GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Enzymes

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Example Questions

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Example Question #1 : Enzymes

A researcher is studying the rate of an enzyme-catalyzed reaction by placing increasing amounts of substrate into a solution containing the enzyme. After a certain concentration, the rate of the reaction plateaus and does not go any higher. What has happened?

Possible Answers:

The concentration of enzyme is so small that the reaction has stopped occurring completely

The reaction rate has only momentarily plateaued; given enough time it will increase

The enzyme has become denatured

The enzyme has become saturated

Correct answer:

The enzyme has become saturated

Explanation:

If the reaction rate has plateaued, this indicates that the enzyme has reached saturation. At this point, every active site on every molecule of enzyme is actively catalyzing the reaction as quickly as it can. The only way to change the reaction rate, at this point, would be to increase the concentration of the enzyme in the solution. Further increasing substrate concentration will have no effect.

We know that the enzyme has not become denatured because the reaction is still occurring. The rate of the reaction is constant during the plateau, and does not drop to zero.

Example Question #1 : Enzymes

In a Lineweaver-Burk plot, what quantity determines the y-intercept?

Possible Answers:

Correct answer:

Explanation:

A Lineweaver-Burk plot is a way to graphically represent enzyme kinetics. It is convenient because several portions of the graph readily display important information, such as rate constants. The y-intercept in particular is useful because it represents the reciprocal of the maximum velocity. The x-intercept describes the negative reciprocal of the Michaelis constant. The slope is the quotient of the Michaelis constant over the maximum velocity.

Example Question #1 : Help With Enzyme Kinetics

What information is contained in a Lineweaver-Burk plot that is not present in a standard Michaelis-Menten plot?

Possible Answers:

None of these answers

Correct answer:

None of these answers

Explanation:

The two plots contain the same information. A Michaelis-Menten plot shows the relationship between initial reaction rate concentration of substrate ( versus ). A Lineweaver-Burk plot shows the relationship between the inverses of these same two variables, however, it is much easier to visualize important data on a Lineweaver-Burk plot. The x-intercept, the y-intercept, and the slope all contain points of interest. A downside of the Lineweaver-Burk plot, however, is that it is more susceptible to inaccuracy if there is some flaw in the accumulated data. 

Example Question #1 : Enzyme Principles

Which of the following changes will alter  of an enzyme-catalyzed reaction?

Possible Answers:

Increasing substrate to supraphysiological concentrations

None of these options; cannot be changed

Addition of a competitive inhibitor

Addition of a non-competitive inhibitor

Correct answer:

Addition of a non-competitive inhibitor

Explanation:

The only option that will alter the is to add a non-competitive inhibitor. The addition of this inhibitor will affect the amount of free enzyme available to catalyze the reaction, and thus lower the by reducing the effective enzyme concentration.

Addition of a competitive inhibitor will alter the , but not the . Increasing the substrate concentration will have no effect once saturation has been reached. 

Example Question #1 : Enzymes

A catalyst is an enzyme that promotes a reaction. In terms of free energy, what does a catalyst change about the reaction to promote the reaction proceeding?

Possible Answers:

Catalysts induce a global increase of entropy to the product state of the reaction, making it more favorable and thus occurring at a faster rate. 

Catalysts increase the rate of the reaction by reducing the free energy of the transition state, which lowers the activation energy. 

Catalysts increase the amount of energy contributed to the reaction through heat from the reaction environment, thus increasing the rate of the reaction. 

Catalysts increase the rate of the reaction by increasing the free energy of the transition state, which increases the activation energy. 

Catalysts increase the rate of reaction by decreasing the free energy of the transition state, which increases the activation energy. 

Correct answer:

Catalysts increase the rate of the reaction by reducing the free energy of the transition state, which lowers the activation energy. 

Explanation:

During a reaction, the reactants must pass through high-energy transition states before they evolve into the products. The catalyst reduces the free energy of this transition state, thus making it 'easier' for the reactant to undergo the chemical reaction since the activation energy has been lowered. 

Example Question #6 : Enzymes

Which of the following best describes when an inhibitor binds an enzyme at a separate site from the active site, but only when the enzyme and substrate are already bound in complex?

Possible Answers:

Non-competitive inhibition 

Uncompetitive inhibition

Competitive inhibition 

Allostery 

Reversible inhibition

Correct answer:

Uncompetitive inhibition

Explanation:

The correct answer is uncompetitive inhibition. The formation of a enzyme-substrate complex creates an alternative site on the enzyme for an inhibitor to bind. This mechanism is considered uncompetitive because the inhibitor and substrate are not competing for the same binding site on the enzyme. 

Example Question #1 : Enzymes

What is the primary mechanism by how enzymes increase the rate of a reaction?

Possible Answers:

They decrease the stability of the transition state.

They lower the activation energy needed in the reaction.

They decrease the reverse reaction rate and increase the forward reaction rate.

They decrease the internal energy of the final product.

Correct answer:

They lower the activation energy needed in the reaction.

Explanation:

Enzymes exert their effect on the reaction rate by decreasing the energy needed for the reaction to proceed. As a result, the enzyme will decrease the activation energy. It should be noted that the forward reaction rate and reverse reaction rate are both increased by an enzyme. If this were not the case, more product would be made compared to the uncatalyzed reaction, and enzymes do not affect equilibrium constants for reactions.

Example Question #2 : Enzymes

Which of the following changes cannot be accomplished by an enzyme in a chemical reaction?

Possible Answers:

Change in enthalpy

Change in activation energy

Change in reverse reaction rate

Change in forward reaction rate

Correct answer:

Change in enthalpy

Explanation:

An enzme is a biological catalyst that increases the rate of a reaction. This is accomplished by lowering the activation energy necessary to start the reaction. The equilibrium of the reaction, however, is not affected. This means that enthalpy and entropy are not affected by an enzyme's presence.

Example Question #9 : Enzymes

The class of enzymes that break bonds by forming a new double bond or ring structure (rather than by hydrolysis or oxidation) is best characterized as which of the following?

Possible Answers:

Ligases

Transferases

Isomerases

Lyases

Kinases

Correct answer:

Lyases

Explanation:

The correct answer is lyases. This class of enzymes only requires one substrate for the forward reaction.

Ligases catalyze the formation of a bond between two molecules, isomerases rearrange the atoms of a molecule, kinases phosphorylate molecules, and transferases transfer or join functional groups from one molecule to another. 

Example Question #1 : Enzyme Principles

The third step of glycolysis converts fructose-6-phosphate to fructose-1,6-bisphosphate. What type of enzyme mediates this?

Possible Answers:

An isomerase

A polymerase

A kinase

A phosphatase

A reductase

Correct answer:

A kinase

Explanation:

Kinases are enzyme that catalyze the transfer of a phosphate group from ATP to a substrate molecule. The phosphorylation of fructose-6-phosphate to fructose-1,6-phosphate is mediated by a kinase phosphofructokinase.

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