Question

\(\text{NAD}^{+}\) dependent enzyme are/is:

A. Glyceraldehyde 3-phosphate dehydrogenase

B. Lactate dehydrogenase

C. Malic enzyme

D. HMG CoA reductase

Choose the correct answer from the options given below:

• A. A and B only \
• B. C and D only \
• C. D and A only \
• D. C only

Hint:

Catabolic glycolysis steps use $\text{NAD}^{+}$ (e.g., Glyceraldehyde 3-phosphate DH and Lactate DH). Biosynthesis pathways (like cholesterol production via HMG-CoA reductase) use $\text{NADPH}$.

Answer 1

The Correct Option is A

Step 1: Concept Dehydrogenases and reductases require specific nicotinamide adenine dinucleotide coenzymes ($\text{NAD}^{+}$/$\text{NADH}$ or $\text{NADP}^{+}$/$\text{NADPH}$) as electron carriers. Generally, pathways that break down molecules for energy (catabolism) use $\text{NAD}^{+}$, while pathways that build molecules (anabolism) use $\text{NADP}^{+}$.

Step 2: Meaning An $\text{NAD}^{+}$-dependent enzyme specifically recognizes the non-phosphorylated dinucleotide core to catalyze oxidation-reduction conversions.

Step 3: Analysis * Statement A (Glyceraldehyde 3-phosphate dehydrogenase): A core catabolic enzyme in glycolysis that converts glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate, reducing $\text{NAD}^{+}$ to $\text{NADH}$. It is strictly $\text{NAD}^{+}$-dependent. * Statement B (Lactate dehydrogenase): Catalyzes the reversible anaerobic reduction of pyruvate to lactate using $\text{NADH}$/$\text{NAD}^{+}$. It is strictly $\text{NAD}^{+}$-dependent. * Statement C (Malic enzyme): Catalyzes the oxidative decarboxylation of malate to pyruvate, preferentially using $\text{NADP}^{+}$ to generate $\text{NADPH}$ for biosynthesis. * Statement D (HMG CoA reductase): The rate-limiting enzyme in cholesterol biosynthesis that reduces HMG-CoA to mevalonate using $\text{NADPH}$.

Step 4: Conclusion Thus, only enzymes A and B are dependent on the $\text{NAD}^{+}$ coenzyme system. Final Answer: (A)