250+ TOP MCQs on Enzyme Kinetics – Mechanism of Enzyme Action – 1 and Answers

Enzyme Technology Multiple Choice Questions on “Enzyme Kinetics – Mechanism of Enzyme Action – 1”.

1. The study of rates of chemical reactions that are catalyzed by enzymes is referred to as ______________
A. first order reaction kinetics
B. zero order reaction kinetics
C. chemical kinetics
D. enzyme kinetics
Answer: D
Clarification: Enzyme kinetics is referred to as the rates of chemical reactions that are catalyzed by enzymes. Whereas chemical kinetics describes the rates of chemical reactions. Zero order reaction kinetics is one in which the rate of the reaction does not depend on reaction concentration. Whereas first order reaction kinetics is one in which the rate of the reaction depends on molar concentration of one of the reactant.

2. Lock and key model was proposed by ______________ in 1890.
A. Henri
B. Miachelis and Menten
C. Emil Fischer
D. Daniel Koshland
Answer: C
Clarification: Lock and key model was proposed by Emil Fischer in 1890. Whereas Daniel Koshland proposed induced fit model in 1958. Henri in 1903 and Miachelis and Menten in 1913 proposed kinetic models for enzyme catalyzed reaction. The main difference is that Miachelis and Menten did a strong experimental work than Henri to prove that an enzyme catalyzing single substrate reaction will possess a hyperbolic curve.

3. The enzyme action model represented in the following diagram is ______________

A. Fluid Mosaic Model
B. Induced fit model
C. Reflective index model
D. Lock and Key Model
Answer: D
Clarification: This model in the diagram shows how a lock fits in its own key, in a similar way enzyme fits into its substrate. Enzyme-substrate complex is formed and it makes changes in the substrate allowing to form products and releasing the products. It was proposed by Emil Fischer in 1898.

4. The lock and key model deals with flexible nature of enzymes.
A. True
B. False
Answer: B
Clarification: The lock and key model proposes that as key fits into the model the substrate fits into the active site of the enzyme. This model doesn’t consider conformational changes during binding of substrate to its active site which is proved by X-ray diffraction and NMR. Thus the above statement is false. The model doesn’t deal with flexible nature of enzymes.

5. In induced fit model, only a specific substrate can bind and produce desirable reaction.
A. True
B. False
Answer: A
Clarification: Thee above statement is true. Induced fit model proposes conformational changes in the enzyme upon binding with substrate without affecting primary structure in turn forming ES complex leading to form products. In induced fit model, only substrate binding site is freely accessible and not the catalytic site. Hence, only one substrate can bind and produce desirable reaction.

6. The target substrate molecules bind to active site of the enzyme transforming into products through a series of steps known as the ____________
A. enzyme kinetics
B. enzymatic mechanism
C. chemical kinetics
D. zero order reaction kinetics
Answer: B
Clarification: Enzymatic mechanism is referred to as series of steps through which products are formed when substrate molecule binds to active site of the enzyme. The rates of chemical reactions that are catalyzed by enzymes is called enzyme kinetics. Whereas chemical kinetics describes the rates of chemical reactions. Zero order reaction kinetics is one in which the rate of the reaction does not depend on reaction concentration.

7. When the free energy occurs at standard conditions, it is termed as ______________
A. Activation energy
B. pK_a
C. Standard free energy change
D. Gibbs free energy
Answer: C
Clarification: Standard free energy change is referred to as the free energy change occurring at standard conditions, i.e., at 25°C, pH 7, 1 mol/L solute concentration and normal gas. Gibbs free energy is the energy that exist for performing work. Activation energy is the energy required to attain transition state. pK_a is the pH at which half of the ions are ionized.

8. Which of the following is not true for substrate collision theory?
A. Both enzyme and substrate stumble upon each other
B. Both enzyme and substrate provide good molecular orbital interactions
C. Specificity of the substrate binding site
D. Both enzyme and substrate should be flexible in forming transition states
Answer: C
Clarification: Specificity of the substrate binding site comes into picture when dealing with active site. The following is essential for forming transition state by substrate collision theory:
* Stumble upon each other through proper diffusion limited collision.
* Good molecular orbital interactions by enduring the changes in the medium.
* Overwhelm the vander waals repulsive forces that happen commonly between them.
* Flexibility in forming transition states through proper interactions.

9. The disruption of interactions between substrate and solvent is referred to as __________
A. desolvation
B. molecular orbital steering
C. protonation
D. deprotonation
Answer: A
Clarification: The removal and addition of protons [H⁺] is referred to as deprotonation and protonation respectively. Molecular orbital steering explains that the active residues should guide the groups in the substrate for catalysis to occur. Desolvation refers to disruption of interactions between substrate and interaction. This may be achieved by multiple non-covalent interactions between hydrophobic active site group and the substrate.

10. Which of the following is not explained by molecular orbital steering?
A. The juxtaposition of the side chain groups of amino-acid and the reactive groups of the substrate is not sufficient for proper catalysis
B. Overwhelm the vander waals repulsive forces that happen commonly between them
C. The active residues should guide the groups in the substrate for exact interaction by thermal vibration due to elevated temperature
D. Thermal vibration create motion in the active site amino acid side chain that direct reactive groups in the substrate to interact
Answer: C
Clarification: Overwhelm the vander waals repulsive forces that happen commonly between them is not explained by molecular orbital steering. It is a part of substrate collision theory. Molecular orbital steering explains the following:
* The juxtaposition of the side chain groups of amino-acid and the reactive groups of the substrate is not sufficient for proper catalysis.
* The active residues should guide the groups in the substrate for exact interaction by thermal vibration due to elevated temperature.
* Thermal vibration create motion in the active site amino acid side chain that direct reactive groups in the substrate to interact.

11. If the catalysis involves participation of small organic molecules, cofactors, and amino-acid side chains from the enzyme is termed as ____________
A. specific acid base catalysis
B. general acid base catalysis
C. substrate collision theory
D. lock and key model
Answer: B
Clarification: If the catalysis involves participation of small organic molecules, co-factors and amino-acids side chains from the enzyme is referred to as general acid-base catalysis. If the catalysis involves water molecules for proton donation or acceptance, the catalysis is referred to as general acid-base catalysis. Lock and key model is proposed by Emil Fischer in 1958. Substrate collision theory deals with binding between enzyme and substrate molecule.

12. ________________ involves substrates forming transient covalent bond with the residues present in the active site
A. Covalent catalysis
B. Specific acid-base catalysis
C. General acid-base catalysis
D. Lock and key model
Answer: A
Clarification: Catalysis which involves substrates forming transient covalent bond with the residues present in the active site is referred to as covalent catalysis. If the catalysis involves participation of small organic molecules, co-factors and amino-acids side chains from the enzyme is referred to as general acid-base catalysis. If the catalysis involves water molecules for proton donation or acceptance, the catalysis is referred to as general acid-base catalysis. Lock and key model proposes that as a key fits into a lock same way, substrate fits into the enzyme.

13. The covalent catalysis is aided by one of the following method?
A. General acid-base catalysis
B. Specific acid-base catalysis
C. Nucleophilic catalysis
D. Substrate collision theory
Answer: C
Clarification: Covalent catalysis involves substrates forming transient covalent bond with the residues present in the active site. The covalent catalysis is aided by nucleophilic and electrophilic catalysis. In most of the enzymatic reaction protons are involved. General and specific acid-base catalysis are types of acid-base catalysis. Substrate collision theory says about the binding between substrate and enzyme.

14. Which is the first step involved in chymotrypsin mediated peptide bond hydrolysis?
A. Acylation
B. Specific acid-base catalysis
C. General acid-base catalysis
D. Deacylation
Answer: D
Clarification: Chymotrypsin reaction with its substrates takes place in 2 stages, an initial burst phase at the beginning of the reaction and steady state phase following MM kinetics. The mode of action is explained in 2 steps:
* Acylation of substrate to form an acyl-enzyme intermediate.
* Deacylation in order to return the enzyme to its original state.
Hence the first step is acylation.