Membrane Protein Structures
Two common structural classes of transmembrane proteins are alpha-helical bundles and beta-barrels. The portion of the membrane proteins that are attached to the lipid bilayer are consisting of hydrophobic amino acids only. This is done so that the peptide bonds' carbonyl and amine will react with each other instead of the hydrophobic surrounding. The portion of the protein that is not touching the lipid bilayer and is protruding out of the cell membrane are usually hydrophilic amino acids.
The structures of membrane proteins are stabilized by weak interactions and influenced by additional interactions with the solubilizing environment. The influence of the environment on membrane protein structures is especially significant. Despite the significant functional importance of membrane proteins, the structural biology has been particularly challenging as shown by the low number of membrane protein structures determined. Integral membrane proteins are present in a heterogeneous environment that poses major obstacles for existing structural methodologies.
Many of the successful membrane protein structures are characterized by X-ray crystallography and are very large structures in which the interactions with the membrane mimetic environments can be anticipated to be small in comparison to those within the protein structures. The small domains are particularly sensitive to the influence of membrane mimetic environments, potentially leading to non-native structures. Fortunately, there are many sample preparation conditions that can be chosen for crystallization and for solution NMR. All membrane protein structural biology should be subjected to careful scrutiny; through a combination of structural methodologies it should be possible to achieve an understanding of the native functional state for membrane protein structures.
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