Oversized ubiquinones as molecular probes for structural dynamics of the ubiquinone reaction site in mitochondrial respiratory complex I

J Biol Chem. 2020 Feb 21;295(8):2449-2463. doi: 10.1074/jbc.RA119.012347. Epub 2020 Jan 17.

Abstract

NADH-quinone oxidoreductase (complex I) couples electron transfer from NADH to quinone with proton translocation across the membrane. Quinone reduction is a key step for energy transmission from the site of quinone reduction to the remotely located proton-pumping machinery of the enzyme. Although structural biology studies have proposed the existence of a long and narrow quinone-access channel, the physiological relevance of this channel remains debatable. We investigated here whether complex I in bovine heart submitochondrial particles (SMPs) can catalytically reduce a series of oversized ubiquinones (OS-UQs), which are highly unlikely to transit the narrow channel because their side chain includes a bulky "block" that is ∼13 Å across. We found that some OS-UQs function as efficient electron acceptors from complex I, accepting electrons with an efficiency comparable with ubiquinone-2. The catalytic reduction and proton translocation coupled with this reduction were completely inhibited by different quinone-site inhibitors, indicating that the reduction of OS-UQs takes place at the physiological reaction site for ubiquinone. Notably, the proton-translocating efficiencies of OS-UQs significantly varied depending on their side-chain structures, suggesting that the reaction characteristics of OS-UQs affect the predicted structural changes of the quinone reaction site required for triggering proton translocation. These results are difficult to reconcile with the current channel model; rather, the access path for ubiquinone may be open to allow OS-UQs to access the reaction site. Nevertheless, contrary to the observations in SMPs, OS-UQs were not catalytically reduced by isolated complex I reconstituted into liposomes. We discuss possible reasons for these contradictory results.

Keywords: bioenergetics; chemical biology; complex I; mitochondria; proton pump; respiratory chain; ubiquinone.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Alkynes / metabolism
  • Animals
  • Cattle
  • Computer Simulation
  • Electron Transport
  • Electron Transport Complex I / metabolism*
  • Membrane Potential, Mitochondrial
  • Mitochondria, Heart / metabolism*
  • Mitochondrial Proteins / metabolism
  • Models, Molecular
  • Molecular Probes / metabolism*
  • NAD / metabolism
  • Oxidoreductases / metabolism
  • Plant Proteins / metabolism
  • Protein Subunits / metabolism
  • Proteolipids / metabolism
  • Protons
  • Submitochondrial Particles / metabolism
  • Ubiquinone / chemistry*
  • Ubiquinone / metabolism*

Substances

  • Alkynes
  • Mitochondrial Proteins
  • Molecular Probes
  • Plant Proteins
  • Protein Subunits
  • Proteolipids
  • Protons
  • proteoliposomes
  • NAD
  • Ubiquinone
  • Oxidoreductases
  • alternative oxidase
  • Electron Transport Complex I

Associated data

  • PDB/5LC5