Q1. Source: CSIR NET June 2025
The reactions of the urea cycle occur in two different cellular compartments and generate the following intermediates.
A. Arginino-succinate
B. Citrulline
C. Ornithine
Which intermediate(s) must be transported across the mitochondrial inner membrane?
a. A and B
b. B and C
c. A and C
d. A only
Explanation
The urea cycle is divided between the mitochondrial matrix and the cytosol.
- Ornithine enters the mitochondrion, where it combines with carbamoyl phosphate to form citrulline.
- Citrulline is then transported out of the mitochondrion into the cytosol.
- In the cytosol, citrulline is converted into argininosuccinate, followed by arginine.
- Finally, arginine is cleaved to produce urea and ornithine, and the newly formed ornithine is transported back into the mitochondrion to begin another cycle.
Correct answer: b. B and C
Q2. Source: ICMR JRF 2025 Biological Sciences
The active site of an enzyme is flexible as per the
a. Competitive inhibition model
b. Induced fit model
c. Lock and key model
d. Transition state model
Correct answer: b. Induced fit model
Explanation
The Induced Fit Model, proposed by Daniel Koshland, states that an enzyme’s active site is flexible, not rigid. When the substrate approaches, the active site undergoes a slight conformational change, allowing a better fit between the enzyme and the substrate. This improves catalytic efficiency and stabilizes the transition state.
Let’s look at the other options:
- Competitive inhibition model: Describes how an inhibitor competes with the substrate for the active site. It does not explain the flexibility of the enzyme.
- Lock and Key Model: Proposed by Emil Fischer, this model assumes that the enzyme’s active site is rigid and perfectly complementary to the substrate.
- Transition State Model: Focuses on the stabilization of the transition state during catalysis rather than the flexibility of the active site.
Correct answer: b. Induced fit model
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