Lipid Bilayer-Membrane Protein Coupling - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 78

Fig. 5.2 The bilayer deforms near the gramicidin A channels where the channels are considered
to be coupled with the host bilayer incurring an energetic cost [ 13 ]. The upper panel shows a lipid
bilayer without any integral membrane protein, which is why the bilayer exists with an average
bilayer thickness d 0 (see Fig. 4.2 ) . The lower panel shows a bilayer with an integral gramicidin A
channel. For simplicity, we use two blocks in the figure to represent a gramicidin A dimer. The
channel's length l can differ, depending on the type of monomers participating in constructing the
channel (see Fig. 4.2 ) . d 0 is the unperturbed thickness of the bilayer, and is on the order of 4-5 nm in
hydrocarbon-containing lipid bilayers. The value of d 0 depends highly on the type of hydrocarbons
residing inside the bilayer. In the model lipid membrane construction, decane or squalene are usually
used. Decane accounts for a relatively thicker bilayer than squalene. By varying the lipid acyl chain
lengths, the bilayer thickness can be varied. The channel length l is on the order of 2 nm depending
on the number of amino acid sequences involved in constructing the gramicidin A monomers. The
channels form a rigid structure which means that the lengths are almost constant
might act as a molecular force transducer. The presence of ion channels inside a
lipid membrane sometimes takes the form of a molecular force transducer, such as
gramicidin A, in a lipid bilayer (see Fig. 5.2 ). The gravitational push (for example by
a drug) on the membrane, or a pull exerted on the lipid layers by integral membrane
proteins due to their hydrophobic coupling with lipid layers, may create considerable
permanent or stable deformations. These deformations may sometimes cause drastic
effects, such as a local breaking in the bilayer structure, or an induction of substan-
tial flow of ions through pores or channels. Figure 5.4 schematically illustrates the
equilibrium condition which can take the deformed form presented in Fig. 5.2 for
gramicidin A channel. It is important to understand whether bilayer elasticity can