Structural and functional analysis of supercomplex I+III

There are several experimental evidences of the supramolecular organization of the mitochondrial respiratory chain in supercom- plexes [1-2], but only few works deal with its functional implica- tion. Indeed supercomplex organization provides a kinetic advantage through the direct channeling of electrons between the enzyme components and it can be postulated that the presence of supercomplexes may limit the formation of radical oxygen species (ROS) from the respiratory chain [3]. A methodological approach by metabolic flux control analysis used in our laboratory allowed to demonstrate that Complex I and Complex III act as one enzy- matic unit in mammalian mitochondria [4]. Starting from these previous observations, our aim is to investi- gate the characteristics of supercomplex I+III and to evaluate the functional implications of its disassembling. For this purpose we analyzed the structure, the kinetic properties and the produc- tion of ROS after inducing supercomplex I+III dissociation in two experimental models: (i) in proteoliposomes enriched in Complex I and Complex III with different protein to lipid ratios, we observed that supercomplex I+III dissociation occurs in the presence of an excess of phospholipids or if lipid peroxidation is chemically induced; (ii) in mitochondrial membranes, the I+III super-assembly is loss after treatment with chaotropic agents. Kinetic analysis showed lower efficiency with loss of channelling; on the other hand preliminary results on ROS production showed changes in the amount and sensitivity to Complex I inhibitors. References: 1. Schägger H. IUBMB Life 2001; 52: 119–28. 2. Schäfer E et al. Biochemistry 2007; 46:12579–85. 3. Lenaz G et al. Antioxidants & Redox Signaling 2010; 12: 961–1008. 4. Bianchi C et al. J Biol Chem 2004; 279: 36562–9.