---
title: "NADH Dehydrogenase | Biochemical Chemistry II"
description: "NADH dehydrogenase is Complex I of the mitochondrial electron transport chain, passing electrons from NADH to start proton pumping in Biochemical Chemistry II."
canonical: "https://fiveable.me/biological-chemistry-ii/key-terms/nadh-dehydrogenase"
type: "key-term"
subject: "Biological Chemistry II"
unit: "Unit 6"
---

# NADH Dehydrogenase | Biochemical Chemistry II

## Definition

NADH dehydrogenase is Complex I, the first enzyme in the mitochondrial electron transport chain. It oxidizes NADH to NAD+ and feeds those electrons into proton pumping for ATP production.

## What It Is

NADH dehydrogenase is Complex I of the mitochondrial electron transport chain in Biological Chemistry II. It sits in the inner mitochondrial membrane and is the first major protein complex to receive electrons from NADH.

Its job is to oxidize NADH back to NAD+, which matters because cells need NAD+ to keep glycolysis, the citric acid cycle, and other redox reactions moving. When NADH donates electrons, those electrons do not stop at the enzyme. They move through a flavin mononucleotide (FMN) group and a chain of iron-sulfur clusters inside the complex.

That electron flow is coupled to proton pumping. Complex I uses the released energy to move protons from the mitochondrial matrix into the intermembrane space, building an electrochemical gradient across the inner membrane. That gradient is the stored energy cells later cash in through ATP synthase.

A simple way to think about it is that NADH dehydrogenase starts the whole oxidative phosphorylation chain. If electrons from NADH cannot enter at this step, less proton motive force is built and less ATP can be made. So even though the enzyme does not synthesize ATP itself, it sets up the conditions that make ATP synthesis possible.

This is also why Complex I is often discussed together with the electron transport chain as a whole. It is not just an electron shuttle. It is a redox enzyme and a proton pump, and those two functions are linked. In problem sets, you may be asked to trace that link from NADH oxidation to proton gradient formation to ATP yield.

One common point of confusion is that NADH dehydrogenase is not the same thing as NADH in general. NADH is the electron carrier. NADH dehydrogenase is the membrane complex that accepts those electrons and passes them onward into the chain.

## Why It Matters

NADH dehydrogenase is the entry point for a large share of the reducing power made during fuel breakdown. In Biochemical Chemistry II, that means it connects metabolism to bioenergetics. When you map how glucose, fatty acids, or amino acids are turned into ATP, Complex I is one of the first places where you can see energy being converted into a proton gradient instead of heat.

It also shows up any time you are tracking redox balance. Because it regenerates NAD+, it supports pathways that depend on NADH turnover. If the enzyme slows down or fails, the ripple effects are easy to predict: electrons back up, proton pumping drops, and oxidative phosphorylation makes less ATP.

This term is especially useful for comparing parts of the electron transport chain. Complex I is the largest and one of the main proton-pumping entry points, while other complexes handle electrons from different carriers. That difference helps you explain why electrons from NADH usually yield more ATP than electrons entering later through other routes.

## Connections

### Electron transport chain

NADH dehydrogenase is the first complex in the chain, so it is the opening step in the flow of electrons through the inner mitochondrial membrane. If you trace the electron transport chain from start to finish, this enzyme is where NADH hands off its electrons and proton pumping begins.

### [Complex I](/biological-chemistry-ii/key-terms/complex-i)

Complex I is the name most biochemistry texts use for NADH dehydrogenase. If a question uses either label, it is pointing to the same membrane complex. The Complex I label is especially useful when comparing it with Complex II, III, and IV in the full pathway.

### [Proton Motive Force](/biological-chemistry-ii/key-terms/proton-motive-force)

Complex I helps build the proton motive force by moving H+ into the intermembrane space. That gradient has both a chemical difference in proton concentration and an electrical difference across the membrane. ATP synthase later uses that stored energy to make ATP.

### ATP synthase

ATP synthase does the ATP-making step, but it depends on the gradient started by complexes like NADH dehydrogenase. A good way to connect them is to remember that Complex I stores energy in the form of a proton gradient, while ATP synthase releases that energy as ATP.

## On the AP Exam

A quiz or problem-set question often asks you to identify where NADH dehydrogenase fits in the electron transport chain or to explain what happens when NADH donates electrons. You may need to trace electrons from NADH to FMN and iron-sulfur centers, then connect that transfer to proton pumping across the inner mitochondrial membrane. If a lab or data question gives reduced ATP production, you should be ready to reason that a defect in Complex I would lower proton motive force and cut oxidative phosphorylation efficiency. In short, use the term to link redox chemistry with membrane transport and ATP yield.

## NADH dehydrogenase vs Complex II

Complex I and Complex II both feed electrons into the electron transport chain, but they do not do the same job. NADH dehydrogenase oxidizes NADH and pumps protons, while Complex II transfers electrons from succinate-related metabolism and does not pump protons. If you mix them up, you lose the main reason Complex I is such a big source of the proton gradient.

## Key Takeaways

- NADH dehydrogenase is Complex I, the first electron-accepting complex in the mitochondrial electron transport chain.
- It oxidizes NADH to NAD+ and passes the electrons through FMN and iron-sulfur clusters.
- The energy released by electron transfer is used to pump protons into the intermembrane space.
- That proton pumping helps create the proton motive force that ATP synthase uses to make ATP.
- If Complex I is damaged, cellular respiration produces less ATP and redox balance can be disrupted.

## FAQs

### What is NADH dehydrogenase in Biological Chemistry II?

It is Complex I of the mitochondrial electron transport chain. The enzyme takes electrons from NADH, passes them through internal carriers, and uses the energy to pump protons across the inner mitochondrial membrane.

### Is NADH dehydrogenase the same as Complex I?

Yes. In most biochemistry courses, NADH dehydrogenase and Complex I refer to the same large membrane complex. If you see either name, think first electron entry point for NADH in oxidative phosphorylation.

### Does NADH dehydrogenase make ATP directly?

No, it does not synthesize ATP itself. It builds the proton gradient that ATP synthase later uses to make ATP, so its job is upstream of ATP production rather than the final step.

### Why does a defect in NADH dehydrogenase lower ATP production?

If Complex I cannot transfer electrons well, fewer protons are pumped into the intermembrane space. That weakens the proton motive force, and ATP synthase has less energy to work with, so ATP output drops.

## Related Study Guides

- [6.1 Electron transport chain: components and function](/biological-chemistry-ii/unit-6/electron-transport-chain-components-function/study-guide/AMTy1JesdF6sXhDT)
- [6.2 Oxidative phosphorylation and chemiosmotic theory](/biological-chemistry-ii/unit-6/oxidative-phosphorylation-chemiosmotic-theory/study-guide/jhggI6yCcbmH8hDo)

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