Zu 2003 J Am Chem Soc

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Zu Y, Shannon RJ, Hirst J (2003) Reversible, electrochemical interconversion of NADH and NAD+ by the catalytic (Ilambda) subcomplex of mitochondrial NADH:ubiquinone oxidoreductase (complex I). J Am Chem Soc 125:6020-1. https://doi.org/10.1021/ja0343961

Β» PMID: 12785808

Zu Y, Shannon RJ, Hirst J (2003) J Am Chem Soc

Abstract: NADH:ubiquinone oxidoreductase (complex I) is the first enzyme of the mitochondrial electron transport chain and catalyzes the oxidation of beta-NADH by ubiquinone, coupled to transmembrane proton translocation. It contains a flavin mononucleotide (FMN) at the active site for NADH oxidation, up to eight iron-sulfur (FeS) clusters, and at least one ubiquinone binding site. Little is known about the mechanism of coupled electron-proton transfer in complex I. This communication demonstrates how the catalytic fragment of complex I, subcomplex Ilambda, can be adsorbed onto a pyrolytic graphite edge electrode to catalyze the interconversion of NADH and NAD+, with the electrode as the electron acceptor or donor. NADH oxidation and NAD+ reduction are completely reversible and occur without the application of an overpotential. The potential of zero current denotes the potential of the NAD+/NADH redox couple, and the dependence of ENAD+ on pH, and on the NADH:NAD+ ratio, is in accordance with the Nernst equation. The catalytic potential of the enzyme, Ecat, is close to one of the two reduction potentials of the active site FMN and to the potential of a nearby [2Fe - 2S] cluster; therefore, either one or both of these redox couples is suggested to be important in controlling NADH oxidation by complex I.

β€’ Bioblast editor: Gnaiger E

Zu 2003 J Am Chem Soc CORRECTION.png

Hydrogen ion ambiguities in the electron transfer system

Communicated by Gnaiger E (2023-10-08) last update 2023-11-10
Electron (e-) transfer linked to hydrogen ion (hydron; H+) transfer is a fundamental concept in the field of bioenergetics, critical for understanding redox-coupled energy transformations.
Ambiguity alert H+.png
However, the current literature contains inconsistencies regarding H+ formation on the negative side of bioenergetic membranes, such as the matrix side of the mitochondrial inner membrane, when NADH is oxidized during oxidative phosphorylation (OXPHOS). Ambiguities arise when examining the oxidation of NADH by respiratory Complex I or succinate by Complex II.
Ambiguity alert e-.png
Oxidation of NADH or succinate involves a two-electron transfer of 2{H++e-} to FMN or FAD, respectively. Figures indicating a single electron e- transferred from NADH or succinate lack accuracy.
Ambiguity alert NAD.png
The oxidized NAD+ is distinguished from NAD indicating nicotinamide adenine dinucleotide independent of oxidation state.
NADH + H+ β†’ NAD+ +2{H++e-} is the oxidation half-reaction in this H+-linked electron transfer represented as 2{H++e-} (Gnaiger 2023). Putative H+ formation shown as NADH β†’ NAD+ + H+ conflicts with chemiosmotic coupling stoichiometries between H+ translocation across the coupling membrane and electron transfer to oxygen. Ensuring clarity in this complex field is imperative to tackle the apparent ambiguity crisis and prevent confusion, particularly in light of the increasing number of interdisciplinary publications on bioenergetics concerning diagnostic and clinical applications of OXPHOS analysis.
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