Mitochondrial supercomplexes do not enhance catalysis by quinone channeling

Justin G Fedor,Judy Hirst

doi:10.1016/j.bbabio.2018.09.152

Justin G Fedor, Judy Hirst

Open Access

https://doi.org/10.1016/j.bbabio.2018.09.152

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Abstract

Summary Mitochondrial respiratory supercomplexes, comprising complexes I, III, and IV, are the minimal functional units of the electron transport chain. Assembling the individual complexes into supercomplexes may stabilize them, provide greater spatiotemporal control of respiration, or, controversially, confer kinetic advantages through the sequestration of local quinone and cytochrome c pools (substrate channeling). Here, we have incorporated an alternative quinol oxidase (AOX) into mammalian heart mitochondrial membranes to introduce a competing pathway for quinol oxidation and test for channeling. AOX substantially increases the rate of NADH oxidation by O2 without affecting the membrane integrity, the supercomplexes, or NADH-linked oxidative phosphorylation. Therefore, the quinol generated in supercomplexes by complex I is reoxidized more rapidly outside the supercomplex by AOX than inside the supercomplex by complex III. Our results demonstrate that quinone and quinol diffuse freely in and out of supercomplexes: substrate channeling does not occur and is not required to support respiration.

Highlights

The close association of the enzymes in supercomplex assemblies has been suggested to confer a kinetic advantage on respiration by trapping or channeling quinone to enhance its transfer between the enzymes in the supercomplex, creating an independent, local quinone pool that does not exchange with the quinone pool outside
Kinetic experiments have shown that CI and complex II (CII, succinate:ubiquinone oxidoreductase) are both able to reduce all the CIII present, suggesting they do not interact with separate quinone pools (Blaza et al, 2014)
Addition of alternative quinol oxidase (AOX) to Mammalian Respiratory Membranes Shows that Quinone/Quinol Is Not Channeled If respiratory-chain supercomplex assemblies contain the quinone/quinol substrate and channel it between their component enzymes, a competing quinol oxidase, outside the supercomplex structure, should not be able to turn over (Bulutoglu et al, 2016; Wheeldon et al, 2016) (Figure 1)

Summary

Introduction

Mitochondrial respiration, catalyzed predominantly by supermolecular assemblies of respiratory complex I (CI, NADH:ubiquinone oxidoreductase), complex III (CIII, ubiquinol:cytochrome c oxidoreductase), and complex IV (CIV, cytochrome c oxidase), is at the center of cellular bioenergetics (Lenaz et al, 2016; Letts and Sazanov, 2017; Lobo-Jarne and Ugalde, 2018; Milenkovic et al, 2017). Kinetic experiments have shown that CI and complex II (CII, succinate:ubiquinone oxidoreductase) are both able to reduce all the CIII present, suggesting they do not interact with separate quinone pools (Blaza et al, 2014)

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Discussion

Conclusion