Possible role of the Complex III - bound cardiolipin
dimer in initiation of mitochondrial lipid peroxidation

D.A. Cherepanov
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky Av. 31, Moscow

The bottleneck of chain free-radical oxidation of lipid membranes is the hydrogen atom transfer in the course of radical propagation reaction
LOO˙ +LH → LOOH + L˙ (1)
It is a slow process, for polyunsaturated linoleate acid its rate constant is as small as 60 M-1s-1. The mitochondrial membrane contains a large amount of native antioxidant ubiquinone, the rate constant of its action
LOO˙ + UQH2 → LOOH + UQH˙ (2)
is as high as 3x105 M-1s-1. Because the concentration of ubiquinone in mitochondrial membrane is comparable with the concentration of polyunsaturated lipid chains, ubiquinone protects the peroxidation of lipids with high efficiency.
The main source of polyunsaturated hydrocarbon chains in the inner mitochondrial membrane is cardiolipin (CL), one molecule of CL carries usually four linoleate chains (eight unsaturated bonds). A large fraction of CL in the inner mitochondrial membrane is not free but tightly bound to various proteins. We analyzed molecular structures of membrane protein complexes containing CL, namely, ADP/ATP carrier, II complex (succinate dehydrogenase), III complex (cytochrome bc1 complex), IV complex (cytochrome c oxidase), and formate dehydrogenase. In all cases, with the exception of III complex, CL is bound at the protein/membrane interface where it is easily reached for the direct interaction with ubiquinone in the lipid phase. The exception is the structure of III complex, where the CL binding site is deeply buried into proteins in the vicinity of the Qi site. In the structure of bovine cytochrome bc1 complex (1PP9), the site contains two CL molecules and one molecule of phosphatidilcholine, whereas in the structure of yeast enzyme (3CX5), the site contains one CL molecule and one molecule of phosphatidilcholine. An essential difference in the two structures is the accessibility of the interior of CL binding sites for the external lipid phase. In the case of bovine enzyme, the binding site is completely covered by protein and represents a well, where the bound CL is protected from the direct interaction with the lipid phase. To the contrary, in the structure of yeast enzyme, the binding site is open and CL is reached for the interaction with ubiquinone diffusing in the lipid phase.
Cytochrome bc1 complex is a major producer of superoxide radical in the mitochondrial membrane, the radicals are generated at the Qi site just in the vicinity of the CL binding site. Some features of the CL binding site in the bovine enzyme point out that it might function as an initiator of the mitochondrial membrane oxidation: (1) eight linoleate chains are tightly packed in the site so that the local concentration of polyunsaturated bonds is an order of magnitude higher than that in the lipid phase on the average (increased oxidation rate); (2) the chains of bound CL could not directly interact with ubiquinone coming from the lipid phase (decreased antioxidant protection). As a consequence, appearance of the first peroxide radical in the CL binding site would result in a fast oxidation of all eight linoleate chains to their peroxide derivatives. It is noteworthy, that in yeast mitochondria such function of the CL binding site is apparently missing, that correlates with the absence of polyunsaturated acids in yeast membranes.
Oxidation of CL in the binding site of the bc1 complex might induce some conformational changes, which could increase the superoxide production by III complex. In such a case peroxidation of CL in III complex is the primary trigger of apoptotic cascade coordinated by mitochondria.

Homo Sapiens Liberatus Workshop, Moscow State University, May 2010