Chapter 6 Practice Problems

Question 1

Both water and glucose share an -OH that can serve as a substrate for a reaction with the terminal phosphate of ATP catalyzed by hexokinase. Glucose, however, is about a million times more reactive as a substrate than water.
The best explanation is that:

A.) The larger glucose binds to the enzyme; it induces a conformational change in hexokinase that brings active-site amino acids into position for catalysis
B.) The -OH group of water is attached to an inhibitory H atom, while the glucose -OH group is attached to a C atom
C.) Water normally will not reach the active site because it is hydrophobic
D.) Glucose has more -OH groups per molecule than does water
E.) Water and the second substrate, ATP, compete for the active site resulting in a competitive inhibition o the enzyme

Answer 1

A.) The larger glucose binds to the enzyme; it induces a conformational change in hexokinase that brings active-site amino acids into position for catalysis

Question 2

Which one of the following statements is TRUE of enzyme catalysts?

A.) They can increase the equilibrium constant for a given reaction by a thousand-fold or more
B.) To be effective, they must be present at the same concentration as their substrate
C.) They can increase the reaction rate for a given reaction by a thousand-fold or more
D.) Their catalytic activity is independent of pH
E.) They are generally equally active on D and L isomers of a given substrate

Answer 2

C.) They can increase the reaction rate for a given reaction by a thousand-fold or more

Question 3

Enzyme X exhibits maximum activity at pH=6.9. X shows a fairly sharp decrease in its activity when the pH goes much lower than 6.4. One likely interpretation of this pH activity is that:

A.) a Glu residue on the enzyme is involved in the reaction
B.) a His residue on the enzyme is involved in the reaction
C.) the enzyme has a metallic cofactor
D.) the enzyme relies on specific acid-base catalysis
E.) the enzyme is found in gastric secretions

Answer 3

B.) a His residue on the enzyme is involved in the reaction

Question 4

The steady-state assumption, as applied to enzyme kinetics, implies;

A.) The Km is equivalent to the cellular substrate concentration
B.) The enzyme is regulated
C.) The maximum velocity occurs when the enzyme is saturated
D.) The ES complex is formed and broken down at equivalent rates
E.) Km=Ks

Answer 4

D.) The ES complex is formed and broken down at equivalent rates

Question 5

Which of the following is TRUE of the binding energy derived from enzyme-substrate interactions?

A.) It is sometimes used to hold two substrates in the optimal orientation for reaction
B.) Most of it derived from covalent bonds between enzyme and substrate
C.) It cannot provide enough energy to explain the large rate accelerations brought about by enzymes
D.) Most of it is used up simply binding the substrate to the enzyme
E.) It is the result of covalent bonds formed between enzyme and substrate

Answer 5

A.) It is sometimes used to hold two substrates in the optimal orientation for reaction

Question 6

Enzymes are potent catalysts because they:

A.) Drive reactions to completion while other catalysts drive reactions to equilibrium
B.) Are very specific and can prevent the conversion of products back to substrates
C.) Lower the activation energy for the reaction they catalyze
D.) Are consumed in the reactions they catalyze
E.) Increase the equilibrium constants for the reactions they catalyze

Question 7

One of the enzymes involved in glycolysis, aldolase, requires Zn2+ for catalysis. Under conditions of zinc deficiency, when the enzyme may lack zinc, it would be referred to as the:

A.) coenzyme
B.) prosthetic group
C.) apoenzyme
D.) holoenzyme
E.) substrate

Answer 7

C.) apoenzyme

Question 8

An enzyme-catalyzed reaction was carried out with the substrate concentration initially a thousand times greater than the Km for that substrate. After 9 minutes, 1% of the substrate has been converted to product, and the amount of product formed in the reaction mixture was 12 mmol. If, in a separate experiment, one-third as much enzyme and twice as much substrate had been combined, how long would it take for the same amount (12 mmol) of product to be formed?

A.) 27 min
B.) 3 in
C.) 1.5 min
D.) 6 min
E.) 13.5 min

Answer 8

A.) 27 min

Question 9

The concept of “induced fit” refers to the fact that:

A.) Enzyme specificity is induced by enzyme-substrate binding
B.) Substrate binding may induce a conformational change in the enzyme, which then brings catalytic groups into proper orientation
C.) enzyme-substrate binding induces movement along the reaction coordinate to the transition state
D.) Enzyme-substrate binding induces an increase in the reaction entropy, thereby catalyzing the reaction
E.) When a substrate binds to an enzyme, the enzyme induces a loss of water (desolvation) from the substrate

Answer 9

B.) Substrate binding may induce a conformational change in the enzyme, which then brings catalytic groups into proper orientation