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The assembly, regulation and function of the mitochondrial respiratory chain

Abstract

The mitochondrial oxidative phosphorylation system is central to cellular metabolism. It comprises five enzymatic complexes and two mobile electron carriers that work in a mitochondrial respiratory chain. By coupling the oxidation of reducing equivalents coming into mitochondria to the generation and subsequent dissipation of a proton gradient across the inner mitochondrial membrane, this electron transport chain drives the production of ATP, which is then used as a primary energy carrier in virtually all cellular processes. Minimal perturbations of the respiratory chain activity are linked to diseases; therefore, it is necessary to understand how these complexes are assembled and regulated and how they function. In this Review, we outline the latest assembly models for each individual complex, and we also highlight the recent discoveries indicating that the formation of larger assemblies, known as respiratory supercomplexes, originates from the association of the intermediates of individual complexes. We then discuss how recent cryo-electron microscopy structures have been key to answering open questions on the function of the electron transport chain in mitochondrial respiration and how supercomplexes and other factors, including metabolites, can regulate the activity of the single complexes. When relevant, we discuss how these mechanisms contribute to physiology and outline their deregulation in human diseases.

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Fig. 1: Overview of the structure and function of the mitochondrial oxidative phosphorylation machinery.
Fig. 2: Structural properties of the electron transport chain.
Fig. 3: Assembly process of respiratory complexes and supercomplexes.
Fig. 4: Regulation of the electron transport chain by supercomplexes.

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I.V. researched data for the article and wrote the article. All authors contributed substantially to discussion of the content and reviewed and/or edited the manuscript before submission.

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Supplementary information

Glossary

Reducing equivalents

Chemical species that transfer the equivalent of one electron in redox reactions.

Quinol

Reduced form of quinone. In the context of the electron transport chain, it is produced by complexes I, II and III2 and utilized by complex III2.

ATP synthase

An enzyme that catalyses the formation of ATP using ADP and inorganic phosphate. Various types of ATPases exist in nature, including A-ATPases (‘A’ stands for ‘archaeal’), F-ATPases (‘F’ stands for ‘(phosphorylation) factor’) and V-ATPases (‘V’ stands for ‘vacuolar’). The mammalian ATP synthase is an F-type ATPase.

Iron–sulfur clusters

(Fe–S clusters). Groups of iron and sulfur atoms coordinated to protein residues acting as cofactors in redox reactions due to the ability to transfer electrons from or to other Fe–S clusters or different types of donors/acceptors while undergoing oxidation and reduction cycles. The xFe–yS nomenclature refers to the number of Fe (x) and S (y) atoms in the cluster.

Haem groups

Redox cofactors featuring an iron atom at the centre of a porphyrin structure. The iron atom can undergo cycles of oxidoreduction, thereby transferring electrons from the donor to acceptors along the electron transport chain. There are three types of haems in the electron transport chain, characterized by different substituents on the porphyrin ring: haems a (a and a3) of complex IV, haems b (bH and bL) of complex III2, haem c of cytochrome c and haem c1 of complex III2.

E-channel

Group of charged residues (mostly glutamates (E)) located within subunits ND1, ND3, ND6 and ND4L of the complex I membrane arm involved in the coupling mechanism of quinone reduction to proton translocation.

Brown adipose tissue

Subtype of fat tissue characterized by a dark colour, as opposed to the normal white appearance, devoted to thermogenesis (via the uncoupling of proton gradient dissipation from ATP synthesis) instead of energy storage. Brown adipose tissue thus contains a lot more (brown) mitochondria than (white) lipid droplets.

Oncometabolite

Substrate of metabolic reactions whose aberrant accumulation triggers cancer-related pathways.

Epithelial–mesenchymal transition

Biological process typical of embryonic development, but also observed in cancer, by which epithelial cells acquire mesenchymal properties (for example, losing apicobasal polarity and increasing their motility).

Paragangliomas and phaeochromocytomas

Paragangliomas are a rare type of neuroendocrine cancer growing around ganglia (groups of neuronal bodies and glial cells) in the head, neck, torso and abdomen. Specifically, when affecting the adrenal glands, they are called ‘phaeochromocytomas’.

Insulin resistance

Pathological condition in which cells do not respond to insulin, thereby not internalizing and utilizing glucose. This phenomenon is correlated with the development of type 2 diabetes.

Caspase

Refers to a family of proteases activated by various stimuli and responsible for the apoptotic response in cells.

Electrostatic wave

Mechanism of signal transduction along the membrane arm of complex I by which the change in the charge status of key polar residues drives proton translocation.

Midpoint potential

Electric potential at which the oxidized and reduced components of a redox reaction are at equilibrium (that is, the midpoint of a redox titration).

Rieske iron–sulfur protein

(ISP). Membrane-anchored catalytic subunit of complex III2 shuttling electrons from quinol bound at haem bL to cytochrome c1 via its iron–sulfur cluster.

Low-potential redox chain

A pathway for electron transfer within complex III2 which goes from haem bL to haem bH. It shows low redox potential.

High-potential redox chain

A pathway for electron transfer within complex III2 that goes from the ISP to cytochrome c1. It shows high redox potential.

Electron paramagnetic resonance

Spectroscopic technique detecting unpaired electrons by the application of a magnetic field. In bioenergetics, it is used to study radicals, such as the quinone intermediates between fully oxidized and fully reduced states, as well as the electron transfer through transition metals in iron–sulfur clusters, haems and copper centres.

AMPK signalling

A pathway of intracellular reactions starting from AMPK (AMP-activated kinase), a sensor of the ATP levels. The signalling cascade starting from AMPK thus responds to the energetic demands of the cell and it triggers a high variety of responses pertaining to metabolism, growth, autophagy and cell polarity.

Copper centre

Prosthetic group composed of copper ions. Among the electron transport chain components, complex IV has two such centres involved in electron transfer.

c-ring

Membrane-embedded domain of ATP synthase formed by multiple copies of subunit c, arranged in the shape of a ring. The number of copies differs across species, thereby changing the diameter of the ring: mammalian ATP synthase has eight copies.

IF1

Inhibitory factor 1 (IF1) of ATP synthase involved in the prevention of ATP hydrolysis due to reverse functioning of the enzyme.

Mitochondrial cristae

The ultrastructure of the inner mitochondrial membrane, characterized by deep invaginations, increasing the overall surface of the inner mitochondrial membrane. It is guided by the arrangement of dimers of ATP synthase in rows that impose the membrane curvature.

Permeability transition pore

(PTP). Channel-like proteinaceous pore located in the inner mitochondrial membrane responsible for leakage of large molecules (up to 1.5 kDa) from the mitochondrial matrix.

Cyclophilin D

Mitochondrial peptidyl-prolyl cistrans isomerase and a member of the cyclophilin family, a group of proteins able to bind the antifungal peptide cyclosporin A. Cyclophilins are involved in protein folding, signal transduction and the immune system. Although the precise role of cyclophilin D is not clear, it is known to interact with ATP synthase and mediate the opening of the permeability transition pore.

Substrate channelling

Metabolic phenomenon by which the reaction product of an enzyme is directly processed as a substrate by another enzyme without being exchanged with the external solution.

Endoplasmic reticulum stress

Aberrant condition characterized by the accumulation of unfolded proteins in the lumen of the endoplasmic reticulum

Hypoxia-inducible factor 1

Transcription factor primarily involved in the cellular response to hypoxia.

Metabolic flux control analysis

Mathematical description of a metabolic path where every enzymatic component is given a coefficient that describes the extent of its control over the pathway by correlating changes in the enzyme activity with changes in the flux rate.

Submitochondrial particles

Inverted vesicles of the inner mitochondrial membrane, obtained after disruption of the outer mitochondrial membrane via different mechanisms, such as osmotic shock, cycles of freezing and thawing or sonication.

Ischaemia–reperfusion injury

Pathological cellular response to reoxygenation of a tissue (reperfusion) after a period of hypoxia (ischaemia). At the molecular level, this phenomenon is characterized by an increase in the production of reactive oxygen species and activation of the caspase pathway, eventually leading to cell death.

Barth syndrome

A rare genetic disease, affecting mainly male individuals, characterized by neuromuscular deficiencies and associated with aberrant cardiolipin metabolism.

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Vercellino, I., Sazanov, L.A. The assembly, regulation and function of the mitochondrial respiratory chain. Nat Rev Mol Cell Biol (2021). https://doi.org/10.1038/s41580-021-00415-0

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