title: Kinetic evidence against partitioning of the ubiquinone pool and the catalytic relevance of respiratory-chain supercomplexes.
abstract: In mitochondria, four respiratory-chain complexes drive oxidative phosphorylation by sustaining a proton-motive force across the inner membrane that is used to synthesize ATP. The question of how the densely packed proteins of the inner membrane are organized to optimize structure and function has returned to prominence with the characterization of respiratory-chain supercomplexes. Supercomplexes are increasingly accepted structural entities, but their functional and catalytic advantages are disputed. Notably, substrate "channeling" between the enzymes in supercomplexes has been proposed to confer a kinetic advantage, relative to the rate provided by a freely accessible, common substrate pool. Here, we focus on the mitochondrial ubiquinone/ubiquinol pool. We formulate and test three conceptually simple predictions of the behavior of the mammalian respiratory chain that depend on whether channeling in supercomplexes is kinetically important, and on whether the ubiquinone pool is partitioned between pathways. Our spectroscopic and kinetic experiments demonstrate how the metabolic pathways for NADH and succinate oxidation communicate and catalyze via a single, universally accessible ubiquinone/ubiquinol pool that is not partitioned or channeled. We reevaluate the major piece of contrary evidence from flux control analysis and find that the conclusion of substrate channeling arises from the particular behavior of a single inhibitor; we explain why different inhibitors behave differently and show that a robust flux control analysis provides no evidence for channeling. Finally, we discuss how the formation of respiratory-chain supercomplexes may confer alternative advantages on energy-converting membranes.
authors.affiliation: Medical Research Council Mitochondrial Biology Unit, Cambridge CB2 0XY, United Kingdom.
authors.forename: James N
authors.initials: JN
authors.lastname: Blaza
authors.affiliation: Medical Research Council Mitochondrial Biology Unit, Cambridge CB2 0XY, United Kingdom.
authors.forename: Riccardo
authors.initials: R
authors.lastname: Serreli
authors.affiliation: Medical Research Council Mitochondrial Biology Unit, Cambridge CB2 0XY, United Kingdom.
authors.forename: Andrew J Y
authors.initials: AJ
authors.lastname: Jones
authors.affiliation: Medical Research Council Mitochondrial Biology Unit, Cambridge CB2 0XY, United Kingdom.
authors.forename: Khairunnisa
authors.initials: K
authors.lastname: Mohammed
authors.affiliation: Medical Research Council Mitochondrial Biology Unit, Cambridge CB2 0XY, United Kingdom jh@mrc-mbu.cam.ac.uk.
authors.forename: Judy
authors.initials: J
authors.lastname: Hirst
mesh_terms: D000818:Animals; D002417:Cattle; D004579:Electron Transport / Q000502:physiology; D042967:Electron Transport Complex I / Q000737:chemistry* / Q000378:metabolism; D007700:Kinetics; D008929:Mitochondria, Heart / Q000201:enzymology*; D008956:Models, Chemical; D009243:NAD / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction; D019802:Succinic Acid / Q000737:chemistry* / Q000378:metabolism; D014451:Ubiquinone / Q000737:chemistry* / Q000378:metabolism
chemical_list: D009243:NAD; D014451:Ubiquinone; D019802:Succinic Acid; D042967:Electron Transport Complex I
keywords: mitochondria; respirasome; respiratory chain; supercomplex; ubiquinone