Membrane Lipid Biology
Cell membranes constitute one of the fundamental structural and functional elements of living organisms. These complex mixtures of lipids and proteins form the outer boundary of the cell. They distinguish between the inside" and the "outside", and allow the cell to communicate with its environment and keep its integrity. While covalent bonds between the atoms in the protein and lipid molecules determine their structural integrity, their macromolecular aggregate that forms the actual membrane retains its association without the benefit of covalent bonds, and is governed largely by the "hydrophobic effect". This force is in turn is based on the unique properties of
water. While water has great solvent power for polar substances such
as ions, it lacks solubility for non-polar substances. Amphipathic molecules,
which contain both polar and non-polar regions, accommodate in a water
environment by maximizing the interaction of the polar parts of the molecules
with the aqueous phase andshield the interaction of the non-polar region
Often a happy solution for phospholipids is a
double layer with the hydrocarbon chains sequestered away from
the water in the interior of the bilayer , while the polar phospholipid
head groups maintain contact with the aqueous phase. Similarly, protein
amino acid regions with hydrophobic side chains sequester away from water
while polar amino acid side chains are exposed to the aqueous phase.
The lipid-lipid and lipid-protein interactions in membranes, determined by these forces are extremely important for the function of the membrane. Consequently research on membrane structure has become an important subject at the most basic level of the discipline of cell biology.
In recent years, it has been recognized that the role of the lipid membrane
bilayer is far from the rather anonymous, passive and structure less embedding
solvent for membrane proteins as depicted in most general textbooks. In
that view, the lipid bilayer is a thin two-dimensional bimolecular sheet,
has no particular transverse or lateral structure or dynamics and no coupling
to the proteins. The otherwise successful paradigm of the Singer/Nicolson
fluid-mosaic model of bio-membranes may be in part responsible. This model
seems to imply that the lipid-bilayer component of cell membranes is a
structure-less fluid characterized by disorder and randomness rather than
by molecular order and structure. In contrast however, lipid bilayers
display order and structure, be it in a different sense than used in connection
with the static structure of macromolecules. Lipid membranes are highly
dynamic structured and complex fluids. They sustain fluctuations, cooperatively,
and correlated dynamical modes that lead to local order out of disorder,
properties important for membrane function. Membrane proteins are embedded
in a lipid bilayer which accommodates by deformation to make a hydrophobic
match. Amino-acids will interact with specific acyl-chains in the hydrophobic
core, and the charged lipid head-groups interact with more hydrophilic
amino acids at the water membrane interface.
Lipids and proteins
form complex mixtures at the plasma membrane, the outer boundary of the
cell. The mixture distinguishes between inside" and "outside",
allowing the cell to communicate with its environment and keep its integrity.
A change in protein conformation will be accommodated by a local change
in lipid organization.