Practice Questions - Week 9 - Enzymes Flashcards
Which of the following about enzymes is true?
a) The E-S complex often dissociates with no reaction taking place.
b) The E-S complex must form before a reaction can take place.
c) Once the E-S complex forms, it can go on to form product or dissociate to E + S.
d) All of the above is are correct.
d)
The amino acids in the active site can be involved in all of these processes, EXCEPT WHICH OF THE FOLLOWING?
a) Binding of the substrate.
b) Becoming part of the product of the reaction.
c) The actual chemical mechanism for the reaction.
d) Binding of some necessary cofactor.
e) All of these can be functions of the amino acids in the active site.
b)
Enzymatic activity has an optimum temperature because
a) the component amino acids have varying melting points
b) the rate of reactions is thermodynamically controlled
c) the side chains of essential residues are chemically degraded at higher temperatures
d) raising the temperature speeds up the reaction, that is, until protein denaturation sets in.
d)
T/F: All catalysts work by lowering the activation energy for a reaction.
True
The active site of an enzyme
a) is frequently located in a cleft in the enzyme
b) is the portion of the enzyme to which the substrate binds
c) contains the reactive groups that catalyse the reaction
d) all of the options are correct
d)
Which of the statement is CORRECT about the induced-fit model of substrate binding to enzymes.
a) The substrate changes its conformation to fit the active site.
b) The active site changes its conformation to fit the substrate.
c) There is a conformational change in the enzyme when the substrate binds.
d) There is aggregation of several enzyme molecules when the substrate binds.
c)
T/F: The ES complex often shows a slight depression in the energy profile for the reaction
True
Which of the following inhibitors binds to the enzyme at a site other than the active site
a) competitive inhibitor
b) noncompetitive inhibitor
c) irreversible inhibitor
d) all of these
b)
Irreversible inhibitors of enzymatic reactions
a) bind to the enzyme only at low temperatures
b) affect only serine side chains
c) react with the enzyme to produce a protein that is not enzymatically active and from which the original enzyme cannot be regenerated
d) are bound to the enzyme by lock and key.
c)
Where do allosteric inhibitors bind on an enzyme?
a) They always bind at a site different from the active site.
b) They always bind at the active site.
c) They can bind anywhere on the enzyme.
d) They bind to the substrate so that an improved fit to the enzyme active site is achieved
a)
Zymogens are
a) inactive precursors of enzymes which can be activated by irreversible cleavage of covalent bonds.
b) inactive forms of enzymes which require phosphorylation.
c) the aggregation of several enzyme molecules when the substrate binds.
d) conformational changes that take place in enzyme synthesis.
a)
An enzyme that catalyzes conversions of L-sugars to D-sugars is called a/an ________.
a) hydrolase
b) lyase
c) synthase
d) synthetase
e) isomerase
e)
An enzyme that catalyzes conversions of L-sugars to D-sugars is called an isomerase.
Isomerases are a general class of enzymes that convert a molecule from one isomer to another.
Isomerases facilitate intramolecular rearrangements in which bonds are broken and formed.
The general form of such a reaction is as follows:
A–B → B–A
Enzymes that catalyse kinase reactions do which of the following?
a) Add phosphate groups to another biomolecule.
b) Oxidise alcohols to aldehydes
c) Use NAD+/NADH in their reactions.
d) Transfer functional groups from one part of a biomolecule to another.
a)
What distinguishes reversible inhibitors from irreversible inhibitors?
a) Reversible inhibitors are not covalently bound to enzymes but irreversible inhibitors are.
b) There is an equilibrium between bound and unbound reversible inhibitor. There usually is little back reaction for the binding of an irreversible inhibitor.
c) Reversible inhibitors are easier to purify from solutions of enzymes than irreversible inhibitors.
d) All of the above options are correct.
d)
All the statements highlight differences between irreversible and reversible inhibitors.
Reversible inhibitors are not covalently bound to enzymes but irreversible inhibitors are.
There is an equilibrium between bound and unbound reversible inhibitor. There usually is little back reaction for the binding of an irreversible inhibitor.
Reversible inhibitors are easier to purify from solutions of enzymes than irreversible inhibitors.
Match the correct class of enzyme for each of these reactions.
- L-aspartic acid + α-ketoglutarate → oxaloacetic acid + L-Glutamate
- Lactate + NAD+ → Pyruvate + NADH + H+
- Pyruvate + H+ → Acetaldehyde + CO2
a)
1. Transferase
2. Oxidoreductase
3. Lyase
b)
1. Hydrolase
2. Oxidoreductase
3. Ligase
c)
1. Transferase
2. Hydrolase
3. Isomerase
d)
1. Hydrolase
2. Oxidoreductase
3. Transferase
e)
1. Isomerase
2.Transferase
3. Lyase
a)
OK this one is a bit more challenging.
- L-aspartic acid + α-ketoglutarate → oxaloacetic acid + L-Glutamate = Transferase
- Lactate + NAD+ → Pyruvate + NADH + H+ = Oxidoreductase
- Pyruvate + H+ → Acetaldehyde + CO2 = Lyase
