Deck 6: How Enzymes Work

Question 1

Different enzymes that catalyze the same reaction are known as _____.

A) transferases
B) isomerases
C) allosteric enzymes
D) holoenzymes
E) isozymes

Answer 1

isozymes

Question 2

The ability for an enzyme to pick out one particular substrate from the myriad of molecules floating around its environment is an example of _____.

A) natural selection
B) specificity
C) high affinity
D) processivity
E) none of the above

Answer 2

specificity

Question 3

How does a catalyst affect the overall $\Delta$ G of an endergonic reaction?

A) it has no effect
B) the reaction becomes more endergonic
C) the reaction has a $\Delta$ G of zero
D) the reaction becomes exergonic
E) none of the above

Answer 3

it has no effect

Question 4

How does a catalyst increase the rate of a reaction?

A) it makes the reaction more exergonic
B) it increases the temperature of the reaction
C) it allows reacting molecules to more easily form the transition state
D) it causes a localized increase in the concentration of reactants
E) none of the above

Question 5

An organic molecule that is tightly bound to an enzyme and participates in an enzyme catalyzed reaction is specifically referred to as a _____.

A) cofactor
B) metal ion
C) coenzyme
D) cosubstrate
E) prosthetic group

Question 6

Which of the following is seen in a reaction coordinate diagram for an enzyme-catalyzed reaction that uses covalent catalysis?

A) an intermediate
B) two distinct transition states
C) reactants that are lower in energy than an intermediate
D) products that are lower in energy than an intermediate
E) all of the above

Question 7

In the enzyme catalyzed decarboxylation of acetoacetate, a Schiff base is formed between the ketone of acetoacetate and a _____ residue in the active site of the enzyme.

A) Arg
B) Asn
C) Lys
D) Glu
E) Ser

Question 8

Of the following ions, which would be most likely to participate in metal ion catalysis?

A) Na⁺
B) Zn²⁺
C) Ag⁺
D) K⁺
E) Ba²⁺

Question 9

In an enzyme mechanism that generates a negative charge in the transition state, which of the following would be most effective to have in the active site of the enzyme?

A) transition metal cation
B) Asp residue
C) Gln residue
D) transition metal anion
E) none of the above

Question 10

During the first half of the chymotrypsin mechanism where the acyl-enzyme intermediate is formed, what role does His play?

A) general acid only
B) general base only
C) general acid then general base
D) general base then general acid
E) nucleophile

Question 11

Of all the species that enzymes bind, they are thought to bind most tightly to _____.

A) substrates
B) products
C) transition states
D) intermediates
E) all are bound very tightly

Question 12

In the chymotrypsin mechanism, what is used to stabilize the negative charge on the carbonyl oxygen of the transition state?

A) hydrogen bonding between the enzyme and the anion from the carbonyl oxygen
B) electrostatic interaction of the positively charged His and carbonyl oxygen
C) electrostatic interaction of the negatively charged Asp and carbonyl oxygen
D) electrostatic interaction of the active site Zn²⁺ and carbonyl oxygen
E) all of the above

Question 13

If an enzyme rate increases 10-fold for every 5.7 kJ/mol decrease in the energy of activation, what rate increase would be observed if three H-bonds were stabilizing the transition state with each H-bond having a strength of 21 kJ/mol?

A) 11-fold increase
B) 37-fold increase
C) 60-fold increase
D) 100-fold increase
E) 10¹¹-fold increase

Question 14

When the transition state is formed, what is the specific interaction observed between Asp and His that helps stabilize the transition state?

A) dipole-dipole interaction
B) electrostatic interaction
C) hydrogen bond
D) low-barrier hydrogen bond
E) covalent bond

Question 15

The enzyme hexokinase catalyzes a reaction where its substrates are completely sequestered from the aqueous environment allowing for interactions to occur between enzyme and substrates that would not normally occur in an aqueous environment. What term describes this type of catalysis?

A) electrostatic catalysis
B) desolvation catalysis
C) non-aqueous catalysis
D) hydrogen-bonded catalysis
E) closed-system catalysis

Question 16

What amino acid residue present in the specificity pocket allows trypsin to bind to peptides containing Arg or Lys?

A) Ser
B) His
C) Lys
D) Val
E) Asp

Question 17

Chymotrypsin has a large specificity pocket to bind the aromatic amino acids while elastase has a very small specificity pocket meant to bind Ala or Gly. Which of the following amino acid substitutions changes the specificity pocket of chymotrypsin to that of elastase?

A) Trp $\rarr$ Ser
B) Leu $\rarr$ Ala
C) Gly $\rarr$ Val
D) Lys $\rarr$ Asp
E) Val $\rarr$ Asn

Question 18

Which protein in the blood is responsible for converting fibrinogen to fibrin?

A) thrombin
B) factor X
C) factor VII
D) factor VIIa
E) prothrombin

Question 19

Conversion of factor X to factor Xa represents a(n) _____ form of activation; binding of antithrombin to thrombin represents a(n) _____ form of inhibition.

A) reversible; reversible
B) irreversible; reversible
C) reversible; irreversible
D) irreversible; irreversible
E) none of the above since antithrombin is not an inhibitor of thrombin

Question 20

Which of the following is true regarding heparin?

A) it is an allosteric activator of antithrombin
B) it is a sulfated polysaccharide
C) it simultaneously binds to antithrombin and the target of antithrombin
D) it is used clinically as an anticoagulant
E) all of the above

Question 21

Zymogens are not enzymatically active because _____.

A) they do not contain the cofactors required for catalysis
B) they are the product of mutated genes
C) their active sites are distorted and incapable of enzymatic activity
D) the pH of their environment is not optimal for activity
E) none of the above

Question 22

Which of the following is a feature of protease inhibitor specific for trypsin?

A) contains a positive charge to mimic the charge of the substrate
B) ability to interact with the Ser of the catalytic triad
C) inability to form the tetrahedral intermediate
D) high affinity for the enzyme active site
E) all of the above

Question 23

Fibrinogen is converted to _____ by the action of _____.

Question 24

Thrombin is similar to ___ as it cleaves peptide bonds following ___ residues.

Question 25

Thrombin circulates in the blood as a(n) ___ that is called ___.

Question 26

The blood coagulation cascade is triggered when ___ is converted to ___.

Question 27

Name one protease inhibited by antithrombin.

Question 28

Heparin enhances the activity of ___ by acting as ___.

Question 29

How does a catalyst increase the rate of a reaction?

A) It makes the reaction more exergonic.
B) It increases the temperature of the reaction.
C) It allows reacting molecules to more easily form the transition state.
D) It causes a localized increase in the concentration of reactants.
E) None of the above