Enzymes Regulation Questions and Answers

Q: The allosteric inhibitor of an enzyme ___________

Correct answer: Participates in feedback regulationAn allosteric inhibitor binds at a regulatory site, not the active site, and changes the enzyme’s shape so that catalysis slows. Typically the inhibitor is the end product of the pathway, so that when enough has accumulated the first enzyme is switched off - which is exactly what feedback regulation means. Nothing is denatured, and the effect is reversible.

Q: Which of the following is false about allosteric feedback inhibition?

Correct answer: Conversion of L-leucine to L-isoleucineThe classic example is the conversion of L-threonine to L-isoleucine, not of leucine. Threonine dehydratase catalyses the first step and is inhibited by isoleucine, the end product; as isoleucine is used up the inhibition lifts and the pathway runs again. That is why the statement naming leucine is the false one.

Q: Which of the following enzymes is responsible for the regulation of biological nitrogen fixation?

Correct answer: Dinitrogenase reductaseDinitrogenase reductase is regulated by ADP-ribosylation: when fixed nitrogen is plentiful, an ADP-ribosyl group is added and the enzyme is switched off; when nitrogen is scarce it is removed and fixation resumes. Regulating it makes sense, since nitrogen fixation is enormously expensive in ATP.

Q: Which of the following enzymes catalyze the ADP-ribosylation of key cellular enzymes or proteins?

Correct answer: Diphtheria toxin and cholera toxinBoth toxins work by ADP-ribosylating a host protein. Diphtheria toxin modifies elongation factor 2 and so halts protein synthesis; cholera toxin modifies a G protein so that adenylate cyclase is locked on, cyclic AMP floods the cell, and the gut pours out water. In each case one covalent modification of a single protein produces the whole disease.

Q: Which of the following enzymes catalyses the reaction that introduces reduced nitrogen into cellular metabolism?

Correct answer: Bacterial glutamine synthaseGlutamine synthetase is the point at which ammonia enters organic combination, being attached to glutamate to make glutamine. From there the nitrogen passes to every other nitrogenous compound the cell makes, which is why the enzyme is among the most elaborately regulated known - by cumulative feedback from nine end products and by adenylylation.

The allosteric inhibitor of an enzyme ___________

A.Causes the enzyme to work faster
B.Binds to the active site
C.Participates in feedback regulation
D.Denatures the enzyme

Answer: C

Correct answer: Participates in feedback regulation

An allosteric inhibitor binds at a regulatory site, not the active site, and changes the enzyme’s shape so that catalysis slows. Typically the inhibitor is the end product of the pathway, so that when enough has accumulated the first enzyme is switched off - which is exactly what feedback regulation means. Nothing is denatured, and the effect is reversible.

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Which of the following is false about allosteric feedback inhibition?

A.Bacterial enzyme system is the first known example
B.Conversion of L-leucine to L-isoleucine
C.Threonine dehydratase is inhibited by isoleucine
D.If the isoleucine concentration decreases, the rate of threonine dehydration increases

Answer: B

Correct answer: Conversion of L-leucine to L-isoleucine

The classic example is the conversion of L-threonine to L-isoleucine, not of leucine. Threonine dehydratase catalyses the first step and is inhibited by isoleucine, the end product; as isoleucine is used up the inhibition lifts and the pathway runs again. That is why the statement naming leucine is the false one.

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Which of the following is false?

A.Methyl, phosphoryl, adenyl, uridylyl and adenosine diphosphate ribosyl groups are the modifying groups
B.An example of an enzyme regulated by methylation is the methyl-donating chemo taxis protein of bacteria
C.The protein is a part of the system that permits a bacterium to swim towards attractant
D.The methylating agent is S- adenosylmethionine

Answer: B

Correct answer: An example of an enzyme regulated by methylation is the methyl-donating chemo taxis protein of bacteria

The protein is the methyl-accepting chemotaxis protein, not methyl-donating - it receives methyl groups, and the donor is S-adenosylmethionine. Its level of methylation is what lets a swimming bacterium compare concentrations over time and so move towards an attractant.

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Which of the following enzymes is responsible for the regulation of biological nitrogen fixation?

A.Dinitrogenase reductase
B.Dinitrogenase oxidase
C.Phosphatase
D.Kinase

Answer: A

Correct answer: Dinitrogenase reductase

Dinitrogenase reductase is regulated by ADP-ribosylation: when fixed nitrogen is plentiful, an ADP-ribosyl group is added and the enzyme is switched off; when nitrogen is scarce it is removed and fixation resumes. Regulating it makes sense, since nitrogen fixation is enormously expensive in ATP.

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Which of the following enzymes catalyze the ADP-ribosylation of key cellular enzymes or proteins?

A.Diphtheria toxin and cholera toxin
B.Dinitrogenase reductase
C.Protein phosphatases
D.Kinase

Answer: A

Correct answer: Diphtheria toxin and cholera toxin

Both toxins work by ADP-ribosylating a host protein. Diphtheria toxin modifies elongation factor 2 and so halts protein synthesis; cholera toxin modifies a G protein so that adenylate cyclase is locked on, cyclic AMP floods the cell, and the gut pours out water. In each case one covalent modification of a single protein produces the whole disease.

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Removal of phosphoryl groups is catalyzed by ___________

A.Diphteria toxin and cholera toxin
B.Dinitrogenase reductase
C.Protein phosphatases
D.Protein kinases

Answer: C

The enzymes that catalyze the ADP-ribosylation and inactivation of key cellular enzymes or proteins are diphteria toxin and cholera toxin. Dinitrogenase reductase is responsible for the regulation of biological nitrogen fixation. The attachment of phosphoryl groups to specific amino acid residues is catalyzed by protein kinases.

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The attachment of phosphoryl groups to specific amino acid residues is catalyzed by ___________

A.Diphteria toxin and cholera toxin
B.Dinitrogenase reductase
C.Protein phosphatases
D.Protein kinases

Answer: D

The enzymes that catalyse the ADP-ribosylation and inactivation of key cellular enzymes or proteins are diphteria toxin and cholera toxin. Dinitrogenase reductase is responsible for the regulation of biological nitrogen fixation. Removal of phosphoryl groups is catalyzed by Protein phosphatases.

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Which of the following takes place due to phosphorylation of isocitrate dehydrogenase?

A.Inhibits the binding of isocitrate at the active site
B.Degrades the enzyme
C.Enhances the substrate-binding affinity
D.No reaction

Answer: A

Correct answer: Inhibits the binding of isocitrate at active site

Phosphorylation of isocitrate dehydrogenase adds a bulky negative phosphate group at the active site, which repels the negatively charged substrate and prevents isocitrate from binding. The enzyme is not degraded, and the change is reversed by a phosphatase - the switch that sends isocitrate into the glyoxylate cycle instead of the TCA cycle.

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Which of the following enzymes catalyses the reaction that introduces reduced nitrogen into cellular metabolism?

A.Bacterial glutamine synthase
B.Bacterial dinitrogenase reductase
C.Bacterial dinitrogenase oxidase
D.Phosphatase

Answer: A

Correct answer: Bacterial glutamine synthase

Glutamine synthetase is the point at which ammonia enters organic combination, being attached to glutamate to make glutamine. From there the nitrogen passes to every other nitrogenous compound the cell makes, which is why the enzyme is among the most elaborately regulated known - by cumulative feedback from nine end products and by adenylylation.

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10.

Which of the following is false about feedback regulation?

A.It is a type of enzymatic activity regulation
B.The synthesis of concerned enzyme is inhibited by interfering with the gene of that enzyme
C.Concentration of enzyme is reduced by the end product
D.The best example is the biosynthesis of L-isoleucine from L-threonine in bacteria

Answer: D

Correct answer: The best example is the biosynthesis of L-isoleucine from L-threonine in bacteria

The threonine-to-isoleucine pathway illustrates feedback inhibition - the end product switching off an enzyme that already exists. The other three statements describe feedback repression, which acts on the gene and reduces how much enzyme is made at all. The two work on different timescales: inhibition in seconds, repression over a generation.

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