Which of the following statements is false with respect to an enzyme's ability to catalyze a reaction?
a) an enzyme provides a reaction surface and a suitable environment for the reaction to take place.
b)an enzyme bind reactant such that they are positioned correctly and can attain their transition-state configurations.
c) an enzyme allows the reaction to go through a less stable transition state than would normally be the case.
d) an enzyme can weaken bonds in the reactants through the binding process.
answer
C
Why? An enzyme allows the reaction to go through a MORE STABLE TRANSITION STATE than would normally be the case. As a result the rate of reaction is increased.
question
Which of the following statements is not true regarding the active site of an enzyme?
a) an active site is normally a hollow or cleft on the surface of an enzyme
b) an active site is normally hydrophilic in nature
c) substrates fit into active sites and bind to functional groups within the active site
d) an active site contains amino acids which are important to the binding process and the catalytic mechanism.
answer
B
Why? An active site is normally HYDROPHOBIC in nature, not hydrophilic. Nevertheless, there are usually hydrophilic amino acids present which are important in binding the substrate in the active site.
question
Which of the following statements best describes an allosteric binding site?
a) It is a binding site containing amino acids with aliphatic side chains
b) It is a binding site that can accept a wide variety of differently shaped molecules
c) It is a binding site, which is separate from the active site, and affects the activity of an enzyme when it is occupied by a ligand
d) It is a description of an active site which has undergone an induced fit
answer
C
Why? An allosteric binding site is separate from the active site. When it is occupied by a ligand, it undergoes an induced fit which affects the shape of the active site. This will affect the ability of the active site to 'recognize' the substrate and hence affect enzyme activity. Both the active site and allosteric binding site are likely to contain amino acids with aliphatic side chains, some of which may be important to the binding process. The ability of a binding site to accept a wide variety of differently shaped molecules will vary depending on the specific binding site involved.
question
Which of the following descriptions best describes an induced fit?
a) The process by which an active site alter shape such that it is ready to accept a substrate
b) The process by which a substrate adopts the correct binding conformation before entering an active site.
c) The process by which a substrate binds to an active site and alters the shape of the active site.
d) The process by which an active site alters the shape of the substrate such that it can adopt the necessary active conformation for binding.
answer
C
Why? An induced fit is a consequence of the binding interactions taking place between a substrate and the active site. The active site cannot undergo the induced fit before these interactions occur and so option a) is wrong. The substrate has to be in the correct conformation for binding to occur but this is not an induced fit, so option b) is wrong. Option d) is wrong since the active site has no control over whether the substrate will be in the active conformation when it enters the active site.
question
Which of the following statements is INCORRECT with respect to an enzyme's ability to catalyze a reaction?
a) An enzyme provides a reaction surface and a hydrophilic environment for the reaction to take place.
b) An enzyme bind reactants such that they are positioned correctly and can attain their transition-state configurations.
c) An enzyme allows the reaction to go through a more stable transition state than would normally be the case.
d) An enzyme can weaken bonds in reactant through the binding process.
answer
A
Why? An enzyme certainly provides a reaction surface for a reaction to take place. This is normally a hollow or cleft in the enzyme which is called the active site, but it is normally hydrophobic in nature rather than hydrophilic.
question
Some enzymes require the presence of a non-protein substance if they are to catalyze a reaction. Which of the following terms is the best general term for such a substance?
a) prosthetic group
b) cofactor
c) co-enzyme
d) modulator
answer
B
Why? A cofactor is the best general term to describe the non-protein substances required by an enzyme to function properly. This term covers both organic molecules and metal ions.
question
What is a co-enzyme?
answer
an organic molecule that acts as a cofactor
question
What is a prothetic group?
answer
a co-enzyme that is covalently bound to the enzyme
question
What is a modulator?
answer
a modulator has nothing to do with cofactors and is a ligand that binds to an allosteric binding site.
question
What term is used for a non-protein organic molecule that is required by some enzymes in order to catalyze a reaction on a substrate?
a)prosthetic group
b)cofactor
c)co-enzyme
d)modulator
answer
C
Why? A co-enzyme is an organic molecule that acts as a cofactor.
question
Some enzymes covalently bind a non-protein organic molecule to the active site. The organic molecule concerned is required if the enzyme is to catalyze a reaction on a substrate. What is the term used for such a molecule?
answer
Prosthetic group
A prosthetic group is a co-enzyme that is covalently bound to the enzyme.
question
Which of the following molecules is involved in the feedback control of the enzyme phosphorylase a?
a) glucose-1-phosphate
b) adrenaline
c) glycogen
d) AMP
answer
A
Why? The products of enzyme-catalyzed reactions are often involved in the feedback control of those enzymes. Glucose-1-phosphate is the product formed from this enzyme-catalyzed reaction. As its concentration increases, it increasingly inhibits the enzyme. Glycogen is the substrate for this reaction. Adrenaline is a hormone that can trigger a cascade of cellular events that will eventually activate phosphorylase a. AMP acts as an enzyme modulator to directly activate the enzyme.
question
What secondary messenger is generated as a result of the action of nitrous oxide?
a) GTP
b) cyclic GMP
c) ATP
d) cyclic AMP
answer
B
Why? Nitrous oxide activates cyclase enzymes to generate the secondary messenger cyclic GMP from GTP. Cyclic AMP is a secondary messenger generated from ATP by a different enzyme that is activated by G-proteins.
question
From which amino acid is nitrous oxide generated?
a) arginine
b) aspartic acid
c) asparagine
d) lysine
answer
A
Why? Arginine contains a guanidine functional group that provides the nitrous oxide. The enzyme NO synthase converts arginine to citrulline with release of nitrous oxide.
question
The Michaelis-Menton equation relates the rate of an enzyme-catalyzed reaction to which of the following?
a) substrate concentration
b) product concentration
c) activation energy
d) inhibitor concentration
answer
A
Why? The Michaelis-Menton equation relates the rate of an enzyme-catalyzed reaction to the substrate concentration where the conversion to product is irreversible.
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