The Membrane as a Transporter, Ion Channels and Membrane Pumps - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 68

4.6.5 The In-Plane Diffusion Model
In this model, the peptides are assumed to enter into phospholipid bilayers and disor-
der the hydrocarbon chains of adjacent phospholipid molecules. This creates bilayer
thinning or local disruption in bilayer packing. Figure 4.11 e is a schematic diagram
representing this model [ 13 ]. This bilayer perturbation requires minimal peptide
aggregation and the peptides are assumed to be aggregated partially. Usually, small
size peptides are the best candidates for this kind of bilayer disruption. Gramicidin
S is such a candidate [ 9 ]. As there is no trans-bilayer association of the peptides,
any stable bilayer pore formation (like those in Fig. 4.10 a-d) can be ruled out using
this model. Peptides either independently or by being aggregated partially reduce the
local bilayer thickness and lead to increased conductance. Therefore, any instanta-
neous conductance events (not stable channel-like) across a bilayer can be created.
Such an event was first reported due to gramicidin S [ 9 ] which has been described in
an earlier section. As the change in lipid packing is an important aspect, the mem-
brane's constituents, thickness, charge properties, etc., have a huge role in creating
any peptide-induced event that follows the in-plane diffusion model.
4.6.6 The Linear β -Helix
The gramicidin A channels, described earlier, do not fit into any of the models
described by Bechinger [ 13 ]. This unique structure which creates a linear dimer
called a 'linear β -helix' where the dimer's longitudinal edges hydrophobically cou-
ple with the lipid bilayer certainly defines an important and unique class. Figure 4.12
explains the model, drawn in light of the structure presented in Fig. 4.2 for the gram-
icidin A channel. Here, the dimer length can be smaller, of the order of the bilayer
thickness, or can even be larger than the bilayer thickness. In the case when the chan-
nel length is smaller than the bilayer thickness, the lipid bilayer thickness is reduced
near the channel. That means, the monolayers bend inward towards the membrane's
hydrophobic inner core. However, when the channel length is smaller than the bilayer,
the bilayer becomes locally thicker, that is, the monolayers bend outwards toward
hydrophilic regions. The bilayer's elastic properties enable it to change its thickness,
but the real cause of this thickness change lies behind complicated energetics, which
will be explained in the next chapter.
4.7 Sodium-Potassium Pumps in Membranes
Electrochemical gradients for sodium and potassium ions, generated by the Na + ,K + -
ATPase, are vital to animal cells, exchanging three sodium ions for two potassium
ions across the plasma membrane during each cycle of ATP hydrolysis. The Na + ,