Cytochrome c oxidase in proteoliposomes visualised by platinum-carbon and by tungsten-tantalum shadowing: image analysis.

Beef heart cytochrome c oxidase complexes incorporated into phospholipid liposomes were examined by freeze-fracture electron microscopy. Enzyme molecules are inserted into the membrane asymmetrically, with larger projections on the 'C' side, where cytochrome c binding occurs, than on the 'M' (matrix-facing) side. Visualisation of the complexes was improved by: (i) image analysis, to determine details of size and shape, and (ii) tungsten-tantalum (W/Ta) rather than platinum-carbon (Pt/C) shadowing, which permits examination of smaller entities. Enzyme molecules are incorporated as dimers in the proteoliposomes. Some surface structural details of the embedded molecules can be discerned. Around each complex is seen a small area of modified lipid, the frozen annulus whose existence has been predicted with other methods.