ADVANTAGES OF THE SUPRAMOLECULAR ORGANISATION OF THE RESPIRATORY CHAIN: KINETIC IMPROVEMENT AND LIMITED PRODUCTION OF REACTIVE OXYGEN SPECIES.

A large proportion of the mitochondrial respiratory chain complexes in a variety of organisms is arranged in supramolecular assemblies called supercomplexes. Supercomplex assembly between Complex I and Complex III is not just a mere structural feature since it confers a kinetic advantage by facilitating the electron transfer in the Coenzyme Q region (substrate channelling) more efficiently then any process based on diffusion and random collision of the acting partners. On the other hand, kinetic evaluation of NADH oxidase activity by Metabolic Control Analysis indicates that cytochrome c obeys pool behaviour in mammalian mitochondria, despite the presence of some molecules of Complex IV comprised in the supercomplex [1]. The forces responsible for the assembly of the respiratory supercomplexes are still little understood. We have shown that formation of supercomplexes by reconstitution of a binary I/III mitochondrial protein fraction strongly depends on the lipid content in the proteoliposomes and that peroxidation of the lipids prior to reconstitution prevents the formation of the supercomplex I1III2 even under the optimal condition of lipid to protein ratio. In addition, several studies demonstrated that supercomplex association is required for the stability and assembly of Complex I. Indirect considerations support the view that supercomplex association may also limit the formation of reactive oxygen species (ROS) from the respiratory chain. We have directly addressed this issue by testing the production of ROS by Complex I in two experimental systems in which the supercomplex I1III2 was dissociated: (i) bovine heart mitochondria or liposome-reconstituted Complex I/III treated with dodecyl-maltoside; (ii) liposome-reconstituted Complex I/III at high phospholipids to protein ratio. The results of our investigation support the view that disruption or prevention of the association between Complex I and Complex III by different means strongly enhances the generation of superoxide from Complex I both in reconstituted proteoliposomes and in bovine heart mitochondrial membranes. It is easy to foresee the implications of these findings in human diseases and in aging, where oxidative stress plays a major etiologic and pathogenic role. [1] C. Bianchi, M.L. Genova, G. Parenti Castelli, G. Lenaz, The mitochondrial respiratory chain is partially organized in a supercomplex assembly: kinetic evidence using flux control analysis, J. Biol. Chem. 279 (2004) 36562-9.