Two separate though interconnected routes underlie NADH and succinate oxidation: kinetic evidence for different functional compartments of Coenzyme Q and/or Complex III

The discovery of respiratory supercomplexes (SCs) led to the proposal that electron transfer between complexes I and III (CI, CIII) is mediated by channelling of Coenzyme Q (Q), with a kinetic advantage on the transfer based on random collisions, whereas electron transfer from CII to CIII obeys to the random collision model. The evidence for Q channelling, however, is highly controversial [1, 2]. We have approached the problem in bovine heart submitochondrial particles and in reconstituted proteoliposomes in which CI and CIII are preserved as SC I1III2. We restricted electron transfer to the Q area by studying NADH and succinate oxidation by exogenous cytochrome c (cyt. c) as acceptor, thus avoiding the bottleneck of endogenous cyt. c. Using this system we found the rates of NADH and succinate oxidation by cyt. c to be almost completely additive. The rate obtained by simultaneous addition of NADH and succinate was much higher than that predicted for a homogeneous Q pool [3], thus suggesting that NADH and succinate oxidation by cyt. c follow two different routes. The NADH route presumably operates through Q channelling in the SC I1III2. However Qpool molecules may exchange with Qbound in SC, approaching the rates predicted for a single pool, when the reducing pressure increases by strong CIII inhibition or when detergents destabilize the SCs. The accessibility of Qpool to SC I1III2 may be a physiological device to control electron fluxes from different substrates and implies a dissociation equilibrium of Qbound with the Q pool, by which the size of the pool determines saturation of the binding site(s) in the SC. Thus bulk Qpool has a role also in oxidation of NAD-linked substrates, providing a rationale for the beneficial effect of exogenous Q supplementation on mitochondrial bioenergetics. References 1. JN Blaza et al. Proc Natl Acad Sci USA 111 (2014) 15735-40. 2. G Lenaz et al. BBA Bioenerg. (2016) Epub ahead of print. 3. A Kröger, M Klingenberg. Eur J Biochem. 34 (1973) 358-68.