Question 1

An inhibitor that must be metabolized by the enzyme before it becomes covalently attached to the enzyme is called a mechanism-based inhibitor. What is another term that has been used for such an inhibitor?

Accepted Answer

A)  Noncompetitive 
B)  Suicide 
C)  Terminal 
D)  Transition State 
E)  Turnover

Question 2

The movement of a tyrosyl residue in carboxypeptidase A when its substrate binds is an example of

Accepted Answer

A)  substrate strain. 
B)  induced fit. 
C)  acid-base catalysis. 
D)  covalent catalysis. 
E)  stabilization of the transition state.

Question 3

Which of the following is NOT an assumption in the Michaelis Menten model for enzyme kinetic behavior?

Accepted Answer

A)  A complex is formed between enzyme and substrate. 
B)  All substrates for the same enzyme will be used at the same rate. 
C)  The concentration of substrate is much larger than that of the enzyme . 
D)  Only initial velocities are measured 
E)  A steady state concentration of enzyme-substrate complex is rapidly established and maintained during measurement.

Question 4

Enzymes which are in an inactive form after the removal of a cofactor or prosthetic group are termed:

Accepted Answer

A)  isozymes 
B)  holoenzymes 
C)  zymogens 
D)  apoenzymes 
E)  proenzymes

Question 5

In a reaction catalyzed by a dehydrogenase, it was observed that NAD+ was required for binding of the second substrate. The reduced form of NAD was found to be a competitive inhibitor against NAD+. Which of the mechanisms listed below describes the action of this enzyme?

Accepted Answer

A)  Random 
B)  Ordered sequential 
C)  Ping Pong 
D)  Concerted 
E)  None of the above

Question 6

The degree of inhibition caused by a competitive inhibitor can be decreased by

Accepted Answer

A)  adding water, thus diluting out the EI complex. 
B)  increasing the enzyme concentration. 
C)  increasing the substrate concentration. 
D)  increasing both the enzyme and inhibitor concentration. 
E)  none of the above.

Question 7

When a reaction reaches equilibrium,

Accepted Answer

A)  delta Go is equal to zero. 
B)  delta G is equal to zero. 
C)  no substrate is converted to product. 
D)  no product can be converted back to substrate. 
E)  the reaction becomes temperature independent.

Question 8

Activation of zymogens is often accomplished by

Accepted Answer

A)  aggregation of monomers to form the active oligomer. 
B)  post-synthetic modification. 
C)  formation of covalent crosslinks. 
D)  hydrolysis of specific peptide bonds. 
E)  addition of a cofactor.

Question 9

All reactions are technically reversible. Why are some considered de facto irreversible?

Accepted Answer

A)  At equilibrium substrate concentration is below the level of detection 
B)  Equilibrium can never be achieved 
C)  Keq for the reaction is so low that it cannot be calculated 
D)  Product inhibits the reverse reaction 
E)  The Keq is 1

Question 10

A competitive inhibitor

Accepted Answer

A)  lowers the maximum velocity of the reaction. 
B)  raises the maximum velocity of the reaction. 
C)  combines with allosteric sites on the enzyme. 
D)  raises the apparent Km of the enzyme for its substrate. 
E)  covalently modifies the enzyme.

Question 11

Serum isoenzyme measurements are useful in diagnosis because

Accepted Answer

A)  the composition of the serum changes specifically with specific diseases. 
B)  different tissues may contain different isoenzymes and any damage to a particular tissue releases only its characteristic isoenzymes into serum. 
C)  different tissues may have the same isoenzyme composition and a given disease specifically releases only certain disease-specific isoenzymes into serum. 
D)  the disease process causes the damaged tissue to synthesize and release specific isoenzymes as a defense mechanism specific for a given disease. 
E)  isoenzyme measurements are expensive and are a good way for hospitals to increase their profits.

Question 12

For an enzyme reaction that requires a coenzyme

Accepted Answer

A)  the coenzyme often reacts with the substrate. 
B)  the enzyme must be multimeric. 
C)  the coenzyme increases the number of active sites on the enzyme. 
D)  the coenzyme is essentially irreversibly bound to the enzyme.

Question 13

A Ramachandran plot is useful in

Accepted Answer

A)  understanding spatial relationships in secondary protein structure. 
B)  making predictions of tertiary and quaternary protein structure. 
C)  identifying which residues are likely to form disulfide bonds. 
D)  determining the best strategy to purify a protein from a heterogeneous mixture. 
E)  comparing conserved amino acids in related primary protein structures.

Question 14

All of the following effects may occur in enzyme catalysis EXCEPT which one?

Accepted Answer

A)  Stabilization of transition states occurring between substrates and products. 
B)  General acid and base catalysis. 
C)  Lowering of the free energy of products, making a forward reaction thermodynamically more favorable. 
D)  Nucleophilic covalent catalysis, creating intermediates not possible in an uncatalyzed reaction. 
E)  Local concentration effects the orienting of substrate in configurations favorable to catalysis by reactive groups on an enzyme.

Question 15

The fact that an enzyme-catalyzed reaction is first order with respect to substrate at low substrate concentration but becomes zero order with respect to substrate at high substrate concentration is explained by

Accepted Answer

A)  the catalytic constant. 
B)  reversibility of formation of the enzyme-substrate complex. 
C)  three point attachment. 
D)  specificity. 
E)  saturation.

Question 16

Each of the following statements about the Michaelis-Menten equation is correct EXCEPT which one?

Accepted Answer

A)  The equation describes a hyperbolic curve. 
B)  The Michaelis constant of an enzyme-substrate pair would be expected to change with a change in pH of the assay mixture. 
C)  For a group-specific enzyme, the substrate with the largest Km is the one which binds most strongly to the active site. 
D)  The Michaelis constant is expressed in units of molarity. 
E)  The Km is equal to the substrate concentration yielding a velocity one-half of the Vmax.

Question 17

The turnover number of an enzyme catalyzed reaction is:

Accepted Answer

A)  used as a measure of enzyme purity. 
B)  related to the equilibrium constant of the reaction. 
C)  changes when the concentration of the enzyme is altered. 
D)  is a measure of the efficiency of the enzyme in catalyzing the reaction.

Question 18

The enzyme hexokinase, which can utilize ATP and glucose to form glucose-6-phosphate and ADP, would be classified as a(an):

Accepted Answer

A)  Transferase 
B)  Hydrolase 
C)  Ligase 
D)  Lyase 
E)  Oxido-reductase

Question 19

In the spectrophotometric assay for lactate dehydrogenase (LDH) activity which of the following reaction components is monitored at a wavelength of nanometers?

Accepted Answer

A)  lactate 
B)  pyruvate 
C)  NADH 
D)  lactate dehydrogenase

Question 20

Select the statement which most correctly describes biotin.

Accepted Answer

A)  It is a coenzyme for certain carboxylation reactions. 
B)  It is a cofactor for kinases. 
C)  It is involved in methyl group transfer. 
D)  It is a cofactor for the malate shuttle. 
E)  It is involved in phosphoryl transfer.

Exam 10: Enzymes: Classification, Kinetics, and Control