Question 1

A biochemist measures the rate of the enzyme Rubisco in the fixation of CO$$_2 on $$RuBP. The following data are obtained:

Substrate added ($$\mu$$mol/l): 217, 315, 400, 500, 900

V (nmol/l/s): 0.8, 1.0, 1.2, 1.4, 1.8

Using the Michaelis-Menten equation, what is the value of $$K_m$$ for Rubisco? (Hint: The $$[S] at $$which $$V_0$$ equals to $$1/2 V_{max}$$ represents the $$K_m.)$$

Accepted Answer

$$K_m = 400\ \mu mol/l$$

Question 2

Which of the following is the correct double reciprocal (Lineweaver-Burk) transformation of the Michaelis-Menten equation?

Accepted Answer

$$\frac{1}{V} = \frac{K_m}{V_{max}} \cdot \frac{1}{[S]} + \frac{1}{V_{max}}$$

Question 3

In a Lineweaver-Burk plot, what does the y-intercept represent?

Accepted Answer

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

Question 4

Why is the Lineweaver-Burk plot considered more useful than the standard Michaelis-Menten plot for determining $$V_{max}$$?

Accepted Answer

Because $$V_{max} is $$never achieved experimentally, but the Lineweaver-Burk plot allows extrapolation to $$1/V_{max}.$$

Question 5

Given the Lineweaver-Burk equation $$\frac{1}{V} = \frac{K_m}{V_{max}} \cdot \frac{1}{[S]} + \frac{1}{V_{max}}$$, what is the slope of the plot?

Accepted Answer

$$\frac{K_m}{V_{max}}$$

Question 6

If an enzyme-catalyzed reaction has a $$V_{max} of$$ $$2.0\ nmol/l/s$$ and a $$K_m of$$ $$400\ \mu mol/l$$, what is the initial velocity $$V_0$$ when $$[S] = 400\ \mu mol/l$$?

Accepted Answer

$$1.0\ nmol/l/s$$

Question 7

A student plots $$1/V$$ versus $$1/[S]$$ and obtains a straight line. What does the x-intercept of this plot represent?

Accepted Answer

$$-\frac{1}{K_m}$$

Question 8

Which of the following statements best explains why the maximum velocity ($$V_{max}) is $$never achieved in a typical enzyme assay?

Accepted Answer

Because substrate concentrations high enough to saturate all enzyme molecules are rarely attainable in practice.

Question 9

If the Lineweaver-Burk plot of an enzyme yields a y-intercept of $$0.5\ (l/nmol/s)$$, what is the $$V_{max}$$?

Accepted Answer

$$2.0\ nmol/l/s$$

Question 10

A mutation in Rubisco increases its $$K_m$$ for CO$$_2$$ without affecting $$V_{max}. $$What is the most likely effect on the enzyme's function in the cell?

Accepted Answer

The enzyme will require a higher concentration of CO$$_2 to $$reach half-maximal velocity.

Question 11

Which of the following best describes the significance of $$K_m in $$enzyme kinetics?

Accepted Answer

$$K_m is $$the substrate concentration at which the reaction velocity is half of $$V_{max}.$$

Question 12

If an enzyme has a low $$K_m$$ for its substrate, what does this indicate about the enzyme's affinity for the substrate?

Accepted Answer

The enzyme has a high affinity for the substrate.

Question 13

A researcher finds that doubling the substrate concentration does not double the reaction velocity. What is the most likely explanation?

Accepted Answer

The enzyme is approaching saturation and is operating near $$V_{max}.$$

Question 14

Which of the following is a limitation of the Lineweaver-Burk plot?

Accepted Answer

It overemphasizes errors at low substrate concentrations.

Question 15

In a clinical scenario, a patient has a mutation that reduces the $$V_{max} of a $$key metabolic enzyme but does not change $$K_m. $$What is the most likely effect on the patient's metabolism?

Accepted Answer

The maximum rate of the metabolic pathway will be reduced, but substrate affinity remains unchanged.

Question 16

A Lineweaver-Burk plot is constructed for an enzyme and the following equation is obtained: $$\frac{1}{V} = 2 \cdot \frac{1}{[S]} + 0.5. $$What are the values of $$K_m$$ and $$V_{max}$$?

Accepted Answer

$$K_m = 1.0\ \mu mol/l$$, $$V_{max} = 2.0\ nmol/l/s$$

Question 17

Which of the following best explains the essence of $$K_m$$ and $$V_{max} in $$the context of enzyme-catalyzed reactions?

Accepted Answer

$$K_m$$ reflects the substrate concentration required for significant activity, while $$V_{max}$$ reflects the catalytic capacity of the enzyme.

Question 18

A new inhibitor is found to increase the slope of the Lineweaver-Burk plot without changing the y-intercept. What type of inhibition is this?

Accepted Answer

Competitive inhibition

Question 19

If the initial velocity ($$V_0) of an $$enzyme-catalyzed reaction is measured at several substrate concentrations and plotted as $$V_0$$ versus $$[S]$$, what is the expected shape of the curve?

Accepted Answer

A hyperbola

Michaelis-Menten Kinetics and the Lineweaver-Burk Plot

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