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Electron Transport Chain: Complex I and II

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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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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.
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Related Experiment Video

Updated: May 18, 2026

Protein Film Infrared Electrochemistry Demonstrated for Study of H2 Oxidation by a [NiFe] Hydrogenase

Protein Film Infrared Electrochemistry Demonstrated for Study of H₂ Oxidation by a [NiFe] Hydrogenase

Published on: December 4, 2017

A missing link between complex I and group 4 membrane-bound [NiFe] hydrogenases.

Bruno C Marreiros¹, Ana P Batista, Afonso M S Duarte

¹Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Av. da República EAN, 2780-157 Oeiras, Portugal.

Biochimica Et Biophysica Acta

|September 25, 2012

Summary

Complex I, a key energy-converting enzyme, shares evolutionary links with hydrogenases and antiporters. New evidence reveals energy-converting hydrogenase-related complexes (Ehr) as a crucial evolutionary intermediate.

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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

Area of Science:

Biochemistry
Molecular Biology
Evolutionary Biology

Background:

Complex I is a crucial energy-transducing enzyme in respiratory chains, vital for cellular energy production.
Previous studies suggested evolutionary links between Complex I, group 4 [NiFe] hydrogenases, and Mrp antiporters.
The modular structure of Complex I involves proteins found in other enzyme complexes.

Purpose of the Study:

To conduct a comprehensive taxonomic analysis of prokaryotic group 4 [NiFe] hydrogenases, Complex I, and related enzymes.
To investigate the gene clustering patterns of these enzyme complexes.
To elucidate the evolutionary relationships between Complex I, hydrogenases, and antiporter systems.

Main Methods:

Taxonomic profiling of prokaryotic group 4 [NiFe] hydrogenases, Complex I, and related enzymes.
Analysis of gene clustering and genomic organization.
Comparative analysis of enzyme structures and functional domains.

Main Results:

Identified novel enzyme complexes related to hydrogenases, termed Ehr (energy-converting hydrogenases related complexes).
These Ehr complexes lack the catalytic center binding site found in typical hydrogenases.
Ehr complexes represent a missing evolutionary link between Complex I and group 4 [NiFe] hydrogenases.

Conclusions:

Ehr complexes provide a new perspective on the evolutionary origins of Complex I.
This finding offers insights into the functional and mechanistic diversification of bioenergetic systems.
The study contributes to understanding the evolutionary aspects of bioenergetic systems.