 14

Serine dehydratase catalyzes the conversion of serine into NH₄⁺ and __________________.

Question 15

The genetic deficiency of the enzyme __________________________ results in a condition referred to as maple syrup urine disease.

Question 16

What enzyme transfers an $\alpha$ -amino group from an amino acid to $\alpha$ -ketoglutarate?

A) reductive aminase
B) oxidative deaminase
C) ketotransferase
D) aminotransferase
E) None of the answers is correct.

Question 17

What term describes a specific sequence of amino acids that indicates the particular protein should be degraded?

A) consensus sequence
B) proteasome recognition sequence
C) degron
D) ubiquitin binding domain
E) None of the answers is correct.

Question 18

What cofactor is required by phenylalanine hydroxylase?

A) flavin mononucleotide
B) pyridoxal phosphate
C) dihydrofolate
D) nicotinamide adenine dinucleotide
E) tetrahydrobiopterin

Question 19

What enzyme catalyzes an oxidative deamination using either NAD⁺ or NADP⁺?

A) glutamate dehydrogenase
B) $\alpha$ -ketoglutarate dehydrogenase
C) glutamate deaminase
D) $\alpha$ -ketoglutarate reductase
E) None of the answers is correct.

Question 20

The C-terminal glycine of ubiquitin is covalently linked to ________________ residues of the protein destined to be degraded.

Question 21

Ammoniotelic organisms excrete excess nitrogen as

A) xanthine.
B) NH₄⁺.
C) urea.
D) uric acid.
E) None of the answers is correct.

Question 22

Uricotelic organisms release nitrogen as

A) NH₄⁺.
B) arginine.
C) uric acid.
D) urea.
E) None of the answers is correct.

Question 23

Which amino acid is solely ketogenic?

A) leucine
B) tryptophan
C) valine
D) arginine
E) isoleucine

Question 24

Which amino acids supply carbons as succinyl CoA for eventual entry into metabolism?

A) Met, Val, Leu
B) Met, Ile, Val
C) Ile, Val, Leu
D) Ile, Leu, Gly
E) None of the answers is correct.

Question 25

What is the function of the acidic environment in the stomach?

Question 26

Four of the five enzymes in the urea cycle are evolutionarily related to enzymes found in

A) glucose transport.
B) electron transport chain.
C) nucleotide biosynthesis.
D) ubiquitination.
E) None of the answers is correct.

Question 27

What are some types of enzyme-catalyzed reactions that require pyridoxal phosphate as a coenzyme?

Question 28

Ketogenic amino acids are degraded to which of the following metabolites?

A) pyruvate
B) acetoacetate
C) oxaloacetate
D) $\alpha$ -ketoglutarate
E) All of the answers are correct.

Question 29

In the urea cycle, the second nitrogen of urea enters the cycle in the form of which of the following metabolites?

A) alanine
B) glutamine
C) ornithine
D) aspartate
E) arginine

Question 30

Which of the following is an allosteric activator of mammalian carbamoyl phosphate synthetase?

A) α-ketoglutarate
B) N-acetylaspartate
C) N-acetylglutamate
D) glutamine
E) None of the answers is correct.

Question 31

The eukaryotic system for ubiquitination appears to have evolved from what prokaryotic precursor?

A) A system for protein degradation and turnover
B) A system for protein phosphorylation
C) A system for thiamine biosynthesis
D) A system for nucleotide degradation
E) None of the answers is correct.

Question 32

The half-life of a cytosolic protein is primarily determined by the

A) length of the protein chain.
B) amino terminal residue.
C) sequence at the carboxyl terminus.
D) presence of repeated sequences.
E) None of the answers is correct.

Question 33

Describe the structure and function of the 26S proteasome.

Question 34

In the urea cycle, the carbon skeleton of aspartate is preserved as

A) succinate.
B) fumarate.
C) ornithine.
D) oxaloacetate.
E) citrulline.

Question 35

Which amino acid is a metabolite in the urea cycle but is not used as a building block of proteins?

A) ornithine
B) aspartate
C) glutamate
D) lysine
E) arginine

Question 36

Surplus amino acids are

A) stored in proteosomes.
B) stored in protein scaffolds.
C) used as metabolic fuel.
D) stored as albumin.
E) None of the answers is correct.

Question 37

What is the significance of the PEST sequence?

Question 38

Which amino acids can be directly deaminated to produce NH₄⁺?

A) serine and threonine
B) serine, asparagine, and threonine
C) proline and threonine
D) proline and valine
E) None of the answers is correct.

Question 39

Why are four or more chains of ubiquitin particularly effective as a signal for degradation?

Question 40

In the urea cycle, free NH₃ is coupled with carboxyphosphate to form

A) ureatic phosphate.
B) pyruvate.
C) carbamic acid.
D) urea.
E) None of the answers is correct.

Question 41

Explain the cause of PKU, and give the structure of the abnormal metabolite that accumulates from which this condition gets its name.

Question 42

Biopterin is a complicated coenzyme required for the conversion of Phe to Tyr. Why is biopterin not considered a vitamin?

Question 43

Write the net equation of the urea cycle and indicate whether energy is produced or consumed by the cycle.

Question 44

What is the fate of sulfur in the metabolism of cysteine?

Question 45

Describe how the urea cycle is linked to the citric acid cycle.

Question 46

Describe the glucose-alanine cycle and its significance in amino acid metabolism.

Question 47

Which amino acids can be converted to pyruvate by a single enzyme-catalyzed reaction?

Question 48

What is the basic strategy by which amino acids are degraded?

Question 49

What is the structure of compound X in the following metabolic reaction? Name the type of reaction.