1. The energy required to attain transition state is referred to as _____________
a) Gibbs free energy
b) Activation energy
c) Standard free energy change
d) pH
Explanation: The key role of any enzyme is to act upon specific substrate and accelerate the reaction. For fastening of any reaction, the amino-acids side chain groups of the active site should properly interact with the substrate molecule and form a transition state. For attaining a transition state, the enzyme needs certain energy which is termed as activation energy (EA).
2. The transition state cannot be achieved by using one of the following methods?
a) Substrate collision theory
b) Lock and key model
c) Acid-base catalysis
d) Covalent catalysis
Explanation: By using lock and key model, the transition state cannot be achieved. Lock and key model proposes the fitting of substrate into the active site of the enzyme like a key fits into its lock. Substrate collision theory, acid-base catalysis and covalent catalysis are used to achieve the transition state required during enzyme catalysis.
3. Which of the following factor has a negative impact on substrate collision theory?
a) Temperature
b) Protonation or Deprotonation
c) Substrate concentration
d) Type of medium
Explanation: Protonation and deprotonation of enzyme or substrate leads to their structural distortion which directs improper orientation to them due to which the interaction may not be accurate which may lead to negative impact. Temperature, substrate concentration and type of medium have positive impact on the substrate collision theory.
4. The type of catalysis wherein the proton donation or acceptance is done by water molecule is referred to as _____________
a) substrate collision theory
b) lock and key model
c) general acid-base catalysis
d) specific acid-base catalysis
Explanation: In most of the enzymatic reactions, some type of proton transfer is involved. If the proton donation or acceptance is done by water molecules during catalysis is called specific acid-base 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. Lock and key deals with the interaction between substrate and enzyme at active site in a way as key fits into its lock. Substrate collision theory deals with collision between substrate and enzyme.
5. In the catalysis of RNA by Bovine pancreatic RNase A, His 12 acts as proton donor.
a) true
b) false
Explanation: In most of the enzymatic reactions, some type of proton transfer is involved. In catalysis of RNA by bovine pancreatic RNase A, His 12 (imidazole) acts as proton acceptor and His 119 acts as a proton donor to attain the transition state. Hence the above statement is false.
6. The phenomenon which involves formation of an additional covalent intermediate in a reaction that helps to reduce the energy of later transition states is referred to as ___________
a) covalent catalysis
b) substrate collision theory
c) general acid-base catalysis
d) specific acid-base catalysis
Explanation: Covalent catalysis involves substrates forming transient covalent bond with the residues present in the active site. The phenomenon which involves formation of an additional covalent intermediate in a reaction that helps to reduce the energy of later transition states is referred to as covalent catalysis. The covalent bond must be broken to regenerate the free enzyme at the later stage of the reaction.
7. Chymotrypsin: Nucleophilic catalysis:: Carboxypeptidase A: __________
a) General acid-base catalysis
b) Specific acid-base catalysis
c) Electrophilic catalysis
d) Nucleophilic catalysis
Explanation: Chymotrypsin mediated peptide bond is an example of nucleophilic catalysis. In the same way, carboxypeptidase A is and an example of electrophilic catalysis. In most of the enzymatic reaction protons are involved. General and specific acid-base catalysis are types of acid-base catalysis.
8. A type of catalysis which involves covalent intermediate transition state formation between the cationic electrophilic group in the enzyme and the electron rich portion of the substrate is referred to as ____________
a) general acid-base catalysis
b) electrophilic catalysis
c) nucleophilic catalysis
d) specific acid-base catalysis
Explanation: In most of the enzymatic reaction protons are involved. General and specific acid-base catalysis are types on acid-base catalysis. Nucleophilic catalysis involves donation of electrons from nucleophilic active site residues to a substrate forming a covalent adduct transition state intermediate. Whereas, electrophilic catalysis involves covalent intermediate transition state formation between the cationic electrophilic group in the enzyme and the electron rich portion of the substrate.
9. _____________ involves donation of electrons from nucleophilic active site residues to a substrate forming a covalent adduct transition state intermediate.
a) General acid-base catalysis
b) Specific acid-base catalysis
c) Electrophilic catalysis
d) Nucleophilic catalysis
Explanation: In most of the enzymatic reaction protons are involved. General and specific acid-base catalysis are types of acid-base catalysis. The catalysis which involves donation of electrons from nucleophilic active site residues to a substrate forming a covalent adduct transition state intermediate is referred to as nucleophilic catalysis. Nucleophilic and electrophilic catalysis are used to aid covalent catalysis.
10. Which of these is not required for electrophilic catalysis?
a) Mg2+
b) Zn2+
c) Mn2+
d) Ser 195
Explanation: Electrophilic catalysis involves covalent intermediate transition state formation between the cationic electrophilic group in the enzyme and the electron rich portion of the substrate. Most of the amino-acid side chains in the active site don’t act as an efficient electrophile. So, most of the enzymes require metal ions such as Mg2+, Zn2+, Mn2+, etc., as an electron deficient electrophile for optimum catalysis. Ser 195 is an amino-acid in the active site chymotrypsin which hydrolyses peptides.