Question 1

Which of the following statements best describes how enzymes affect the activation energy ($$\Delta G$$^{\ddagger}$$) and the standard free energy change ($$\Delta G^\circ$$) of a biochemical reaction?

Accepted Answer

Enzymes lower the activation energy but do not change the standard free energy change.

Question 2

A researcher is studying an enzyme-catalyzed reaction and finds that doubling the substrate concentration does not increase the reaction rate. What is the most likely explanation for this observation?

Accepted Answer

The enzyme is saturated with substrate and is operating at $$V_{max}.$$

Question 3

Which of the following best describes the meaning of $$k_{cat}/K_m$$ for an enzyme?

Accepted Answer

It is a measure of catalytic efficiency and substrate specificity.

Question 4

A Lineweaver-Burk plot of an enzyme-catalyzed reaction in the presence of a competitive inhibitor will show which of the following changes compared to the uninhibited reaction?

Accepted Answer

The $$x-$$intercept moves closer to zero (less negative), but the $$y-$$intercept remains unchanged.

Question 5

Which of the following is a key feature of the induced fit model of enzyme action?

Accepted Answer

The enzyme undergoes a conformational change to better accommodate the substrate.

Question 6

Which catalytic mechanism is primarily used by lysozyme to cleave polysaccharides?

Accepted Answer

General acid/base catalysis and stabilization of an oxocarbenium ion transition state.

Question 7

A patient is prescribed a drug that acts as a noncompetitive inhibitor of an enzyme. Which of the following changes would you expect to see in the enzyme kinetics?

Accepted Answer

Decrease in $$V_{max}$$ with no change in $$K_m.$$

Question 8

Which of the following best explains why enzymes are highly specific for their substrates?

Accepted Answer

Enzymes have active sites with shapes and chemical properties complementary to their specific substrates.

Question 9

Given the Michaelis-Menten equation $$v = \frac{V_{max}[S]}{K_m + [S]}$$, what is the initial velocity $$v$$ when $$[S] = K_m$$?

Accepted Answer

$$v = \frac{1}{2} V_{max}$$

Question 10

A mutation in chymotrypsin replaces Ser195 with alanine. What is the most likely effect on enzyme activity?

Accepted Answer

The enzyme will lose its ability to form the tetrahedral intermediate, greatly reducing activity.

Question 11

Which of the following statements about allosteric enzymes is TRUE?

Accepted Answer

They often display sigmoidal (S-shaped) $$v vs$$. $$[S]$$ plots due to cooperative substrate binding.

Question 12

Which of the following is a common effect of phosphorylation on enzyme activity?

Accepted Answer

It can either activate or inhibit the enzyme by inducing conformational changes.

Question 13

A Lineweaver-Burk plot of an enzyme-catalyzed reaction in the presence of an uncompetitive inhibitor will show which of the following changes?

Accepted Answer

Both $$K_m$$ and $$V_{max}$$ decrease.

Question 14

Which of the following best describes the function of the oxyanion hole in chymotrypsin catalysis?

Accepted Answer

It stabilizes the negative charge on the tetrahedral intermediate during catalysis.

Question 15

Which of the following is a correct statement about enzyme cofactors?

Accepted Answer

Coenzymes are often derived from vitamins and are required for some enzyme functions.

Question 16

A patient is exposed to an irreversible inhibitor of acetylcholinesterase. What is the most likely biochemical consequence?

Accepted Answer

The enzyme will be covalently modified and permanently inactivated.

Question 17

Which of the following is a distinguishing feature of mixed inhibition compared to competitive and uncompetitive inhibition?

Accepted Answer

The inhibitor can bind to both the free enzyme and the enzyme-substrate complex.

Question 18

Which of the following best explains why $$k_{cat} is $$also called the turnover number?

Accepted Answer

It is the number of substrate molecules converted to product per enzyme molecule per unit time.

Question 19

A new enzyme is discovered with a $$K_m of$$ $$1 \ \mu M$$ and a $$k_{cat} of$$ $$10^6$$ \ $$s^{-1}. $$What does this suggest about the enzyme's efficiency?

Accepted Answer

It is likely to be a highly efficient enzyme, possibly approaching the diffusion limit.

Enzymes: Biological Catalysis (Biochemistry Chapter 8 Study Notes)

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