# GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Help with Enzyme Kinetics

## Example Questions

### Example Question #1 : Help With Enzyme Kinetics

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?

The enzyme has become denatured

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 saturated

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 #2 : Help With Enzyme Kinetics

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

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 #3 : Help With Enzyme Kinetics

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

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 #4 : Help With Enzyme Kinetics

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

Increasing substrate to supraphysiological concentrations

None of these options; cannot be changed

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 #5 : Help With Enzyme Kinetics

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?

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.

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.