Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Free energy requirment for DNA ligase enzyme is obtained in a species dependant manner.
Reason (R) : In E.coli, DNA ligase enzyme utilizes ATP and catalyzes the sealing of single strand nicks.
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

To easily remember the cofactors:
- Eukaryotes, Viruses, and T4 phage DNA ligase $\rightarrow$ ATP-dependent.
- Bacteria (such as *E. coli*) DNA ligase $\rightarrow$ $\text{NAD}^+$-dependent.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
DNA ligases are essential enzymes that catalyze the formation of a phosphodiester bond between the 3'-hydroxyl group of one nucleotide and the 5'-phosphate group of another, sealing nicks in the sugar-phosphate backbone of DNA.
To drive this thermodynamically unfavorable reaction, DNA ligase requires a cofactor as a source of free energy.
Detailed Explanation:
- Evaluating Assertion (A): The energy source utilized by DNA ligases depends on the species:
- Eukaryotic DNA ligases, as well as those from bacteriophages (such as T4 DNA ligase) and some archaea, utilize Adenosine Triphosphate (ATP) as the energy cofactor.
- In contrast, most bacterial DNA ligases, such as the DNA ligase from E. coli, utilize Nicotinamide Adenine Dinucleotide ($\text{NAD}^+$) as the cofactor to drive the reaction.
- Therefore, the free energy requirement is indeed species-dependent, making Assertion (A) true.
- Evaluating Reason (R): The DNA ligase from E. coli is a classic $\text{NAD}^+$-dependent enzyme.
It cleaves the pyrophosphate bond of $\text{NAD}^+$ to release Nicotinamide Mononucleotide (NMN) and form an enzyme-AMP intermediate, rather than utilizing ATP.
Therefore, stating that E. coli DNA ligase utilizes ATP is incorrect, making Reason (R) false.

Step 2: Final Answer:
Assertion (A) is true but Reason (R) is false. This corresponds to option (C).