NAD+ is reduced to NADH + H+ by dehydrogenases of all the following substrates, Except
## Core Concept
The question tests understanding of **redox reactions** in cellular respiration, specifically focusing on the reduction of **NAD+ to NADH + H+** by various dehydrogenases. This process is crucial in glycolysis, fatty acid oxidation, and the citric acid cycle, generating reducing equivalents for the electron transport chain.
## Why the Correct Answer is Right
The correct answer, , involves understanding that **isocitrate dehydrogenase** and **malate dehydrogenase** in the citric acid cycle, **glyceraldehyde-3-phosphate dehydrogenase** in glycolysis, and **fatty acid dehydrogenases** in fatty acid oxidation all reduce NAD+ to NADH + H+. However, **succinate dehydrogenase** (Complex II or Succinate:Quinone Oxidoreductase) in the citric acid cycle and the electron transport chain reduces **FAD** to **FADH2**, not NAD+ to NADH.
## Why Each Wrong Option is Incorrect
- **Option A:** Isocitrate dehydrogenase indeed reduces NAD+ to NADH + H+, making it an incorrect choice for the exception.
- **Option B:** Glyceraldehyde-3-phosphate dehydrogenase reduces NAD+ to NADH + H+ in glycolysis, so it's not the correct exception.
- **Option C:** Malate dehydrogenase also reduces NAD+ to NADH + H+, which rules it out as the exception.
## Clinical Pearl / High-Yield Fact
A key point to remember is that **succinate dehydrogenase** is unique because it participates directly in both the citric acid cycle and the electron transport chain by reducing FAD to FADH2, bypassing the first complex of the electron transport chain.
## Correct Answer Line
**Correct Answer: D. Succinate**