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Study Area 2

Enzymes

1. Which of the following situations does NOT represent a transformation of one type of energy to another?

A) the burning of gasoline in a car engine B) the production of sugar by photosynthesis C) the production of electrical power by damming a river D) cracking a nut by using a nutcracker E) plugging a stereo into a wall socket to play music

2. What is energy?

A) the capacity to perform work B) the amount of food eaten C) movement D) the rearrangement of chemical molecules within matter E) the capacity to produce heat

3. Energy is most commonly stored in plants as ____________.

A) electrical energy B) mechanical energy C) electromagnetic energy D) kinetic energy E) chemical energy

4. Which one of the following has the most free energy per molecule?

A) a sugar molecule B) an amino acid molecule C) a starch molecule D) a fatty acid molecule E) a cholesterol molecule

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5. What must be true if the reaction AB + CD > AC + BD occurs spontaneously?

A) The DG of the reaction must be negative. B) The reaction must be exergonic. C) The environment has adequate thermal energy to meet the activation energy requirement. D) The bonds must have absorbed enough energy to become unstable. E) All of the above are true.

6. Which of the following reactions would be endergonic?

A) HCl > H+ + Cl- B) C6H12O6 + 6 O2 > 6 CO2 + 6 H2O C) ATP > ADP + Pi D) maltose + fructose > sucrose E) all of the above

7. An exergonic (spontaneous) reaction is a chemical reaction that ____________.

A) occurs only when an enzyme is present B) occurs within living cells but not in a test tube C) releases energy when proceeding in the forward direction D) occurs in all living cells E) all of the above

8. When G is positive, the reaction or process can be termed ___________.

A) endergonic B) exergonic C) exothermic D) endothermic E) enthalpic

9. A reaction is designated as exergonic rather than endergonic when ___________.

A) activation energy is necessary B) no kinetic energy is released C) activation energy exceeds net energy release D) the potential energy of the products is less than the potential energy of the reactants E) it absorbs more energy

10. What best characterizes the role of ATP in cellular metabolism?

A) The release of free energy during the hydrolysis of ATP heats the surrounding environment. B) Its free energy is coupled to an endergonic process via the formation of a phosphorylated

intermediate.

C) It is catabolized to carbon dioxide and water. D) The G associated with its hydrolysis is positive. E) The polar phosphate groups assist in the alignment of polar substrates as they enter an

enzyme's active site.

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11. Much of the suitability of ATP as an energy intermediary is related to the instability of the bonds between the phosphate groups. These bonds are unstable because _____.

A) the valence electrons in the phosphorus atom have less energy on average than those of other atoms

B) the negatively charged phosphate groups vigorously repel one another C) they are hydrogen bonds, which are only about 10% as strong as covalent bonds D) the phosphate groups are polar and are attracted to the water in the cell's interior E) all of the above

12. Which monomer could be most easily modified to form ATP?

A) the RNA nucleotide adenosine B) the amino acid tryptophan C) the DNA nucleotide adenosine D) cholesterol E) the monosaccharide galactose

13. ___________ is an example of the cellular work accomplished with the free energy of ATP hydrolysis.

A) Mechanical work, such as the beating of cilia, B) Transport work, such as the movement of glucose into an adipose cell, C) Chemical work, such as the synthesis of new protein, D) Mechanical work, such as pumping blood through the circulatory system, E) all of the above

14. In general, the hydrolysis of ATP drives cellular work by _____.

A) changing to ADP and phosphate B) transferring a phosphate group to some other molecule C) releasing heat D) acting as a catalyst E) lowering the free energy of the reaction

15. The mechanism of enzyme action is _____.

A) to create an energy barrier between substrates B) to lower the energy of the activation of a reaction C) to change the direction of thermodynamic equilibrium D) to change endergonic into exergonic reactions E) to allow substrates to move more freely in solution

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16. Which of the following statements about enzymes is true?

A) Enzymes raise the activation energy for reactions. B) Enzymes react with substrates (form chemical bonds) to form an enzyme-substrate complex,

which irreversibly alters the enzyme.

C) Most enzymes are chains of amino acids. D) Only the most efficient enzymes can catalyze reactions in either direction. E) The more a reaction is heated, the faster the enzymes will function.

17. What is meant by the induced fit of an enzyme?

A) The substrate can be altered so it is induced to fit into the enzyme's active site. B) The enzyme is altered so it is induced to fit many different types of substrate. C) Several sites on an enzyme can be induced to act on a substrate. D) The enzyme changes its shape slightly as it binds to the substrate. E) All of the above.

18. Three or four of the following statements concerning enzymes are true and correct. Which one, if any, is NOT correct? If all the statements are correct, choose "All of the above."

A) Most enzymes are proteins. B) An enzyme is not consumed by the catalytic process. C) An enzyme is very specific in terms of which substrates it can bind to. D) An enzyme lowers the activation energy of a chemical reaction. E) All of the above.

19. The active site of an enzyme _____.

A) has a rigid shape, which does not change B) is where the coenzyme will never be found C) resembles a groove or crevice into which the substrate fits D) is the place where both apoenzymes join E) fits the substrate as a lock and key

20. Enzymes can __________, but they cannot __________ because they cannot __________.

A) greatly speed up reactions ... change the net energy output ... change the activation energy B) change the equilibrium point of reactions ... speed up reactions ... change the net energy

output

C) greatly speed up reactions ... change the activation energy ... change the net energy output D) lower the activation energy of reactions ... change the equilibrium point ... change the net

energy output

E) none of the above

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21. A plot of reaction rate (velocity) against temperature for an enzyme indicates little activity at 0C and 45C, with peak activity at 35C. The most reasonable explanation for the low

velocity at 0C is that _____.

A) the hydrogen bonds that define the enzyme's active site are unstable B) the substrate becomes an allosteric regulator C) the enzyme was denatured D) the co-factors required by the enzyme system lack the thermal energy required to activate the

enzyme

E) there is too little activation energy available

22. Succinylcholine is structurally almost identical to acetylcholine, but if combined with the enzyme that normally hydrolyzes acetylcholine; the enzyme is no longer able to hydrolyze

acetylcholine. This suggests that _____.

A) succinylcholine must be a competitive inhibitor with acetylcholine B) succinylcholine must be an allosteric regulator for this enzyme C) the active site must have the wrong configuration to permit succinylcholine binding D) succinylcholine must be a noncompetitive inhibitor E) the activation energy barrier for succinylcholine hydrolysis is higher than for acetylcholine

hydrolysis

23. Which of the following environments or actions does NOT affect enzyme activity?

A) heating the enzyme B) cooling the enzyme C) salt concentration D) pH E) All of the above can affect enzyme activity.

24. Which of these statements about enzyme inhibitors is true?

A) A competitive inhibitor binds to the enzyme outside the active site. B) The action of competitive inhibitors may be reversible or irreversible. C) A noncompetitive inhibitor does not change the shape of the active site. D) When the product of an enzyme or an enzyme sequence acts as its inhibitor, this is known as

positive feedback.

E) Antibiotics and pesticides generally do not act on enzymes, but rather affect the genetic code of their victims.

25. pH can affect enzyme activity because _____.

A) most substrates don't function well at high or low pH B) high or low pH may disrupt hydrogen bonding and change the shape of the active site C) high or low pH may cause the active site to lose its energy D) excess hydrogen ions can combine with the substrate and cause the reaction to go more

slowly

E) hydrogen ions absorb energy and thus there may not be enough energy to get the reaction started.

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26. A competitive inhibitor competes with the __________ at the __________ of an enzyme.

A) product ... active site B) product ... allosteric site C) substrate ... active site D) substrate ... allosteric site E) substrate ... active site and allosteric site

27. The process of stabilizing the quaternary structure of an enzyme in its active form by the binding of a molecule is an example of _____.

A) feedback inhibition B) competitive inhibition C) allosteric regulation D) the participation of a co-factor E) cooperativity

28. Which, if any, of the following statements is NOT true about allosteric proteins?

A) They are sensitive to environmental conditions. B) They are acted on by inhibitors. C) They exist in active and inactive conformations. D) They have more than one subunit. E) All of the above statements are true of allosteric proteins.

29. In an enzyme molecule with multiple subunits, the binding of one substrate molecule to the active site of one subunit causes all the subunits to assume their active conformation, via the

mechanism of induced fit. This is called ____________.

A) allosteric activation B) allosteric inhibition C) competitive inhibition D) cooperativity E) feedback inhibition

30. Cellular respiration is an example of _____.

A) an anabolic pathway

B) a catabolic pathway

C) bioenergetics

D) thermodynamics

E) entropy

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31. Which of these reactions is endergonic?

A) the cooling that results from water mixing with urea

B) the heating that results when water mixes with sodium hydroxide

C) the burning of wood

D) the generation of light by fireflies

E) none of the above

32. The molecule most directly involved in energy transfer within cells is ____________.

A) ATP

B) O2

C) CO2

D) NH3

E) CH4

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

A) are proteins

B) provide activation energy for the reactions they facilitate

C) change the rate of a reaction without being consumed by the reaction

D) can make an endergonic reaction exergonic

E) elevate the activation energy barrier so the molecules will not spontaneously degrade

34. What stabilizes an allosteric enzyme in the active conformation?

A) an activator

B) a competitive inhibitor

C) a noncompetitive inhibitor

D) an end product

E) a substrate

35. In your body, what process converts the chemical energy found in glucose into the chemical

energy found in ATP?

A) potentiation

B) cellular respiration

C) digestion

D) anabolism

E) redox

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

A) anabolism

B) hydrolysis

C) dehydration decomposition

D) dehydration synthesis

E) entropic

37. In this reaction _____.

A) the products have less potential energy than the reactants

B) CD is a product

C) entropy has decreased

D) the products have been rearranged to form reactants

E) AC is a reactant

38. In this reaction _____.

A) the chemical energy of the products is greater than that of the reactants

B) the kinetic energy of the reactants is less than that of the products

C) entropy has decreased

D) disorder has decreased

E) heat has been released to the environment

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Question 1

The following graph shows the effect of pH on the activity of a digestive enzyme.

(a) (i) Name one organ in the human digestive system in which the enzyme

would work effectively.

_________________________________________________________

(1 mark)

(ii) Explain how changes in pH can affect the activity of the enzyme.

_________________________________________________________

_________________________________________________________

_________________________________________________________

(2 marks)

(iii) State two variables that would need to be kept constant in the experiment

above.

_________________________________________________________

(1 mark)

Proteins are digested in the following manner:

Protein polypeptides amino acids

(b) Why is more than one enzyme needed for a protein to be completely digested?

_______________________________________________________________

_______________________________________________________________

(1 mark)

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(c) Name the source of energy required to start an enzyme-controlled reaction.

_______________________________________________________________

(1 mark)

(d) (i)`Describe the induced-fit model of enzyme action.

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

(2 marks)

Another model that describes the action of enzyme on substrates is the lock and key model.

(ii) Draw a labeled diagram showing the lock and key model of an enzyme acting on its

substrates.

(2 marks)

(e) State one reason why enzyme-substrate binding lowers the initial input of energy required to

start a reaction.

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

(1 mark)

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(f) Explain how a rise in temperature can cause an enzyme-substrate binding reaction to stop.

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

(2 marks)

(g) Explain why enzyme-catalyzed reactions reach a constant rate when substrate concentration is

increased beyond a certain point.

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

(2 marks)

Total 15 marks

Question 2

The diagram below illustrates the metabolism of phenylalanine.

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(a) (i) Name an enzyme in the metabolic pathway.

_________________________________________________________

(1 mark)

(ii) Describe the role enzymes play in a metabolic pathway.

_________________________________________________________

_________________________________________________________

(2 marks)

(iii) List three final products of the metabolism of phenylalanine.

_________________________________________________________

_________________________________________________________

(3 marks)

(b) If the enzyme homogentisic acid oxidase is faulty, the disease alkaptonuria occurs.

(i) What kind of molecule is homogentisic acid oxidase?

______________________________________________________

(1 mark)

(ii) What is the most likely cause of the faulty enzyme?

______________________________________________________

(1 mark)

(iii) Name the chemical that would be present in excess.

______________________________________________________

(1 mark)

(iv) Name the chemical that would be absent.

______________________________________________________

(1 mark)

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(c) An experiment was performed to establish the optimum pH for the enzyme homogentisic acid oxidase. In such an experiment, identify

(i) The experimental variable

______________________________________________________

(1 mark)

(ii) Name two variables that would have to be kept constant.

______________________________________________________

(2 marks)

(iii) Label the below axes and draw a graph of the expected results.

(3 marks)

(Total 16 marks)