Architecture of Human Mitochondrial Respiratory Megacomplex I2III2IV2

Abstract

The respiratory megacomplex represents the highest-order assembly of respiratory chain complexes, and it allows mitochondria to respond to energy-requiring conditions. To understand its architecture, we examined the human respiratory chain megacomplex-IIIIIV (MCIIIIIV) with 140 subunits and a subset of associated cofactors using cryo-electron microscopy. The MCIIIIIV forms a circular structure with the dimeric CIII located in the center, where it is surrounded by two copies each of CI and CIV. Two cytochrome c (Cyt.c) molecules are positioned to accept electrons on the surface of the c state CIII dimer. Analyses indicate that CII could insert into the gaps between CI and CIV to form a closed ring, which we termed the electron transport chain supercomplex. The structure not only reveals the precise assignment of individual subunits of human CI and CIII, but also enables future in-depth analysis of the electron transport chain as a whole.

Mitochondrial Supercomplexes Do Not Enhance Catalysis by Quinone Channeling

“…In contrast, recent structures of the respirasome not only show minimal inter-protein contact between CI and CIII, but also that the quinone/quinol-binding sites are a substantial ∼100 Å distance apart and not oriented to facilitate channeling (Bauler et al., 2010, Gu et al., 2016, Guo et al., 2017, Letts et al., 2016). …”

“…An even larger assembly than the respirasome, the so-called megacomplex, CI 2 :CII 2 :CIII 2 :CIV 2 , was recently proposed and suggested to contain a sealed-in quinone pool for maximum catalytic effectiveness (Guo et al., 2017). In a less restrictive model it has also been proposed that, although exchange with the outside is not precluded, under high turnover conditions quinone/quinol react preferentially within the supercomplex (Lenaz et al., 2016, Letts et al., 2016) due to the enzyme proximities.…”

“…In a less restrictive model it has also been proposed that, although exchange with the outside is not precluded, under high turnover conditions quinone/quinol react preferentially within the supercomplex (Lenaz et al., 2016, Letts et al., 2016) due to the enzyme proximities. Alternatively, recent structures of the mammalian respirasome clearly show that the substrate-binding sites in CI and CIII are separated by ∼100 Å, with no mediating protein to facilitate channeling between them (Gu et al., 2016, Guo et al., 2017, Letts et al., 2016, Milenkovic et al., 2017, Wu et al., 2016). Similarly, the structures reveal no barriers to the free diffusion of cytochrome c (Letts et al., 2016, Milenkovic et al., 2017), which has been shown by biophysical methods to diffuse freely along the membrane and not be localized to a single supercomplex (Trouillard et al., 2011).…”

Respiratory supercomplexes act as a platform for complex III ‐mediated maturation of human mitochondrial complexes I and IV

“…The quaternary structure of the complex is always dimeric (cIII 2 ), with each monomer being composed of ten different subunits (Iwata et al, 1998;Berry et al, 1999Berry et al, , 2000, only one of which (MT-CYB) is encoded by the mitochondrial genome (mtDNA). Structurally, cIII 2 is part of all known respiratory supercomplexes (SCs), where it physically interacts with both cI and cIV in the SCs cI+cIII 2 +cIV n (Schagger & Pfeiffer, 2000), structures known as "respirasomes" because they are in principle able to transfer electrons from NADH to O 2 (Acin-Perez et al, 2008;Gu et al, 2016;Letts et al, 2016;Wu et al, 2016;Guo et al, 2017). It is well known that severe cIII 2 deficiency in patients carrying null mutations in genes encoding some cIII 2 structural components and assembly factors are associated with a concomitant decrease in cI activity (Lamantea et al, 2002;Bruno et al, 2003;Acin-Perez et al, 2004;Barel et al, 2008;Tucker et al, 2013;Carossa et al, 2014;Feichtinger et al, 2017) and, in some cases, in cIV activity as well (Carossa et al, 2014).…”

Electron cryomicroscopy as a powerful tool in biomedical research

“…Many structures were determined for a large number of medically important channels and transporters that are closely linked to various diseases [63–65]. In addition, extraordinarily large membrane protein complexes, in particular those ones involved in respiration and photosynthesis, were structurally characterized by cryo-EM [66, 67]. A good example of both a channel and a large membrane protein complex is the ryanodine receptor, a key mediator of calcium release from the sarcoplasmic reticulum, initiating muscle contraction.…”