Lipid Bilayer-Membrane Protein Coupling - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 74

Chapter 5
Lipid Bilayer-Membrane Protein Coupling

Lipid organization in membranes forms liquid crystalline structures. Membrane pro-
teins, like all other proteins, exist with solid structure, if not generally, then at
least relate to the structure of lipid membranes. The coupling between these two
different structural components is rather complicated. Biological membranes are
dynamical macromolecular assemblies, composed of lipid bilayers with embedded
bilayer-spanning proteins that move within the plane of the membrane. This seminal
membrane concept was originally proposed as the fluid mosaic membrane model
[ 82 ]. A lipid bilayer's primary function is to serve as a semipermeable barrier for
solute movement between different, membrane-separated fluid compartments. This
barrier function depends on the bilayer's hydrophobic core being a poor “solvent” for
polar solutes. The bilayer permeability coefficient for solute X ( P X ) can be approx-
imated as:

α X D X
ζ

P X = , (5.1)

where α X is the solute partition coefficient between the bilayer core and the aque-
ous phase, D X is the solute diffusion coefficient in the bilayer core (which varies
little among small solutes [ 29 ]), and ζ denotes the bilayer hydrophobic thickness

( 30 Å for hydrocarbon-free bilayers [ 54 , 81 ], or 40-60 Å for hydrocarbon-containing

bilayers [ 16 ]). Experimental results obtained for a wide variety of solutes show that
P X is proportional to X , as approximated by the solutes' oil/water partition coef-
ficient [ 29 , 70 , 90 ], and that the solute diffusion coefficient in the bilayer core is
similar to the diffusion coefficient in bulk hydrocarbons (10 − 6 to 10 − 5 cm 2

∼

s[ 79 ]).
The elucidation of the role of membrane proteins requires a specific mechanism
of lipid regulation of the membrane protein function which is extremely impor-
tant, but poorly understood so far. The free kinetic characteristics of membrane
proteins inside a lipid bilayer are imaginary. In reality, the dynamical properties
of membrane proteins are a result of various contributions from their interactions
with host phospholipid layers and other integral constituents such as other mem-
brane proteins, hydrocarbons, cholesterols, etc. The background dielectric proper-
ties also play important roles. In this chapter, we focus on different components that

/