bims-cytox1 Biomed News
on Cytochrome oxidase subunit 1
Issue of 2022‒07‒24
four papers selected by
Gavin McStay
Liverpool John Moores University

  1. Biochim Biophys Acta Bioenerg. 2022 Jul 15. pii: S0005-2728(22)00064-0. [Epub ahead of print] 148595
      The cytochrome c oxidase complex, complex VI (CIV), catalyzes the terminal step of the mitochondrial electron transport chain where the reduction of oxygen to water by cytochrome c is coupled to the generation of a protonmotive force that drive the synthesis of ATP. CIV evolution was greatly accelerated in humans and other anthropoid primates and appears to be driven by adaptive selection. However, it is not known if there are significant functional differences between the anthropoid primates CIV, and other mammals. Comparison of the high-resolution structures of bovine CIV, mouse CIV and human CIV shows structural differences that are associated with anthropoid-specific substitutions. Here I examine the possible effects of these substitutions in four CIV peptides that are known to affect proton pumping: the mtDNA-coded subunits I, II and III, and the nuclear-encoded subunit VIa2. I conclude that many of the anthropoid-specific substitutions could be expected to modulate the rate and/or the efficiency of proton pumping. These results are compatible with the previously proposed hypothesis that the accelerated evolution of CIV in anthropoid primates is driven by selection pressure to lower the mitochondrial protonmotive force and thus decrease the rate of superoxide generation by mitochondria.
    Keywords:  Anthropoid primates; Cytochrome c oxidase; Human; Proton pumping; Protonmotive force
  2. Biophys Physicobiol. 2022 ;19 e190022
      Most mitochondrial proteins are synthesized as precursor proteins (preproteins) in the cytosol and imported into mitochondria. The translocator of the outer membrane (TOM) complex functions as a main entry gate for the import of mitochondrial proteins. The TOM complex is a multi-subunit membrane protein complex composed of a β-barrel channel Tom40 and six single-pass membrane proteins. Recent cryo-EM studies have revealed high-resolution structures of the yeast and human TOM complexes, which enabled us to discuss the mechanism of protein import at an amino-acid residue level. The cryo-EM structures show that two Tom40 β-barrels are surrounded by two sets of small Tom subunits to form a dimeric structure. The intermembrane space (IMS) domains of Tom40, Tom22, and Tom7 form a binding site for presequence-containing preproteins in the middle of the dimer to achieve their efficient transfer of to the downstream translocase, the TIM23 complex. The N-terminal segment of Tom40 spans the channel from the cytosol to the IMS to interact with Tom5 at the periphery of the dimer, where downstream components of presequence-lacking preproteins are recruited. Structure-based biochemical analyses together with crosslinking experiments revealed that each Tom40 channel possesses two distinct paths and exit sites for protein translocation of different sets of mitochondrial preproteins. Here we summarize the current knowledge on the structural features, protein translocation mechanisms, and remaining questions for the TOM complexes, with particular emphasis on their determined cryo-EM structures. This article is an extended version of the Japanese article, Structural basis for protein translocation by the translocase of the outer mitochondrial membrane, published in SEIBUTSU BUTSURI Vol. 60, p. 280-283 (2020).
    Keywords:  Cryo-EM; TOM complex preprotein; mitochondria; protein translocation
  3. EMBO J. 2022 Jul 20. e110784
      The mitochondrial intermembrane space protein AIFM1 has been reported to mediate the import of MIA40/CHCHD4, which forms the import receptor in the mitochondrial disulfide relay. Here, we demonstrate that AIFM1 and MIA40/CHCHD4 cooperate beyond this MIA40/CHCHD4 import. We show that AIFM1 and MIA40/CHCHD4 form a stable long-lived complex in vitro, in different cell lines, and in tissues. In HEK293 cells lacking AIFM1, levels of MIA40 are unchanged, but the protein is present in the monomeric form. Monomeric MIA40 neither efficiently interacts with nor mediates the import of specific substrates. The import defect is especially severe for NDUFS5, a subunit of complex I of the respiratory chain. As a consequence, NDUFS5 accumulates in the cytosol and undergoes rapid proteasomal degradation. Lack of mitochondrial NDUFS5 in turn results in stalling of complex I assembly. Collectively, we demonstrate that AIFM1 serves two overlapping functions: importing MIA40/CHCHD4 and constituting an integral part of the disulfide relay that ensures efficient interaction of MIA40/CHCHD4 with specific substrates.
    Keywords:  AIFM1; MIA40-CHCHD4; NDUFS5; complex I; mitochondrial disulfide relay
  4. Proc Natl Acad Sci U S A. 2022 Jul 26. 119(30): e2205228119
      The mitochondrial electron transport chain maintains the proton motive force that powers adenosine triphosphate (ATP) synthesis. The energy for this process comes from oxidation of reduced nicotinamide adenine dinucleotide (NADH) and succinate, with the electrons from this oxidation passed via intermediate carriers to oxygen. Complex IV (CIV), the terminal oxidase, transfers electrons from the intermediate electron carrier cytochrome c to oxygen, contributing to the proton motive force in the process. Within CIV, protons move through the K and D pathways during turnover. The former is responsible for transferring two protons to the enzyme's catalytic site upon its reduction, where they eventually combine with oxygen and electrons to form water. CIV is the main site for respiratory regulation, and although previous studies showed that steroid binding can regulate CIV activity, little is known about how this regulation occurs. Here, we characterize the interaction between CIV and steroids using a combination of kinetic experiments, structure determination, and molecular simulations. We show that molecules with a sterol moiety, such as glyco-diosgenin and cholesteryl hemisuccinate, reversibly inhibit CIV. Flash photolysis experiments probing the rapid equilibration of electrons within CIV demonstrate that binding of these molecules inhibits proton uptake through the K pathway. Single particle cryogenic electron microscopy (cryo-EM) of CIV with glyco-diosgenin reveals a previously undescribed steroid binding site adjacent to the K pathway, and molecular simulations suggest that the steroid binding modulates the conformational dynamics of key residues and proton transfer kinetics within this pathway. The binding pose of the sterol group sheds light on possible structural gating mechanisms in the CIV catalytic cycle.
    Keywords:  complex IV; cryo-EM; electron transport chain; kinetics; molecular simulations