Cell and Molecular Biology: Concepts and Experiments

Gerald Karp

Chapter 3

Bioenergetics, Enzymes, and Metabolism - all with Video Answers

Educators

Chapter Questions

Problem 1

How would you expect a drop in $\mathrm{pH}$ to affect a reaction catalyzed by chymotrypsin? How might an increase in $\mathrm{pH}$ affect this reaction?

Bryan Valdivia

Bryan Valdivia

Numerade Educator

Problem 2

Feedback inhibition typically alters the activity of the first enzyme of a metabolic pathway rather than one of the latter enzymes of the pathway. Why is this adaptive?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 3

After reviewing the reactions of glutamine formation on page $92,$ explain why each of the following statements concerning the third (or overall) reaction is either true or false.
a. If the reaction were written in reverse, its $\Delta G^{\circ \prime}$ would be $+3.9 \mathrm{kcal} / \mathrm{mol}$
b. If all reactants and products were at standard conditions at the beginning of an experiment, after a period of time, the $\left[\mathrm{NH}_{3}\right] /[\mathrm{ADP}]$ ratio would decrease.
c. As the reaction proceeds, the $\Delta G^{\circ \prime}$ moves closer to zero.
d. At equilibrium, the forward and reverse reactions are equal and the $[\mathrm{ATP}] /[\mathrm{ADP}]$ ratio becomes one.
e. In the cell, it is possible for glutamine to be formed when the [glutamine]/[glutamic acid] ratio is greater than one.

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 4

You have just isolated a new enzyme and have determined the velocity of reaction at three different substrate concentrations. You find that the slope of the product versus time curve is the same for all three concentrations. What can you conclude about the conditions in the reaction mixture?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 5

Lysozyme is a slow-acting enzyme, requiring approximately two seconds to catalyze a single reaction. What is the turnover number of lysozyme?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 6

In the reaction $\mathrm{R} \rightleftharpoons \mathrm{P}$, if one mole of product (P) has the same free energy as one mole of reactant (R), what is the value for the $K_{\mathrm{eq}}^{\prime}$ of this reaction? What is the value of the $\Delta G^{\mathrm{or}}$

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 7

What is meant in terms of concentration ratios when it is said that the $\Delta G$ of ATP hydrolysis in the cell is approximately $-12 \mathrm{kcal} / \mathrm{mol},$ whereas the $\Delta G^{\circ \prime}$ is $-7.3 \mathrm{kcal} / \mathrm{mol}^{2}$

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 8

The enzymes under cellular regulation are those whose reactions typically proceed under nonequilibrium conditions. What would be the effect of allosteric inhibition of an enzyme that operated close to equilibrium?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 9

In the reaction $A=B$, if the $K_{e q}^{\prime}$ is $10^{3},$ what is the $\Delta G^{\circ \prime}$ ? What is the $\Delta G^{\circ \prime}$ if the $K_{\mathrm{eq}}^{\prime \prime}$ had been determined to be $10^{-3 p}$ ? What is the $K_{\mathrm{eq}}^{\prime}$ of the hexokinase reaction indicated in Figure $3.24(\operatorname{step} 1) ?$

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 10

In $1926,$ James Sumner concluded that urease was a protein based on the fact that crystals of the enzyme tested positive for reagents that reacted with proteins and negative for reagents that reacted with fats, carbohydrates, and other substances. His conclusion was attacked by other enzymologists, who found that their highly active enzyme solutions failed to contain evidence of protein. How can these seemingly opposing findings be reconciled?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 11

If the reaction $\mathrm{XA}+\mathrm{Y} \rightleftharpoons \mathrm{XY}+\mathrm{A}$ has a $\Delta G^{\circ \prime}$ of $+7.3 \mathrm{kcal} / \mathrm{mol}$
could this reaction be driven in the cell by coupling it to ATP hydrolysis? Why or why not?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 12

In a series of reactions, $A \rightarrow B \rightarrow C \rightarrow D,$ it was determined that the equilibrium constant for the second reaction $(B \rightarrow C)$ is$0.1 .$ You would expect the concentration of $\mathrm{C}$ in a living cell to be:
(1) equal to $\mathrm{B}$, ( 2 ) one-tenth of $\mathrm{B}$, (3) less than one-tenth of
B, (4) 10 times that of $B$, (5) more than 10 times that of B. (Circle any correct answer.)

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 13

The reaction of compound $X$ with compound $Y$ to produce compound $Z$ is an unfavorable reaction $\left(\Delta G^{\circ \prime}=+5 \mathrm{kcal} / \mathrm{mol}\right)$ Draw the chemical reactions that would occur if ATP was utilized to drive the reaction.

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 14

ATP has evolved as the central molecule in energy metabolism. Could 1,3 -bisphosphoglycerate serve the same function? Why or why not?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 15

Calculate the $\Delta G$ for ATP hydrolysis in a cell in which the $[\mathrm{ATP}] /[\mathrm{ADP}]$ ratio had climbed to $100: 1,$ while the $\mathrm{Pi}$ con centration remained at $10 \mathrm{mM}$. How does this compare to the ratio of $[\mathrm{ATP}] /[\mathrm{ADP}]$ when the reaction is at equilibrium and the $P_{i}$ concentration remains at 10 mM? What would be the value for $\Delta G$ when the reactants and products were all at standard conditions $(1 \mathrm{M})$ ?

Rashmi Sinha

Numerade Educator

Problem 16

Consider the reaction:
\[
\begin{aligned}
\text { Glucose }+\mathrm{P}_{\mathrm{i}} \rightleftharpoons_{8} \text { lucose } 6 \text { -phosphate }+\mathrm{H}_{2} \mathrm{O} \\
& \Delta G^{* \prime}=+3 \mathrm{kcal} / \mathrm{mol}
\end{aligned}
\]
What is the equilibrium constant, $K_{\mathrm{eq}}^{\prime},$ for this reaction? (Note:
the concentration of water is ignored.) Does the positive $\Delta G^{0 \prime}$ in the above reaction mean that the reaction can never go spontaneously from left to right?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 17

Under physiologic conditions, [Glucose] = 5 $\mathrm{mM}$, [Glucose 6-phosphate $]=83 \mathrm{mM},$ and $\left[\mathrm{P}_{1}\right]=1 \mathrm{mM} .$ Under these conditions, will the reaction of problem 16 go spontaneously from left to right? If not, what would the concentration of glucose need to be for the reaction to go from left to right, if the concentrations of the other reactants and products are as stated above?

Sana Riaz

Numerade Educator

Problem 18

Consider the reaction:
\[
\begin{array}{l}
\text { Glutamate }+\text { ammonia } \rightleftharpoons \text { glutamine }+\mathrm{H}_{2} \mathrm{O} \\
\qquad \Delta G^{* \prime}=+3.4 \mathrm{kcal} / \mathrm{mol}
\end{array}
\]
If the concentration of ammonia $\left(\mathrm{NH}_{3}\right)$ is $10 \mathrm{mM}$, what is the ratio of glutamate/glutamine required for the reaction to proceed spontaneously from left to right at $25^{\circ} \mathrm{C}$ ?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 19

It should be clear that the synthesis of glutamine cannot occur in a cell by the reaction described in Problem 18 . The actual reaction couples glutamine synthesis to ATP hydrolysis:
\[
\text { Glutamate }+\text { ammonia }+\mathrm{ATP} \rightleftharpoons \text { glutamine }+\mathrm{ADP}+\mathrm{P}
\]
What is the $\Delta G^{\circ \prime}$ for this reaction? Suppose that all reactants and products, except ammonia, are present at $10 \mathrm{mM}$. What concentration of ammonia would be required to drive the reaction to the right, that is, to drive the net synthesis of glutamine?

Sana Riaz

Numerade Educator

Problem 20

A noncompetitive inhibitor does not prevent the enzyme from binding its substrate. What will be the effect of increasing the substrate concentration in the presence of a noncompetitive in hibitor? Do you expect a noncompetitive inhibitor to affect the enzyme $V_{\max }$ ? $K_{\mathrm{M}}$ ? Explain briefly.

Danielle Ashley

Danielle Ashley

Numerade Educator