Substrate channeling versus free quinone pool exchange in supercomplexes

Sousa and colleagues (2016) summarised the case for channeling. They wrote that advantages from partitioning of the quinol pool had been postulated and were supported by several independent studies.¹ They also noted that other results did not fit a partitioned pool. In the bovine respirasome they found that only one complex III monomer sits well placed to accept quinol from complex I. They suggested this may explain why only that monomer is active.² Channeling was therefore treated as an open question, not a settled one.

Fedor and Hirst (2018) argued that the hypothesis survived because it gave a satisfying reason for supercomplexes to exist, and because the kinetic experiments against it were hard for many readers to follow.³ They offered a simpler test. They added an alternative quinol oxidase to heart mitochondrial membranes, giving quinol a second route out. NADH oxidation sped up, and the supercomplexes stayed intact. They concluded that quinone and quinol diffuse freely in and out of supercomplexes and that channeling does not occur.⁴

Letts and colleagues (2019) worked with a purified, defined system. They reported that complex I runs better when quinone can exchange with the membrane pool, and that trapping would limit complex I turnover.⁵⁶ They added that the Q-cycle requires complex III to oxidise two quinols for each quinone that complex I reduces. Matching the two rates strictly may be hard.⁵ Caruana and Stroud (2020) read the structures the same way. They noted that the quinone sites of complexes I and III lie about 100 Å apart.⁷

Brzezinski, Moe and Ädelroth (2021) stated that early channeling hypotheses had been rejected.⁸ They still allowed that cytochrome c may move in two dimensions along the supercomplex surface. They held that this is compatible with unrestricted diffusion, because the binding is weak and cytochrome c stays in equilibrium with its pool.⁹ Letts and colleagues listed other possible roles for supercomplexes, such as stabilising the complexes and reducing reactive oxygen species. They noted that closeness of active sites could help without any trapping.⁶