In TCA cycle, NADH is produce at all sites except –
First, the core concept is the TCA cycle, specifically the steps where NADH is generated. I should mention that the TCA cycle is part of cellular respiration and occurs in the mitochondria. The key points are the dehydrogenase enzymes involved in oxidation reactions that produce NADH.
Next, the correct answer. The question is asking which step does NOT produce NADH. The TCA cycle has several steps where NAD+ is reduced to NADH. Let me recall the steps: isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, and succinate dehydrogenase. Wait, succinate dehydrogenase uses FAD, not NAD+. So that step produces FADH2 instead. That must be the exception. So the correct answer is the step involving succinate dehydrogenase.
Now, the wrong options. The other steps that do produce NADH are isocitrate dehydrogenase (step 3) and alpha-ketoglutarate dehydrogenase (step 4). Also, there's the step where malate is converted to oxaloacetate by malate dehydrogenase (step 8), which also produces NADH. So any options pointing to these steps would be incorrect.
For the clinical pearl, I should highlight that remembering the enzymes that produce NADH versus FADH2 is crucial. Maybe a mnemonic like "Isocitrate, Alpha-ketoglutarate, Malate" for NADH, and "Succinate" for FADH2. Also, knowing that the TCA cycle generates 3 NADH molecules and 1 FADH2 per acetyl-CoA.
I need to check the formatting rules. The user wants sections labeled with bold, no markdown headings. Keep each section concise. Let me structure each part step by step, making sure to explain why each incorrect option is wrong and why the correct one is right. Also, ensure the total length is under 2500 characters. Let me count the characters as I go to stay within the limit.
**Core Concept**
The tricarboxylic acid (TCA) cycle generates NADH via oxidation reactions catalyzed by specific dehydrogenases. NADH is produced at three key steps: isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase. Succinate dehydrogenase uniquely produces **FADH₂**, not NADH.
**Why the Correct Answer is Right**
The TCA cycle step **succinate → fumarate** is catalyzed by **succinate dehydrogenase**, which uses **FAD** as a cofactor, reducing it to FADH₂. This is the **only step** in the cycle that does **not** produce NADH. The enzyme is embedded in the mitochondrial inner membrane and directly links to the electron transport chain via FADH₂.
**Why Each Wrong Option is Incorrect**
**Option A:** *Isocitrate →