Mitochondrial lipid-protein complexes

Mitochondria are the main source of free radicals in the cell and, in turn, ROS can cause inhibition of complex enzymes in the electron transport chain of the mitochondria leading to the shutdown of energy production and amplifying generation of mitochondrial free radicals (Orrenius, 2007). Free radicals can then cause extensive cellular damage by causing oxidation of lipids, proteins, and DNA. 

The first condition is met by having a series of four protein complexes, inserted into the mitochondrial inner membrane, each made up of a number of electron (and sometimes proton) acceptors of increasing redox potential. Three of them (Complexes I, III, and IV) are presented in cartoon form in Figure 5.18. Complex I, referred to more prosaically as NADH-Coenzyme Q oxidoreductase, transfers electrons stepwise from NADH, through a flavo-protein (containing FMN as cofactor) to a series of iron—sulfur clusters (of which more in Chapter 13) and ultimately to coenzyme Q, a lipid-soluble quinone, which transfers its electrons to Complex III. The AE o for the couple NADH/CoQ is 0.36 V, corresponding to a AG° of —69.5 kJ/mol, and in the process of electron transfer, protons are exported into the intermembrane space (between the mitochondrial inner and outer membranes). 

A newly characterized apoA-I high-affinity binding site in the plasma membrane was the fS-chain of human ATP-synthase, a major protein complex of the mitochondrial inner membrane also present in the plasma membrane, involved in ATP synthesis (Martinez et al. 2003). It has two major domains, Fq and Fi, the latter containing five different subunits among which the fS-chain interacts with apoA-I. It has also been reported that both, the p-chain and a-chain of ATP synthase, are receptors for apoE-enriched HDL in the plasma membrane (Beisiegel et al. 1988). The possible involvement of the Fo/lj -ATPase in the lipid influx/efflux rheostat together with ABCAl and the participating receptors is shown in Fig. 4. 

The crystal structure of bovine cytochrome c oxidase (Complex IV) homodimer has been determined to a resolution of 1.8 A (Shinzawa-Itoh et al., 2007). This integral membrane protein complex composed of thirteen different subunits per monomer is responsible for the reduction of molecular oxygen to water during aerobic respiration, with concomitant proton pumping across the mitochondrial inner membrane. A combination of high resolution X-ray structure analysis of the integral lipids in bovine Complex IV with mass spectroscopy analysis of their chain lengths and the positions of the unsaturated bonds of the... 

In the electron transport chain, electrons donated by NADH or FAD(2H) are passed sequentially through a series of electron carriers embedded in the inner mitochondrial membrane. Each of the components of the electron transfer chain is oxidized as it accepts an electron, and then reduced as it passes the electrons to the next member of the chain. From NADH, electrons are transferred sequentially through NADH dehydrogenase (complex 1), CoQ (coenzyme Q), the cytochrome b-Ci complex (complex 111), cytochrome c, and finally cytochrome c oxidase (complex IV). NADH dehydrogenase, the cytochrome b-Ci complex and cytochrome c oxidase are each multisubunit protein complexes that span the inner mitochondrial membrane. CoQ is a lipid soluble quinone that is not protein-bound and is free to diffuse in the lipid membrane. It transports electrons from complex 1 to complex 111 and is an intrinsic part of the proton pumps for each of these complexes. Cytochome c is a small protein in the inner membrane space that transfers electrons from the b-Ci... 

Functionally and strucmrally, the components of the respiratory chain are present in the inner mitochondrial membrane as four protein-lipid respiratory chain complexes that span the membrane. Cytochrome c is the only soluble cytochrome and, together with Q, seems to be a more mobile component of the respiratory chain connecting the fixed complexes (Figures 12-7 and 12-8). 

Defects of nuclear DNA also cause mitochondrial diseases. As mentioned above, the vast majority of mitochondrial proteins are encoded by nDNA, synthesized in the cytoplasm and imported into the mitochondria through a complex series of steps. Diseases can be due to mutations in genes encoding respiratory chain subunits, ancillary proteins controlling the proper assembly of the respiratory chain complexes, proteins controlling the importation machinery, or proteins controlling the lipid composition of the inner membrane. All these disorders will be transmitted by mendelian inheritance. From a biochemical point of view, all areas of mitochondrial metabolism can be affected (see below). 

FIGURE 19-9 IMADH ubiquinone oxidoreductase (Complex I). Complex I catalyzes the transfer of a hydride ion from NADH to FMN, from which two electrons pass through a series of Fe-S centers to the iron-sulfur protein N-2 in the matrix arm of the complex. Electron transfer from N-2 to ubiquinone on the membrane arm forms QH2, which diffuses into the lipid bilayer. This electron transfer also drives the expulsion from the matrix of four protons per pair of electrons. The detailed mechanism that couples electron and proton transfer in Complex I is not yet known, but probably involves a Q cycle similar to that in Complex III in which QH2 participates twice per electron pair (see Fig. 19-12). Proton flux produces an electrochemical potential across the inner mitochondrial membrane (N side negative, P side positive), which conserves some of the energy released by the electron-transfer reactions. This electrochemical potential drives ATP synthesis. 

Cytochromes, as components of electron transfer chains, must interact with the other components, accepting electrons from reduced donor molecules and transferring them to appropriate acceptors. In the respiratory chain of the mitochondria, the ubiquinolxytochrome c oxidoreductase, QCR or cytochrome bc complex, transfers electrons coming from Complexes 1 and 11 to cytochrome c. The bc complex oxidises a membrane-localised ubiquinol the redox process is coupled to the translocation of protons across the membrane, in the so-called proton-motive Q cycle, which is presented in a simplified form in Figure 13.14. This cycle was first proposed by Peter Mitchell 30 years ago and substantially confirmed experimentally since then. The Q cycle in fact consists of two turnovers of QH2 (Figure 13.14). In both turnovers, the lipid-soluble ubiquinol (QH2) is oxidized in a two-step reoxidation in which the semiquinone CoQ is a stable intermediate, at the intermembrane face of the mitochondrial inner membrane. It transfers one electron to the Rieske iron—sulfur protein (ISP), one electron to one of the two cytochrome b haems (bi), while two protons are transferred to the intermembrane space. In both of the Q cycles, the cytochrome bi reduces cytochrome bfj while the Reiske iron—sulfur cluster reduces cytochrome c/. The cytochrome ci in turn reduces the water-soluble cytochrome c, which transfers its electrons to the terminal oxidase, cytochrome c oxidase, described above. In one of the two Q cycles, reduced cytochrome bf reduces Q to the semiquinone, which is then reduced to QH2 by the second reduced cytochrome bn- The protons required for this step are derived from the matrix side of the membrane. The overall outcome of the two CoQ cycles (10) (/ — matrix o — intermembrane space) is... 

---