Incorporation of Membrane Proteins into Polymeric Membranes

3 Incorporation of Membrane Proteins into Polymeric Membranes 

Biological membranes are always pictured as being very selective barriers separating different biochemical reaction compartments. This high performance transport specificity solely depends on the presence of membrane proteins embedded in the lipid matrix. On the other hand, most membrane proteins cease to function in the absence of lipids. In order to introduce biological transport abilities into artificial membrane systems protein-lipid interactions are of vital interest. The question is how the activity of membrane proteins is affected if they are placed into a polymeric environment. 

As an example of an asymmetric membrane integrated protein, the ATP synthetase complex (ATPase from Rhodospirillum Rubrum) was incorporated in liposomes of the polymerizable sulfolipid (22)24). The protein consists of a hydrophobic membrane integrated part (F0) and a water soluble moiety (Ft) carrying the catalytic site of the enzyme. The isolated ATP synthetase complex is almost completely inactive. Activity is substantially increased in the presence of a variety of amphiphiles, such as natural phospholipids and detergents. The presence of a bilayer structure is not a necessary condition for enhanced activity. Using soybean lecithin or diacetylenic sulfolipid (22) the maximal enzymatic activity is obtained at 500 lipid molecules/enzyme molecule. With soybean lecithin, the ATPase activity is increased 8-fold compared to a 5-fold increase in the presence of (22). There is a remarkable difference in ATPase activity depending on the liposome preparation technique (Fig. 41). If ATPase is incorporated in- 

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