Di Trani, J. | bioRxiv Science

bioRxiv2021

Rieske head domain dynamics and indazole-derivative inhibition of Candida albicans complex III

During cellular respiration, electron transfer between the integral membrane protein complexes of the electron transport chain is coupled to proton translocation across the inner mitochondrial membrane, which in turn powers synthesis of ATP and transmembrane transport processes. The homodimeric electron transport chain Complex III (CIII2) oxidizes ubiquinol (UQH2) to ubiquinone (UQ), transferring electrons to cytochrome c, and translocating protons through a mechanism known as the Q cycle. The Q cycle involves UQH2 oxidation and UQ reduction at two different sites within each CIII monomer, as well as movement of the head domain of the Rieske subunit. We used cryoEM to determine the structure of CIII2 from Candida albicans, revealing density for endogenous UQ in the structure and allowing us to directly visualize the continuum of conformations of the Rieske head domain. Analysis of these conformations does not indicate cooperativity in the position of the Rieske head domains or binding of ligands in the two CIIIs of the CIII2 dimer. CryoEM with the indazole derivative Inz-5, which inhibits fungal CIII2 and is fungicidal when administered with fungistatic azole drugs, showed that inhibition by Inz-5 alters the equilibrium of the Rieske head domain positions.

biochemistry↗

bioRxiv2020

Cryo-EM structure and kinetics reveal electron transfer by 2D diffusion of cytochrome c in the yeast III-IV respiratory supercomplex

Energy conversion in aerobic organisms involves an electron current from low-potential donors, such as NADH and succinate, to dioxygen through the membrane-bound respiratory chain. Electron transfer is coupled to transmembrane proton transport that maintains the electrochemical proton gradient used to produce ATP and drive other cellular processes. Electrons are transferred between respiratory complexes III and IV (CIII and CIV) by water-soluble cyt. c. In S. cerevisiae and some other organisms, these complexes assemble into larger CIII2CIV1/2 supercomplexes, the functional significance of which has remained enigmatic. In this work, we measured the kinetics of the S. cerevisiae supercomplexs cyt.c-mediated QH2:O2 oxidoreductase activity under various conditions. The data indicate that the electronic link between CIII and CIV is confined to the surface of the supercomplex. Cryo-EM structures of the supercomplex with cyt. c reveal distinct states where the positively-charged cyt. c is bound either to CIII or CIV, or resides at intermediate positions. Collectively, the structural and kinetic data indicate that cyt. c travels along a negatively-charged surface patch of the supercomplex. Thus, rather than enhancing electron-transfer rates by decreasing the distance cyt. c must diffuse in 3D, formation of the CIII2CIV1/2 supercomplex facilitates electron transfer by 2D diffusion of cyt. c. This mechanism enables the CIII2CIV1/2 supercomplex to increase QH2:O2 oxidoreductase activity and suggests a possible regulatory role for supercomplex formation in the respiratory chain. Significance StatementIn the last steps of food oxidation in living organisms, electrons are transferred to oxygen through the membrane-bound respiratory chain. This electron transfer is mediated by mobile carriers such as membrane-bound quinone and water-soluble cyt. c. The latter transfers electrons from respiratory complex III to IV. In yeast these complexes assemble into III2IV1/2 supercomplexes, but their role has remained enigmatic. This study establishes a functional role for this supramolecular assembly in the mitochondrial membrane. We used cryo-EM and kinetic studies to show that cyt. c shuttles electrons by sliding along the surface of III2IV1/2 (2D diffusion). The structural arrangement into III2IV1/2 supercomplexes suggests a mechanism to regulate cellular respiration.

biochemistry↗