Biochemistry
Biochemistry
9th Edition
ISBN: 9781305961135
Author: Mary K. Campbell, Shawn O. Farrell, Owen M. McDougal
Publisher: Cengage Learning
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Chapter 6, Problem 28RE

MATHEMATICAL The enzyme β -methylaspartase catalyzes the deamination of β -methylaspartate

Chapter 6, Problem 28RE, MATHEMATICAL The enzyme -methylaspartase catalyzes the deamination of -methylaspartate [V. Williams

[V. Williams and J. Selbin, J. Biol. Chem. 239, 1636 (1964)]. The rate of the reaction was determined by monitoring the absorbance of the product at 240 nm ( A 240 ) . From the data in the following table, determine K M for the reaction. How does the method of calculation differ from that in Questions 26 and 27?

Expert Solution & Answer
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Interpretation Introduction

Interpretation:

The Km value for the reaction in which “enzyme b-methyl aspartate catalyzes the deamination of b-methyl aspartate” is to be determined.

Concept introduction:

In an enzymatic reaction, Vmax is the maximum rate of the reaction, Km is the substrate concentration at which the rate of the reaction is half of Vmax. Km also defines substrate affinity of the enzyme.

To determine Vmax and Km, kinetic data obtained from the reaction needs to be put into the Lineweaver–Burk plot.

To draw this plot, substrate concentration 1[S] and its reciprocal velocity value 1V, needs to be plotted on the y- and x- axes.

The yintercept will be the 1Vmax of the reaction. From Vmax value, Km can be determined. The xintercept is 1Km. From that, Km can also be obtained.

Answer to Problem 28RE

Plotting the data gives the following straight line:

Biochemistry, Chapter 6, Problem 28RE

From this straight-line graph, the value of Km is 2.86×102M.

Explanation of Solution

Given information:

Following data containing substrate concentrations and velocity of the reaction at that substrate concentration, is given.

Substrate Concentration (mmol/Lit)Velocity (ΔA240min1)0.0020.0450.0050.1150.200.2850.0400.3800.0600.4600.0800.4750.1000.505.

In enzyme kinetics, to determine Km and Vmax of a reaction, the Michaels–Menten equation can be used.

V=KmVmax (This is the actual Michaelis–Menten equation.)

But Km and Vmax of a reaction cannot be accurately determined from this equation. To do that, this equation needs to be modified, so that it can be represented as a straight line and the straight-line equation, y=mx+b, can be used.

Modification of the equation is

1V=KmVmax(1[S])+1Vmax

This equation can be plotted on the graph, 1V represents the y-axis and KmVmax is the slope m. 1[S] represents the x-axis and b is 1Vmax. The y intercept is 1Vmax. And the x-intercept is 1Km. This plot is known as the Lineweaver–Burk plot. From this plot, Km and Vmax can be determined easily.

Vmax=1yintercept and Km=1xintercept

Conclusion

The enzyme, b-methyl aspartate, catalyzes the deamination of b-methyl aspartate. By plotting the given data into a Lineweaver–Burk plot, Km for this reaction is 2.86 × 10-2 mole. Although in this case, concentration cannot be determined directly. Absorbance values were used instead, as a matter of convenience.

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Chapter 6 Solutions

Biochemistry

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