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Electron Transport Chain: Complex I and II

Electron Transport Chain: Complex I and II

The mitochondrial electron transport chain (ETC) is the main energy generation system in the eukaryotic cells. However, mitochondria also produce cytotoxic reactive oxygen species (ROS) due to the large electron flow during oxidative phosphorylation. While Complex I is one of the primary sources of superoxide radicals, ROS production by Complex II is uncommon and may only be observed in cancer cells with mutated complexes.
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During the electron transport chain, electrons from NADH and FADH2 are first transferred to complexes I and II, respectively. These two complexes then transfer the electrons to ubiquinol, which carries them further to complex III. Complex III passes the electrons across the intermembrane space to Cyt c, which carries them further to complex IV. Complex IV donates electrons to oxygen and reduces it to water. As electrons pass through complexes I, III, and IV, the energy released aids the pumping...

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The Supercomplexes in the Crista Membrane

The mitochondrial cristae membrane is the primary site for the oxidative phosphorylation (OXPHOS) process of energy conversion mediated through respiratory complexes I to V. These complexes have been widely studied for decades, and it has been proven that they form supramolecular structures called respiratory supercomplexes (SC). These higher-order complexes may be crucial in maintaining the biochemical structure and improving the physiological activity of the individual complexes while...

Electron Transport Chains

Electron Transport Chains

The final stage of cellular respiration is oxidative phosphorylation that consists of two steps: the electron transport chain and chemiosmosis. The electron transport chain is a set of proteins found in the inner mitochondrial membrane in eukaryotic cells. Its primary function is to establish a proton gradient that can be used during chemiosmosis to produce ATP and generate electron carriers, such as NAD+ and FAD, that are used in glycolysis and the citric acid cycle.
The ETC is comprised of...

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Related Experiment Video

Updated: May 31, 2026

Analyzing Supercomplexes of the Mitochondrial Electron Transport Chain with Native Electrophoresis, In-gel Assays, and Electroelution

Analyzing Supercomplexes of the Mitochondrial Electron Transport Chain with Native Electrophoresis, In-gel Assays, and Electroelution

Published on: June 1, 2017

Why does mitochondrial complex I have so many subunits?

¹Medical Research Council Mitochondrial Biology Unit, Wellcome Trust/MRC Building, Cambridge, UK. jh@mrc-mbu.cam.ac.uk

The Biochemical Journal

|June 30, 2011

Summary

Eukaryotic complex I enzymes, crucial for energy transduction, are larger than prokaryotic versions. New research suggests accessory subunits enhance complex I enzyme stability.

Area of Science:

Biochemistry
Molecular Biology
Cellular Respiration

Background:

Complex I (proton-pumping NADH:quinone oxidoreductase) is vital for energy transduction in both prokaryotes and eukaryotes.
Eukaryotic complex I is significantly larger and has more subunits than its prokaryotic counterpart, despite sharing 14 conserved core subunits sufficient for catalysis.

Purpose of the Study:

To investigate the functional role of supernumerary subunits in eukaryotic complex I.
To provide context for new evidence suggesting these subunits contribute to enzyme stability.

Main Methods:

The study by Angerer et al. is discussed, providing new evidence.
Commentary contextualizes these findings within the broader understanding of complex I structure and function.

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Isolation of Mitochondria for Mitochondrial Supercomplex Analysis from Small Tissue and Cell Culture Samples

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An Improved Method to Isolate Mitochondrial Contact Sites

Published on: June 16, 2023

Related Experiment Videos

Last Updated: May 31, 2026

Analyzing Supercomplexes of the Mitochondrial Electron Transport Chain with Native Electrophoresis, In-gel Assays, and Electroelution

Analyzing Supercomplexes of the Mitochondrial Electron Transport Chain with Native Electrophoresis, In-gel Assays, and Electroelution

Published on: June 1, 2017

Isolation of Mitochondria for Mitochondrial Supercomplex Analysis from Small Tissue and Cell Culture Samples

Isolation of Mitochondria for Mitochondrial Supercomplex Analysis from Small Tissue and Cell Culture Samples

Published on: May 3, 2024

An Improved Method to Isolate Mitochondrial Contact Sites

An Improved Method to Isolate Mitochondrial Contact Sites

Published on: June 16, 2023

Main Results:

New evidence suggests that supernumerary subunits in eukaryotic complex I are important for enzyme stability.
The conserved core subunits are sufficient for the catalytic activity of complex I.

Conclusions:

The increased size and subunit count of eukaryotic complex I, beyond the core catalytic components, likely serve a role in maintaining enzyme stability.
Understanding the function of accessory subunits is key to fully elucidating the differences between prokaryotic and eukaryotic complex I.