Electron transfer pathway

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Electron transfer pathway


In the mitochondrial electron transfer pathway (ET pathway) electrons are transferred from externally supplied reduced fuel substrates to oxygen. Based on this experimentally oriented definition (see ET capacity), the ET pathway consists of (1) the membrane-bound ET pathway with respiratory complexes located in the inner mt-membrane, (2) TCA cycle and other mt-matrix dehydrogenases generating NADH and succinate, and (3) the carriers involved in metabolite transport across the mt-membranes. » MiPNet article

Abbreviation: ET pathway

Reference: Gnaiger 2009 Int J Biochem Cell Biol, Gnaiger 2020 BEC MitoPathways

Electron transfer pathway versus electron transport chain

Publications in the MiPMap
Gnaiger E (2017) Electron transfer pathway versus electron transport chain. Mitochondr Physiol Network (2010-08-17) last update 2020-06-02.


Oroboros (2020) MiPNet

Abstract: Conventionally, the 'electron transport chain' has been considered as the sequence of membrane-bound respiratory complexes, mainly CI and CII feeding electrons into the Q-junction, and CIII and CIV linked by cytochrome c. Emphasis on the term electron transfer pathway (Gnaiger 2020 BEC MitoPathways) clarifies (1) the convergent structure of the mitochondrial pathways, (2) the upstream modules of electron transfer from externally supplied fuel substrates, transport into the matrix space, and matrix dehydrogenases, including the TCA cycle and the N-junction.

O2k-Network Lab: AT Innsbruck Gnaiger E

The well established terms 'respiratory chain' or 'electron transfer chain' suggest erroneously that the convergent electron transfer pathway may be designed as a simple chain. But the term electron transport chain (or electron transfer chain, ETC) is a misnomer. Understanding mitochondrial respiratory control has suffered greatly from this inappropriate terminology, although textbooks using the term ETC (Lehninger 1970) make it sufficiently clear that electron transfer is not arranged as a chain: the „ETC‟ is in fact not a simple chain but an arrangement of electron transfer complexes in a non-linear, convergent electron transfer pathway. The classically introduced term Electron transfer pathway (Hatefi et al 1962) is more accurate. Since the enzyme-catalyzed steps form a metabolic pathway, the term electron transfer pathway is accurate and sufficient (IUB 1991).
The established convention of defining the 'electron transport chain' as being comprised of four Complexes has conceptual weaknesses.
(a) In fact, there are more than six Complexes of mitochondrial electron transfer (not including Complex V, which is not part of the ET pathway): CI to CIV, and additional respiratory complexes linked to pathways converging at the Q-junction (see »Electron transfer pathway state).
(b) The term „chain‟ suggests a linear sequence, whereas the functional structure of the electron transfer pathway can only be understood by recognizing the convergence of electron flow at the Q-junction, followed by a chain of Complexes III and IV, mediated by cytochrome c (Gnaiger 2014). Electrons flow to oxygen from either Complex I with a total of three coupling sites, or from Complex II and other flavoproteins, providing multiple entries into the Q-junction with two coupling sites downstream (Gnaiger 2014).

Electron transfer versus transport

Electron transfer and electron transport are used synonymously. A general distinction, however, is helpful:
(i) Transfer (inter- or intramolecular) of a reactant involves a chemical reaction.
(ii) Transport (from one location to another) of an entity is a (vectorial) process in contrast to a chemical reaction (IUPAC Green Book).

Related MitoPedia pages

  • Electron transfer pathway, ET pathway
» Electron transfer pathway
» Q-junction
  • ET-pathway states
» ET-pathway state
  • Coupling-control state E
E.jpg ET capacity
» Noncoupled respiration
» Is respiration uncoupled - noncoupled - dyscoupled?


Bioblast linkReferenceYear
Gnaiger E (2009) Capacity of oxidative phosphorylation in human skeletal muscle. New perspectives of mitochondrial physiology. Int J Biochem Cell Biol 41:1837-45.2009
Gnaiger E (2020) Mitochondrial pathways and respiratory control. An introduction to OXPHOS analysis. 5th ed. Bioenerg Commun 2020.2: 112 pp. doi:10.26124/bec:2020-0002.2020
Gnaiger Erich et al ― MitoEAGLE Task Group (2020) Mitochondrial physiology. Bioenerg Commun 2020.1. doi:10.26124/bec:2020-0001.v1.2020
Hatefi Y, Haavik AG, Fowler LR, Griffiths DE (1962) Studies on the electron transfer system XLII. Reconstitution of the electron transfer system. J Biol Chem 237:2661-9.1962


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