Lipids hydrophobic interactions with

FIGURE 11-3 Fluid mosaic model for membrane structure. The fatty acyl chains in the interior of the membrane form a fluid, hydrophobic region. Integral proteins float in this sea of lipid, held by hydrophobic interactions with their nonpolar amino acid side chains. Both proteins and lipids are free to move laterally in the plane of the... 

Integral Proteins Are Held in the Membrane by Hydrophobic Interactions with Lipids... 

It appears that most of the protons that do not exchange are not shielded from water by hydrophobic interaction with lipids since the fraction of exchangeable protons in the amide bonds of residual protein after pronase treatment is approximately the same as in intact ghosts. Indeed this result, together with ORD data, suggests that there is no difference in the gross conformations of the enzymatically accessible and inaccessible protein. Proton exchange is probably inhibited because... 

Protein Release. Biomembranes consist of lipids and proteins. The latter may be subdivided into so-called intrinsic and extrinsic proteins (49). Intrinsic proteins supposedly are integrated into the membrane phase primarily by the hydrophobic interaction with lipids. Extrinsic proteins are attached to the membranes. Ionic interactions are believed to be important in the binding of extrinsic proteins. When these proteins dissociate from the membrane, they may be sufficiently hydrophilic to be soluble in the aqueous phase. When freeze-aggregated thylakoids are sedimented, a number of membrane proteins are found in the supernatant fluid. Among them are catalytic proteins involved in energy conservation and electron transport (42,48). The total amount of proteins released depends on freezing conditions and the solute environment, but may be as much as 5% of the total membrane protein (48). When frozen in the presence of a cryoprotective solute, at a sufficient concentration, thylakoids remain functional and do not release proteins in significant amounts. Protein release thus accompanies membrane injury and, in fact, is an indication of such injury. 

Soluble proteins exhibit hundreds of distinct localized folded structures, or motifs (see Figure 3-6). In comparison, the repertoire of folded structures in Integral membrane proteins Is quite limited, with the hydrophobic a helix predominating. Integral proteins containing membrane-spanning ct-hellcal domains are embedded in membranes by hydrophobic Interactions with specific lipids and probably also by Ionic Interactions with the polar head groups of the phospholipids. 

We simultaneously incorporate both lipid and protein by using dialysis to remove detergent from a solubilized lipid-protein mixture in the presence of the alkylsilanated substrate. Under our conditions, from the evidence in this paper and elsewhere (9), the surface structures appear to be single bilayer membranes. Our hypothesis is that the hydrocarbon chains attached to the surface serve as initiation sites for a lipid bilayer membrane to form as the detergent is slowly removed. The model is of a membrane that is anchored to the surface by hydrophobic interactions with the surface-bound hydrocarbon layer. Integral membrane proteins are retained in these structures by their interaction with the hydrophobic core of the membrane without being directly attached to the electrode surface. 

Neuraminidase isolated from the culture filtrate of Arthrobacter ureafaciens has been characterized in detail with respect to its action on glycolipids. Strong electrolytes had a reversible inhibitory effect on the action of the enzyme on brain gangliosides in accordance with Debye-Hiickel effect of ionic environment on ionic activity, and resulted in an acidic shift and a broadening of the pH optimum. Both ionic and non-ionic detergents markedly enhanced the activity of the thiol-sensitive enzyme on the gangliosides, and caused an acidic shift of the pH optimum. It was suggested that the hydrophobic ceramide moiety increases affinity of the lipid substrate to the enzyme, but inhibits hydrolysis of the substrate, possibly due to its hydrophobic interaction with hydrophobic portions of the enzyme molecule. 

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