Structures of membrane proteins.

The possible conformations of integral membrane proteins are restricted by the nature of their environment. In order to satisfy the requirement of maximum hydrogen bonding, those portions of the polypeptide chain which are in contact with lipid hydrocarbon must be organized into regions of regular secondary structure. As possible models of the intramembranous regions of integral membrane proteins, three types of regular structures are discussed. Two, the alpha helix and the beta-pleated sheet, are regularly occurring structural features of soluble proteins. The third is a newly proposed class of conformations called beta helices. These helices have unique features which make them particularly well-suited to the lipid bilayer environment. The central segment of the membrane-spanning protein glycophorin can be arranged into a beta helix with a hydrophobic exterior and a polar interior containing charged amino-acid side chains. Such structures could function as transmembrane ion channels. A model of the activation process based on a hypothetical equilibrium between alpha and beta helical forms of a transmembrane protein is presented. The model can accurately reproduce the kinetics and voltage dependence of the channels in nerve.