Cytochrome oxidases interaction with lipids

The role of these interesting plasma membrane-dependent, vanadate-stimulated NAD(P)H oxidation reactions in cellular metabolism remains to be elucidated, although multiple interactions with cellular metabolism and components are possible including interactions with xanthine oxidase and lipid peroxidation [24], Decavanadate has been shown to enhance cytochrome c reduction [31], and cytochrome c release from mitochondria is associated with initiation of apoptosis. Perhaps the reduced cytochrome c is more readily released from the mitochondria. With increasing emphasis on the redox properties of vanadium being important in its pharmacological effects, it is quite possible that these reactions, either protein dependent or not, may play a role in therapeutic actions of vanadium. 

Rytomaa, M., and Kinnunen, P.KJ., 1995, Reversibility of the binding of cytochrome c to liposomes. Implications for lipid-protein interactions. /. Biol. Chem., 270 3197-3202 Salamon, Z., and Tollin, G., 1996, Surface plasmon resonance studies of complex formation between cytochrome c and bovine cytochrome c oxidase incorporated into a supported planar lipid bilayer. II. Binding of cytochrome c to oxidase-containing cardiolipin /phosphatidylcholine membranes. Biophys. J., 71 858-867 Salamon, Z., and Tollin, G., 1997, Interaction ofhorse heart cytochrome c with lipid bilayer membranes effects on redox potentials. J. Bioenerg. Biomembr. 29 211-221 Scarlett, J.L., and Murphy, M.P., 1997, Release of apoptogenic proteins from the... 

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... 

Fig. 6, left shows an end view of a type-I crystal formed by stacking two-dimensional crystal layers, ordered sheets of proteins. Many proteins, but not all, can form such a two-dimensional crystal layer, in which the hydrophobic regions of the proteins interact with the hydrocarbon tails of the lipids, the two-dimensional structure being stabilized by both hydrophobic and polar interactions. In each two-dimensional crystal layer no detergent is present and only the polar domains are exposed at the surface. These two-dimensional crystal layers then stack up to form a three-dimensional crystal through polar attractions between the layers. In three-dimensional crystals, the successive two-dimensional crystal layers need to be ordered in the third dimension with respect to translation, rotation and up-down orientation. Examples of type-I crystals which have been prepared are mitochondrial cytochrome oxidase, chloro-plastChl-a/ proteins, and a protein from the purple membrane ofhalobacteria. Two-dimensional crystals are usually rather small and useful only for examination by electron microscopy. 

Purified preparations of cytochrome oxidase are unstable and researchers have to deal, as a rule, with submitochondrial particles including, together with cytochrome oxidase, a part of the lipid membrane. The enzyme contains heme and copper in equimolar quantities as prosthetic groups. It apparently reacts with cytochrome c due to electrostatic interaction interaction with oxygen is limited by the latter s rate of diffusion. 

The situation is even worse for membrane lipids. Not a single, naturally occurring phospholipid with unsaturated hydrocarbon chains has yet been crystallized. However, nearly 40 crystal structures of closely related synthetic glycerolipids with saturated hydrocarbon chains have been solved by X-ray. On the structural level, little is known about the interactions of proteins with lipid bilayer environments. Detergent molecules have been detected in some of the X-ray structures, and a small number of studies discuss lipids bound to proteins. An example is cytochrome C oxidase crystals, where the lipids were found to be arranged in a bilayer structure. 

The interaction of cytochrome C oxidase with lipid membranes has been investigated by means of spin-label... 

At low coverages, thiol molecules are mobile on gold and silver. With increasing surface coverage, islands with the alkane carbon chain tails parallel to the surface plane form and nucleate to cover the surface. Islands with the alkane tails normal to the surface plane consequently form and nucleate to form a complete mono-layer structure. If thiol islands exist on the substrates used in this work prior to lipid deposition, lateral hydrophobic interactions between alkane carbon tails and the lipids, deoxycholate, and cytochrome c oxidase may disperse the islands, leading to a thiol submonolayer with the thiol molecules more evenly spaced over the electrode surface. 

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