Mastering Biology Ch. 8

During a laboratory experiment, you discover that an enzyme-catalyzed reaction has a G of -20 kcal/mol. If you double the amount of enzyme in the reaction, what will be the G for the new reaction?

-20 kcal/mol

Which of the following statements about equilibrium of chemical reactions is correct?

A reaction that is at equilibrium is not capable of doing any work.

Select the highest energy form of adenosine from the following images.

ATP adenosine triphosphate, contains most phosphate groups

You have an enzymatic reaction proceeding at the optimum pH and optimum temperature. You add a competitive inhibitor to the reaction and notice that the reaction slows down. What can you do to speed the reaction up again?

Add more substrate; it will outcompete the inhibitor and increase the reaction rate.

How does an enzyme increase the rate of the chemical reaction it catalyzes?

An enzyme reduces the free energy of activation (EA) of the reaction it catalyzes.

Living organisms increase in complexity as they grow, resulting in a decrease in the entropy of an organism. How does this relate to the second law of thermodynamics?

As a consequence of growing, organisms cause a greater increase in entropy in their environment than the decrease in entropy associated with their growth.

What will happen to the rates of the forward and reverse reactions when a catalyst is added?

Both forward and reverse rates increase.

Which of the following statements is representative of the second law of thermodynamics?

Cells require a continuous input of energy to maintain their high level of organization.

How do cells use the ATP cycle shown in the figure?

Cells use the cycle to recycle ADP and phosphate.

Consider a situation in which the enzyme is operating at optimum temperature and pH, and has been saturated with substrate. What is your best option for increasing the rate of the reaction?

Increase the enzyme concentration.

Succinate dehydrogenase catalyzes the conversion of succinate to fumarate. The reaction is inhibited by malonic acid, which resembles succinate but cannot be acted upon by succinate dehydrogenase. Increasing the ratio of succinate to malonic acid reduces the inhibitory effect of malonic acid. What is malonic acid's role with respect to succinate dehydrogenase?

It is a competitive inhibitor.

For the hydrolysis of ATP to ADP + i, the free energy change is -7.3 kcal/mol under standard conditions (1 M concentration of both reactants and products). In the cellular environment, however, the free energy change is about -13 kcal/mol. What can we conclude about the free energy change for the formation of ATP from ADP and i under cellular conditions?

It is about +13 kcal/mol.

Which of the following is true of metabolism in its entirety in all organisms?

Metabolism consists of all the energy transformation reactions in an organism.

Rank these by reaction rate, as measured by the rate of product formation per unit time, from lowest reaction rate to highest reaction rate. To rank items as equivalent, overlap them.

Order: Uncatalyzed > Enzyme A > Enzyme B Enzymes lower the activation energy of a chemical reaction. This means that a catalyzed reaction is more likely to proceed than an uncatalyzed reaction, and it forms products more rapidly than an uncatalyzed reaction.

Which of the following statements is most likely to be true in the case of the feedback-regulated enzymatic pathway shown?

P4 binds E1 and deactivates it.

When 10,000 molecules of ATP are hydrolyzed to ADP and i in a test tube, about half as much heat is liberated as when a cell hydrolyzes the same amount of ATP. Which of the following is the best explanation for this observation?

Reactant and product concentrations in the test tube are different from those in the cell.

Succinate dehydrogenase catalyzes the conversion of succinate to fumarate. The reaction is inhibited by malonic acid, which resembles succinate but cannot be acted upon by succinate dehydrogenase. Increasing the ratio of succinate to malonic acid reduces the inhibitory effect of malonic acid. Based on this information, which of the following is correct?

Succinate is the substrate, and fumarate is the product.

Which of the following is NOT a way in which an enzyme can speed up the reaction that it catalyzes?

The active site can provide heat from the environment that raises the energy content of the substrate.

The binding of a compound to an enzyme is observed to slow down or stop the rate of the reaction catalyzed by the enzyme. Increasing the substrate concentration reduces the inhibitory effects of this compound. Which of the following could account for this observation?

The compound is a competitive inhibitor.

Which of the following statements about the role of ATP in cell metabolism is true?

The energy from the hydrolysis of ATP may be directly coupled to endergonic processes by the transfer of the phosphate group to another molecule.

Which of the following statements about the combustion of glucose with oxygen to form water and carbon dioxide (C6H12O6 + 6 O2 → 6 CO2 + 6 H2O) is correct?

The entropy of the products is greater than the entropy of the reactants.

What would happen to the rate of the forward reaction if the concentration of nitrogen were decreased?

The reaction rate would decrease.

Which of these is ATP?

Triphosphate and one ribose

Which statement about the binding of enzymes and substrates is correct?

When substrate molecules bind to the active site of the enzyme, the enzyme undergoes a slight change in shape.

Which of the following is an example of cooperativity?

a molecule binding at one unit of a tetramer, allowing faster binding at each of the other three

Which of these is exhibiting kinetic energy?

a space station orbiting Earth

Reactants capable of interacting to form products in a chemical reaction must first overcome a thermodynamic barrier known as the reaction's

activation energy.

Which of the following is most similar in structure to ATP?

an RNA nucleotide

Allosteric enzyme regulation is usually associated with

an enzyme with more than one subunit.

Which of the following types of reactions would decrease the entropy within a cell?

anabolic reactions

Which of the following represents the activation energy required for the enzyme-catalyzed reaction in the figure?

b

How does a noncompetitive inhibitor decrease the rate of an enzyme reaction?

by changing the shape of the enzyme's active site

Which term most precisely describes the cellular process of breaking down large molecules into smaller ones?

catabolism

In your body, what process converts the chemical energy found in glucose into the chemical energy found in ATP?

cellular respiration

When you have a severe fever, what grave consequence may occur if the fever is not controlled?

change in the tertiary structure of your enzymes

Zinc, an essential trace element for most organisms, is present in the active site of the enzyme carboxypeptidase. The zinc most likely functions as a(n)

cofactor necessary for enzyme activity.

Which of the following represents the ΔG of the reaction in the figure?

d

A chemical reaction that has a positive ΔG is best described as

endergonic

"Conservation of energy" refers to the fact that _____.

energy cannot be created or destroyed but can be converted from one form to another

What is the correct label for "A"?

energy of activation

Which of the following terms best describes the forward reaction in the figure?

exergonic, Δ G < 0

Which of these are by-products of cellular respiration?

heat, carbon dioxide, and water

What type of reaction breaks the bonds that join the phosphate groups in an ATP molecule?

hydrolysis

Some of the drugs used to treat HIV patients are competitive inhibitors of the HIV reverse transcriptase enzyme. Unfortunately, the high mutation rate of HIV means that the virus rapidly acquires mutations with amino acid changes that make them resistant to these competitive inhibitors. Where in the reverse transcriptase enzyme would such amino acid changes most likely occur in drug-resistant viruses?

in or near the active site

Enzymes are described as catalysts, which means that they _____.

increase the rate of a reaction without being consumed by the reaction

As a result of its involvement in a reaction, an enzyme _____.

is unchanged

Besides turning enzymes on or off, what other means does a cell use to control enzymatic activity?

localization of enzymes into specific organelles or membranes

Mutations that result in single amino acid substitutions in an enzyme

may change the enzyme's optimal temperature or optimal pH.

Chemical energy is a form of _____ energy.

potential

In general, enzymes are what kinds of molecules?

proteins

Enzymes work by _____.

reducing EA

What name is given to the reactants in an enzymatically catalyzed reaction?

substrate

Which of the following best describes enthalpy (H)?

the heat content of a chemical system

Which of the following is true for all exergonic reactions?

the reaction proceeds with a net release of free energy

An important group of peripheral membrane proteins are enzymes such as the phospholipases that cleave the head groups of phospholipids. What properties must these enzymes exhibit?

water solubility

The mathematical expression for the change in free energy of a system is ΔG =ΔH - TΔS. Which of the following is (are) correct?

ΔG is the change in free energy.

Which part of the adenosine triphosphate molecule is released when it is hydrolyzed to provide energy for biological reactions?

γ-phosphate (the terminal phosphate)

Complete this vocabulary exercise relating to enzymes. Match the words in the left-hand column to the appropriate blank in the sentences in the right-hand column.

1. An enzyme is denatured when it loses its nature conformation and its biological activity. 2. An enzyme is considered a catalyst because it speeds up chemical reactions without being used up. 3. An enzyme is considered specific because of its ability to to recognize the shape of a particular molecule. 4. A cofactor, such as a vitamin, binds to an enzyme and plays a role in catalysis. 5. When properly aligned, the enzyme and substrate form an enzyme-substrate (ES) complex. 6. A substrate binds to an enzyme at the active site, where the reaction occurs. 7. In a catalyzed reaction a reactant is often called a substrate.

Complete this vocabulary exercise relating to the three types of enzyme inhibitors.

1. competitive 2. noncompetitive 3. irreversible 4. active site 5. enzyme 6. substrate Competitive inhibitors compete physically and structurally with the substrate for an enzyme's active site; they can be outcompeted by adding extra substrate. Noncompetitive inhibitors do not compete for the active site, but inhibit the enzyme by binding elsewhere and changing the enzyme's shape. Irreversible inhibitors bind directly to the active site by covalent bonds, which change the structure of the enzyme and inactivate it permanently. Most medications are enzyme inhibitors of one kind or another.

A number of systems for pumping ions across membranes are powered by ATP. Such ATP-powered pumps are often called ATPases although they don't often hydrolyze ATP unless they are simultaneously transporting ions. Because small increases in calcium ions in the cytosol can trigger a number of different intracellular reactions, cells keep the cytosolic calcium concentration quite low under normal conditions, using ATP-powered calcium pumps. For example, muscle cells transport calcium from the cytosol into the membranous system called the sarcoplasmic reticulum (SR). If a resting muscle cell's cytosol has a free calcium ion concentration of 10-7 while the concentration in the SR is 10-2, then how is the ATPase acting?

ATPase activity must be pumping calcium from the cytosol to the SR against the concentration gradient.

According to the induced fit hypothesis of enzyme catalysis, which of the following is correct?

The binding of the substrate changes the shape of the enzyme's active site.

Which of the following would be unlikely to contribute to the substrate specificity of an enzyme?

The enzyme has an allosteric regulatory site.

You have added an irreversible inhibitor to a sample of enzyme and substrate. At this point, the reaction has stopped completely. What can you do to regain the activity of the enzyme?

The enzyme is inactive at this point. New enzyme must be added to regain enzyme activity.

What is the free energy change (ΔG) of the hydrolysis of ATP to ADP?

The free-energy change (ΔG) of the hydrolysis of ATP to ADP and Pi may vary considerably with variations in pH, temperature, atmospheric pressure, and concentrations of reactants and products.

For living organisms, which of the following is an important consequence of the first law of thermodynamics?

The organism ultimately must obtain all of the necessary energy for life from its environment.

The Haber process is typically carried out at a temperature of approximately 500∘C. What would happen to the rate of the forward reaction if the temperature were lowered to 100∘C?

The reaction rate would decrease.

Which of the following statements is true concerning catabolic pathways?

They supply energy, primarily in the form of ATP, for the cell's work.

A series of enzymes catalyze the reaction X → Y → Z → A. Product A binds to the enzyme that converts X to Y at a position remote from its active site. This binding decreases the activity of the enzyme. With respect to the enzyme that converts X to Y, substance A functions as

an allosteric inhibitor.

Which of the following represents the activation energy required for a noncatalyzed reaction in the figure?

c

Which temperature and pH profile curves on the graphs were most likely generated from analysis of an enzyme from a human stomach where conditions are strongly acid?

curves 1 and 4

Which curve(s) on the graphs may represent the temperature and pH profiles of an enzyme taken from a bacterium that lives in a mildly alkaline hot springs at temperatures of 70°C or higher?

curves 3 and 5

Which of the following in the figure would be the same in either an enzyme-catalyzed or a noncatalyzed reaction?

d

Which of the following would increase the rate of the reverse reaction?

increasing the concentration of ammonia

An enzyme _____.

is an organic catalyst

In which region is the enzyme saturated with substrate?

region C

Look at the graph of reaction rate versus substrate concentration for an enzyme. (Figure 1) In which region does the reaction rate remain constant?

region C