Dielectric properties and membrane function
This chapter describes the fundamental electrostatic concepts that underlie the behaviour of intramembrane charge, which may regulate membrane function, and begins by considering simple, uncoupled, first-order systems, which provide fundamental physical expectations with which the behaviour of biological systems might be compared. The dielectric behaviour of biological systems is, however, very complex and cannot be adequately described by such simple systems. Some indication of the limiting behaviour that might be expected from regulatory membrane proteins could be derived from the known behaviour of simple polymer systems and, at a higher level, from parallels with soluble allosteric enzymes. This approach originates from the expectation that the orientation of molecular dipoles is often slow relative to changes in the applied field and may not involve a uniform switch in all the molecules. Regulatory membrane proteins are large molecules that possess a long range order and whose constituent dipoles are mechanically coupled. A convenient starting point for the analysis of responses of intramembrane charge to applied electric fields is to consider the behaviour of an insulating material within a parallel-plate capacitor. In striated muscle, contractile activation is voltage-dependent. However, in this case, passage of Ca2+ across the intracellular, sarcoplasmic reticular membrane system is modulated through potential changes across the anatomically close, but electrically remote, transverse tubular membrane. A biological membrane consists of a two-dimensional array of protein molecules, some of which (integral membrane proteins) are embedded in the lipid bilayer matrix. The simplest possible dielectric behaviour arising from a biological membrane subject to an electric field results from an intramembrane charge allowed to occupy either of only two positions within the membrane. Dielectric properties in striated muscle membrane arise from the lipid bilayer and its contained intramembrane proteins. The dielectric behaviour of the smaller proteins is dominated by the orientation of their contained permanent dipoles.
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