Q: An enzyme has a $k_{cat}$ of $5 \times 10^7\ \mathrm{s}^{-1}$ and a $K_m$ of $2 \times 10^{-5}\ \mathrm{M}$. What is the catalytic efficiency ($\frac{k_{cat}}{K_m}$) of this enzyme?

$2.5 \times 10^{12}\ \mathrm{M}^{-1}\mathrm{s}^{-1}$

Question 1

Which of the following statements best describes how enzymes increase the rate of a chemical reaction?

Accepted Answer

Enzymes lower the activation energy barrier ($E_a$) for the reaction, stabilizing the transition state.

Question 2

A researcher is studying an enzyme-catalyzed reaction and finds that at high substrate concentrations, the reaction rate no longer increases with increasing substrate. What is the most likely explanation for this observation?

Accepted Answer

The enzyme has reached its maximal velocity ($V_{max}$) because all active sites are saturated with substrate.

Question 3

Which of the following enzyme classes catalyzes the transfer of a functional group from one molecule to another?

Accepted Answer

Transferases

Question 4

The Michaelis-Menten equation is given by $v_0 = \frac{V_{max}[S]}{K_m + [S]}$. What does the Michaelis constant ($K_m$) represent?

Accepted Answer

$K_m$ is the substrate concentration at which the reaction rate is half-maximal ($v_0 = \frac{V_{max}}{2}$).

Question 5

Which of the following is a correct description of a lyase enzyme?

Accepted Answer

It catalyzes the addition or removal of groups to form double bonds, without oxidation or hydrolysis.

Question 6

An enzyme has a $k_{cat}$ of $5 	imes 10^7\ \mathrm{s}^{-1}$ and a $K_m$ of $2 	imes 10^{-5}\ \mathrm{M}$. What is the catalytic efficiency ($\frac{k_{cat}}{K_m}$) of this enzyme?

Accepted Answer

$2.5 	imes 10^{12}\ \mathrm{M}^{-1}\mathrm{s}^{-1}$

Question 7

Which of the following best describes the effect of an allosteric activator on an enzyme's Michaelis-Menten plot?

Accepted Answer

It decreases $K_m$ and may increase $V_{max}$, resulting in a leftward shift of the curve.

Question 8

A Lineweaver-Burk plot is a double reciprocal plot of $\frac{1}{v_0}$ versus $\frac{1}{[S]}$. What is the y-intercept of this plot?

Accepted Answer

$\frac{1}{V_{max}}$

Question 9

Which of the following statements about enzyme specificity is correct?

Accepted Answer

Enzymes are highly specific for their substrates and the reactions they catalyze.

Question 10

Which enzyme class is responsible for catalyzing the joining of two molecules with the input of energy from a nucleotide triphosphate (e.g., ATP)?

Accepted Answer

Ligases

Question 11

A patient has a genetic mutation that causes a key hydrolase enzyme to be nonfunctional. Which of the following reactions would be most directly affected?

Accepted Answer

Cleavage of a peptide bond by addition of water

Question 12

During an enzyme assay, you observe that the initial velocity ($v_0$) increases linearly with substrate concentration at very low $[S]$. Which kinetic parameter is most relevant in this regime?

Accepted Answer

$\frac{k_{cat}}{K_m}$ (catalytic efficiency)

Question 13

Which of the following is true regarding the steady-state assumption in Michaelis-Menten kinetics?

Accepted Answer

The concentration of the enzyme-substrate complex ($[ES]$) remains constant over the time interval measured.

Question 14

Which of the following best describes a 'kinetically perfect' enzyme?

Accepted Answer

An enzyme for which every substrate binding event results in product formation, and the rate is limited by diffusion.

Question 15

A mutation in an enzyme increases its $K_m$ but does not affect $V_{max}$. What is the most likely effect of this mutation?

Accepted Answer

The enzyme has a lower affinity for its substrate.

Question 16

Which of the following is a property of allosteric enzymes that distinguishes them from Michaelis-Menten enzymes?

Accepted Answer

They often display a sigmoidal $v$ vs. $[S]$ curve due to cooperative substrate binding.

Question 17

Which of the following best describes the function of isomerase enzymes?

Accepted Answer

They catalyze the interconversion of isomers, often between cis and trans forms.

Question 18

A scientist is analyzing a Lineweaver-Burk plot and finds that the x-intercept is $-0.02\ \mathrm{mM}^{-1}$. What is the value of $K_m$?

Accepted Answer

$K_m = 50\ \mathrm{mM}$

Question 19

Which of the following is NOT a general property of enzymes?

Accepted Answer

Enzymes can induce reactions with a positive free energy change ($\Delta G > 0$).

Question 20

Which of the following best explains why enzymes are able to function under physiological conditions (e.g., pH 7, moderate temperature)?

Accepted Answer

Enzymes lower the activation energy, allowing reactions to proceed rapidly at mild conditions.

Enzyme Kinetics: Properties, Classification, and Kinetic Analysis

Study Guide - Practice Questions

Study Guide - Flashcards