Question 1

Which enzyme activity would be directly inhibited by the addition of methotrexate?
A) thymidine kinase
B) thymidylate synthase
C) dihydrofolate reductase
D) serine hydroxymethyltransferase

Accepted Answer

Methotrexate is a drug that inhibits the activity of dihydrofolate reductase, an enzyme that plays a key role in the synthesis of nucleotides, including thymidine. By inhibiting this enzyme, methotrexate prevents the production of thymidine, which is necessary for DNA synthesis and cell proliferation. Therefore, the correct answer is C, dihydrofolate reductase.

Question 2

Relate the broad substrate specificity of nucleoside diphosphate kinase to its role in the consumption and synthesis of nucleoside triphosphates.

Accepted Answer

The answer of Relate the broad substrate specificity of nucleoside...

Question 3

In severe combined immunodeficiency disease (SCID), ribonucleotide reductase activity is __________ because of an overabundance of _.
A) activated; dATP
B) activated; cAMP
C) inhibited; dATP
D) inhibited; cAMP

Accepted Answer

In severe combined immunodeficiency disease (SCID), ribonucleotide reductase activity is inhibited because of an overabundance of dATP. The accumulation of dATP inhibits the enzyme, which is necessary for DNA synthesis and cell proliferation, leading to the characteristic immunodeficiency seen in SCID.

Question 4

Explain how a general inhibitor of endonucleases would alter the homeostasis of mRNA and affect the nucleotide salvage pathway.

Accepted Answer

The answer of Explain how a general inhibitor of endonucleases...

Question 5

If an alteration occurred in the activity site of the R1 subunit of E. coli ribonucleotide reductase that allowed dATP to bind but precluded ATP binding to the site, a possible result would be that ribonucleotide reductase
A) would remain active under all conditions.
B) would be decreased when dATP bound both the activity and specificity sites.
C) activity would be increased when ATP bound the specificity site.
D) activity would be decreased when dCDP bound the catalytic site.

Accepted Answer

E. coli ribonucleotide reductase has two binding sites, the activity site and the specificity site. The activity site binds ATP or dATP and controls the enzyme's activity. The specificity site binds dATP or ATP and controls which substrate is reduced. If an alteration occurred in the activity site that allowed dATP to bind but precluded ATP binding to the site, it means that the enzyme's activity would be controlled by dATP instead of ATP. This would result in decreased activity when both dATP bound the activity site and the specificity site, as the enzyme would be less likely to reduce other substrates. Therefore, option B is correct. The other options are incorrect because they do not accurately predict the behavior of the altered enzyme.

Question 6

Reduction in dTMP biosynthesis is NOT dependent upon _.
A) methotrexate
B) aminopterin
C) raltitrexed
D) dihydrofolate

Accepted Answer

dTMP biosynthesis is dependent on dihydrofolate, which is a precursor of tetrahydrofolate, a cofactor required for thymidylate synthase activity. Methotrexate, aminopterin, and raltitrexed all inhibit dihydrofolate reductase, an enzyme required for the conversion of dihydrofolate to tetrahydrofolate, thereby reducing dTMP biosynthesis.

Question 7

Contrast the energy required to drive the synthesis of AMP from IMP with that required to drive the synthesis of GMP from IMP.

Accepted Answer

The answer of Contrast the energy required to drive the...

Question 8

The importance of stable free radicals in the mechanism of E. coli ribonucleotide reductase was identified in the early 1970s. The existence of stable radicals in a protein had not been previously observed. Which of the following amino acids can exist as a stable radical in ribonucleotide reductase?
A) Cys
B) His
C) Ser
D) Glu

Accepted Answer

The answer of The importance of stable free radicals in...

Question 9

An E. coli strain that is unable to synthesize purines because of a mutation in the gene encoding the AIR synthetase is transfected with the human gene coding for hTrifGART. Would this strain would be able to undergo mitosis in purine-depleted media? Explain your answer.

Accepted Answer

The answer of An E. coli strain that is unable...

Question 10

The enzymes required to convert PRPP to IMP in animal cells form a large protein complex called a(n) _. Describe the experimental evidence proving the existence of this
complex.

Accepted Answer

The answer of The enzymes required to convert PRPP to...

Question 11

When analyzing the production of dTMP by thymidylate synthase in a cell-free system, which of the following compounds would be found as an additional product of this enzyme? A) 7,8-dihydrofolate B) NADP⁺ C) N⁵,N¹⁰-methylenetetrahydrofolate D) glycine

Accepted Answer

Thymidylate synthase uses 5,10-methylenetetrahydrofolate as a substrate to make dTMP, and in the process, it converts 5,10-methylenetetrahydrofolate to dihydrofolate. Dihydrofolate is then reduced to 7,8-dihydrofolate by dihydrofolate reductase. Therefore, in a cell-free system producing dTMP by thymidylate synthase, 7,8-dihydrofolate would be found as an additional product. NADPH is not involved in the reaction, and the other compound listed is a complex mixture and unclear what it pertains to. Glycine is not involved in the biosynthesis of dTMP.

Question 12

Contrast the binding of nucleotides to the activity site and specificity site on the R1 subunit of E. coli ribonucleotide reductase by choosing the nucleotide that can bind to the specificity site but NOT the activity site.
A) ATP
B) dTTP
C) dATP
D) dCDP

Accepted Answer

dTTP can bind to the specificity site of the R1 subunit of E. coli ribonucleotide reductase, but it cannot bind to the activity site because it is not a substrate for the enzyme. In contrast, ATP, dATP, and dCDP can bind to both the activity and specificity sites because they can serve as substrates for the enzyme.

Question 13

Which enzyme activity would be directly reduced if fluorodeoxyuridine-5 ^' -monophosphate is present? A) thymidine kinase B) thymidylate synthase C) dihydrofolate reductase D) serine hydroxymethyltransferase

Accepted Answer

Fluorodeoxyuridine-5F1F1F1S1^F1F1F10-monophosphate is an inhibitor of thymidylate synthase, which is the enzyme responsible for catalyzing the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), a key step in DNA synthesis. Therefore, if this inhibitor is present, thymidylate synthase activity would be directly reduced.

Question 14

Contrast the role of kinases and nucleotidases in the nucleotide salvage pathway.

Accepted Answer

The answer of Contrast the role of kinases and nucleotidases...

Question 15

Place the following steps in their proper order starting with dUMP and N⁵,N¹⁰-methylenetetrahydrofolate as substrates. 1. Formation of tetrahydrofolate 2) Formation of N⁵,N¹⁰-methylenetetrahydrofolate 3) Formation of 7,8-dihydrofolate A) 2; 1; 3 B) 1; 3; 2 C) 1; 2; 3 D) 3; 1; 2

Accepted Answer

The answer of Place the following steps in their proper...

Question 16

A compound is added to a cell and dihydrofolate reductase activity is reduced. If this compound is , it can be predicted that a similar result would be seen if were added instead. A) methotrexate; aminopterin B) methotrexate; raltitrexed C) aminopterin; raltitrexed D) aminopterin; fluorodeoxyuridine-5 ^' -monophosphate

Accepted Answer

Methotrexate and aminopterin are both inhibitors of dihydrofolate reductase, so if one reduces activity, it is likely that the other would have a similar effect. Raltitrexed is also a dihydrofolate reductase inhibitor but works through a different mechanism, so it is not as likely to have the same effect as methotrexate or aminopterin. Fluorodeoxyuridine-5-monophosphate is not a dihydrofolate reductase inhibitor, so it would not have the same effect.

Question 17

To study the allosteric regulation of E. coli ribonucleotide reductase in a cell-free in vitro system, which of the following would be a good choice to add to the system? A) glucose B) ATP C) Ca²⁺ D) IP₃

Accepted Answer

ATP is a known allosteric regulator of E. coli ribonucleotide reductase and would be a good choice to add to the in vitro system for studying its allosteric regulation. Glucose, Ca²⁺, and IP₃ are not known to be allosteric regulators of this enzyme.

Question 18

Describe how the levels of cAMP would change in a liver cell before and after exposure to glucagon. Compare these with the levels expected if the cell were previously exposed to an inhibitor of adenylate cyclase.

Accepted Answer

The answer of Describe how the levels of cAMP would...

Question 19

Several cysteines play important roles in the mechanism of E. coli ribonucleotide reductase. Which is NOT a role carried out by cysteine in this enzyme?
A) donation of a hydrogen atom to the substrate
B) removal of a hydrogen atom from the substrate
C) donation of a proton to ultimately form water
D) reduction of thioredoxin

Accepted Answer

While cysteines play roles in donating and removing hydrogen atoms and donating protons, they do not play a role in the reduction of thioredoxin in E. coli ribonucleotide reductase.

Question 20

Identify the carbon atom that is attacked by Cys439 in the first step of the E. coli ribonucleotide reductase mechanism.

A) A
B) B
C) C
D) D

Accepted Answer

The answer of Identify the carbon atom that is attacked...

Quiz 18: Nucleotide Metabolism

Which enzyme activity would be directly inhibited by the addition of methotrexate?

Relate the broad substrate specificity of nucleoside diphosphate kinase to its role in the consumption and synthesis of nucleoside triphosphates.

In severe combined immunodeficiency disease (SCID) , ribonucleotide reductase activity is __________ because of an overabundance of _.

Explain how a general inhibitor of endonucleases would alter the homeostasis of mRNA and affect the nucleotide salvage pathway.

If an alteration occurred in the activity site of the R1 subunit of E. coli ribonucleotide reductase that allowed dATP to bind but precluded ATP binding to the site, a possible result would be that ribonucleotide reductase

Reduction in dTMP biosynthesis is NOT dependent upon _.

Contrast the energy required to drive the synthesis of AMP from IMP with that required to drive the synthesis of GMP from IMP.

The importance of stable free radicals in the mechanism of E. coli ribonucleotide reductase was identified in the early 1970s. The existence of stable radicals in a protein had not been previously observed. Which of the following amino acids can exist as a stable radical in ribonucleotide reductase?

An E. coli strain that is unable to synthesize purines because of a mutation in the gene encoding the AIR synthetase is transfected with the human gene coding for hTrifGART. Would this strain would be able to undergo mitosis in purine-depleted media? Explain your answer.

The enzymes required to convert PRPP to IMP in animal cells form a large protein complex called a(n) _. Describe the experimental evidence proving the existence of this complex.

When analyzing the production of dTMP by thymidylate synthase in a cell-free system, which of the following compounds would be found as an additional product of this enzyme?

Contrast the binding of nucleotides to the activity site and specificity site on the R1 subunit of E. coli ribonucleotide reductase by choosing the nucleotide that can bind to the specificity site but NOT the activity site.

Which enzyme activity would be directly reduced if fluorodeoxyuridine-5 ^' -monophosphate is present?

Contrast the role of kinases and nucleotidases in the nucleotide salvage pathway.

Place the following steps in their proper order starting with dUMP and N⁵,N¹⁰-methylenetetrahydrofolate as substrates. 1. Formation of tetrahydrofolate 2) Formation of N⁵,N¹⁰-methylenetetrahydrofolate 3) Formation of 7,8-dihydrofolate

A compound is added to a cell and dihydrofolate reductase activity is reduced. If this compound is , it can be predicted that a similar result would be seen if were added instead.

To study the allosteric regulation of E. coli ribonucleotide reductase in a cell-free in vitro system, which of the following would be a good choice to add to the system?

Describe how the levels of cAMP would change in a liver cell before and after exposure to glucagon. Compare these with the levels expected if the cell were previously exposed to an inhibitor of adenylate cyclase.

Several cysteines play important roles in the mechanism of E. coli ribonucleotide reductase. Which is NOT a role carried out by cysteine in this enzyme?

Identify the carbon atom that is attacked by Cys439 in the first step of the E. coli ribonucleotide reductase mechanism.

Quiz 18: Nucleotide Metabolism

Which enzyme activity would be directly inhibited by the addition of methotrexate?

Relate the broad substrate specificity of nucleoside diphosphate kinase to its role in the consumption and synthesis of nucleoside triphosphates.

In severe combined immunodeficiency disease (SCID) , ribonucleotide reductase activity is __________ because of an overabundance of _.

Explain how a general inhibitor of endonucleases would alter the homeostasis of mRNA and affect the nucleotide salvage pathway.

If an alteration occurred in the activity site of the R1 subunit of E. coli ribonucleotide reductase that allowed dATP to bind but precluded ATP binding to the site, a possible result would be that ribonucleotide reductase

Reduction in dTMP biosynthesis is NOT dependent upon _.

Contrast the energy required to drive the synthesis of AMP from IMP with that required to drive the synthesis of GMP from IMP.

An E. coli strain that is unable to synthesize purines because of a mutation in the gene encoding the AIR synthetase is transfected with the human gene coding for hTrifGART. Would this strain would be able to undergo mitosis in purine-depleted media? Explain your answer.

The enzymes required to convert PRPP to IMP in animal cells form a large protein complex called a(n) _. Describe the experimental evidence proving the existence of this complex.

When analyzing the production of dTMP by thymidylate synthase in a cell-free system, which of the following compounds would be found as an additional product of this enzyme?

Contrast the binding of nucleotides to the activity site and specificity site on the R1 subunit of E. coli ribonucleotide reductase by choosing the nucleotide that can bind to the specificity site but NOT the activity site.

Which enzyme activity would be directly reduced if fluorodeoxyuridine-5 ' -monophosphate is present?

Contrast the role of kinases and nucleotidases in the nucleotide salvage pathway.

Place the following steps in their proper order starting with dUMP and N5,N1 0-methylenetetrahydrofolate as substrates. 1. Formation of tetrahydrofolate 2) Formation of N5,N1 0-methylenetetrahydrofolate 3) Formation of 7,8-dihydrofolate

A compound is added to a cell and dihydrofolate reductase activity is reduced. If this compound is , it can be predicted that a similar result would be seen if were added instead.

To study the allosteric regulation of E. coli ribonucleotide reductase in a cell-free in vitro system, which of the following would be a good choice to add to the system?

Describe how the levels of cAMP would change in a liver cell before and after exposure to glucagon. Compare these with the levels expected if the cell were previously exposed to an inhibitor of adenylate cyclase.

Several cysteines play important roles in the mechanism of E. coli ribonucleotide reductase. Which is NOT a role carried out by cysteine in this enzyme?

Identify the carbon atom that is attacked by Cys439 in the first step of the E. coli ribonucleotide reductase mechanism.

Which enzyme activity would be directly reduced if fluorodeoxyuridine-5 ^' -monophosphate is present?

Place the following steps in their proper order starting with dUMP and N⁵,N¹⁰-methylenetetrahydrofolate as substrates. 1. Formation of tetrahydrofolate 2) Formation of N⁵,N¹⁰-methylenetetrahydrofolate 3) Formation of 7,8-dihydrofolate