Hydrophobic mismatch, membrane-protein interactions

One major source of membrane-protein interactions that has been discussed in the literature for many decades is hydrophobic mismatch [20-22, 24, 26, 28, 29, 226-233]. If the width of the hydrophobic transmembrane domain of a protein is larger than the thickness of the lipid bilayer, the system can respond in two ways either the protein tilts [234-236] or the membrane deforms [18, 23, 24]. Both responses have biologically relevant consequences. On the one hand, the... 

The entire thermodynamic system of the membrane and TM protein must be considered to understand how the protein and bilayer achieve their native state. We have summarized four of the mechanisms, hydrophobic matching, tilt angles, and specific protein/lipid and protein/protein interactions that are important in determining the stability (Fig. 5). Other important factors, such as the stability of lipid/lipid interactions, have been left out of our protein-centric view. We describe a hydrophobic mismatch as an unfavorable interaction that can be relieved by the other three processes, but we would expect all these properties of the system to interact. We could easily describe the same equilibria by saying that a strain in curvature is relieved by a hydrophobic mismatch or that strong protein/protein packing interactions might help relieve the hydrophobic mismatch or curvature stress. The complex interplay between all these interactions is at the heart of what determines membrane protein stability and will no doubt be difficult to quantify. 

These fields differ quite substantially in their theoretical description concentrations are scalar variables, orientations are vectors, and differential geometry is at heart a tensor theory but, aU of them are known to mediate interactions. For instance, the fact that proteins might prefer one lipid composition over another and thus aggregate [217-220] is central to an important mechanism attributed to lipid rafts. Tilt-mediated protein interactions have also been studied in multiple contexts [32, 33, 159, 221-223]. It is even possible to describe all these phenomena within a common language [224], using the framework of covariant surface stresses [154, 155, 157-161]. However, in the present review we will restrict the discussion to only two examples, both related to membrane elasticity in Sect. 3.1 we will discuss interactions due to hydrophobic mismatch, and in Sect. 3.2 we will look at interactions mediated by the large-scale curvature deformation of the membrane. 

To reduce the hydrophobic mismatch, either the structure of membrane protein or the lipid membrane has to be altered [12]. In this chapter, QCM-D is used to investigate the adsorption of poly(ethylene glycol) (PEG) with different hydrophobic end groups onto the phospholipid membranes to clarify the hydro-phobic interactions between the end group of polymer chains and the core of hpid membranes. 

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