Mitochondrial Electron Transport Chain

The electron transport chain is a series of protein complexes in the inner mitochondrial membrane that passes electrons from reduced cofactors, step by step, toward molecular oxygen. As electrons move down this energetic gradient, the complexes pump protons across the membrane, storing energy that ATP synthase later uses. The chain is the respiratory core of oxidative phosphorylation.

Definition

The mitochondrial electron transport chain is the set of inner-membrane redox complexes and mobile carriers that transfer electrons from NADH and FADH2 to oxygen, coupling this electron flow to the pumping of protons that builds the proton-motive force.

Scope

The topic covers the respiratory complexes (I-IV), the mobile electron carriers coenzyme Q and cytochrome c, the flow of electrons to oxygen, the coupled pumping of protons, and the organisation of complexes into supercomplexes. It is a biochemical reference and not clinical guidance.

Core questions

• Which complexes make up the respiratory chain and what do they do?
• How do electrons flow from reduced cofactors to oxygen?
• How is electron transfer coupled to proton pumping?
• How are the complexes organised into supercomplexes?

Key concepts

• Complex I (NADH dehydrogenase)
• Complex II (succinate dehydrogenase)
• Complex III (cytochrome bc1)
• Complex IV (cytochrome c oxidase)
• Coenzyme Q (ubiquinone)
• Cytochrome c
• Respiratory supercomplexes
• Redox potential gradient

Mechanisms

Electrons enter the chain from NADH at Complex I or from FADH2 (via succinate dehydrogenase) at Complex II, are carried by ubiquinone to Complex III, then by cytochrome c to Complex IV, where they reduce oxygen to water. Complexes I, III, and IV pump protons from the matrix into the intermembrane space as electrons traverse them, converting the energy released by the favourable redox steps into a transmembrane proton gradient. Mitchell's chemiosmotic framework explains why electron transport and proton pumping are coupled. Evidence indicates that the complexes can assemble into higher-order supercomplexes, an organisation reported to influence how electrons are partitioned through the chain.

Clinical relevance

Defects in respiratory-chain function impair the cell's capacity to generate ATP and are studied across many tissues and disease models. This entry describes the chain's biochemistry for reference and is not a basis for diagnosis or treatment.

History

The cytochromes and the broad sequence of respiratory carriers were worked out across the early twentieth century, and the coupling of this electron flow to ATP synthesis was explained by Mitchell's chemiosmotic hypothesis in 1961. Structural and biochemical work later resolved the individual complexes, and twenty-first-century studies described their assembly into supercomplexes and debated the functional consequences.

Debates

Do respiratory supercomplexes regulate electron flux?

Reports that complexes assemble into supercomplexes raised the proposal that this organisation channels electrons and shapes respiratory efficiency, but whether supercomplexes are required for normal flux or are one of several arrangements remains discussed.

Key figures

• Peter Mitchell
• Matti Saraste
• José Antonio Enríquez

Seminal works

• saraste-1999
• mitchell-1961
• lapuente-brun-2013

Frequently asked questions

What is the final electron acceptor of the chain?

Molecular oxygen, which is reduced to water at Complex IV (cytochrome c oxidase); this is why the process is called aerobic respiration.

How does electron transport help make ATP?

Electron flow drives the complexes to pump protons across the inner membrane, and the resulting proton gradient powers ATP synthase — the electron transport chain does not make ATP directly.