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Related Concept Videos

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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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Electron Transport Chain: Complex I and II01:46

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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 energy released from the breakdown of the chemical bonds within nutrients can be stored either through the reduction of electron carriers or in the bonds of adenosine triphosphate (ATP). In living systems, a small class of compounds functions as mobile electron carriers, molecules that bind to and shuttle high-energy electrons between compounds in pathways. The principal electron carriers that will be considered originate from the B vitamin group and are derivatives of nucleotides; they are...
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Chemiosmosis and ATP Synthesis01:22

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The electron transport chain is a critical component of cellular respiration, occurring in the inner mitochondrial membrane. It facilitates the transfer of high-energy electrons from reduced cofactors NADH and FADH₂ to molecular oxygen, the final electron acceptor. This transfer of electrons through a series of protein complexes is tightly coupled to the translocation of protons across the membrane, generating a proton gradient essential for ATP synthesis.Electron Flow and Proton...
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After glycolysis, the charged pyruvate molecules enter the mitochondria via active transport and undergo three enzymatic reactions. These reactions ensure that pyruvate can enter the next metabolic pathway so that energy stored in the pyruvate molecules can be harnessed by the cells.
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Electron Transport Chain: Complex III and IV01:43

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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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Updated: Mar 16, 2026

Simultaneous Measurement of Superoxide/Hydrogen Peroxide and NADH Production by Flavin-containing Mitochondrial Dehydrogenases
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Mitochondrial nitric oxide production supported by reverse electron transfer.

Silvina S Bombicino1, Darío E Iglesias1, Tamara Zaobornyj1

  • 1Institute of Biochemistry and Molecular Medicine, Physical Chemistry Division, School of Pharmacy and Biochemistry, University of Buenos Aires (IBIMOL, UBA-CONICET), Junín 956, C1113AAD, Buenos Aires, Argentina.

Archives of Biochemistry and Biophysics
|August 16, 2016
PubMed
Summary

Mitochondria generate nitric oxide (NO) via reverse electron transfer (RET), independent of NADPH. This NO production is linked to Complex I, suggesting a functional association between Complex I and mitochondrial nitric oxide synthase (mtNOS).

Keywords:
Complex IFMNHbO(2)Inside-out particlesMitochondrial nitric oxide synthaseNitric oxideO(2)(•-)RETReverse electron transferflavin mononucleotidemitochondrial nitric oxide synthasemtNOSoxyhemoglobinproton electrochemical potentialproton motive forcereverse electron transfersuperoxide anionΔpΔμH(+)

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Area of Science:

  • Mitochondrial biochemistry
  • Cellular respiration
  • Nitric oxide signaling

Background:

  • Mitochondria are key regulators of cellular energy metabolism and signaling.
  • Nitric oxide (NO) plays diverse physiological roles, and its mitochondrial production is increasingly recognized.
  • The precise mechanisms and localization of mitochondrial NO synthesis remain under investigation.

Purpose of the Study:

  • To investigate the role of reverse electron transfer (RET) in supporting mitochondrial NO production.
  • To determine the localization and functional association of mitochondrial nitric oxide synthase (mtNOS) with respiratory complexes.

Main Methods:

  • Measurement of NO production in heart phosphorylating electron transfer particles (ETPH) and coupled mitochondria under various conditions.
  • Assay of NAD(+) reductase activity to confirm RET.
  • Inhibition studies using rotenone to probe the electron transport chain's involvement.
  • Immunological detection of mtNOS and Complex I subunits.

Main Results:

  • Heart ETPH produced NO via mtNOS, sustained by RET using ATP and succinate, even without NADPH.
  • Rotenone inhibited RET-supported NO production, indicating electron flow dependence, not direct mtNOS inhibition.
  • NO production via RET constituted approximately 20% of total mitochondrial NO release.
  • A mitochondrial fraction enriched in Complex I showed significant NO production and co-localized with mtNOS antibodies.

Conclusions:

  • Mitochondrial NO production can be significantly supported by reverse electron transfer.
  • Mitochondrial nitric oxide synthase (mtNOS) is likely located in close proximity to Complex I.
  • These findings support a functional association between Complex I and mtNOS in heart mitochondria.