Biophysical studies of membrane proteins/peptides. Interaction with ...

OUTLINE
OUTLINE
The last two decades, through a massive collective effort, allowed some insight into
the nature of the interactions between the components of biomembranes and also the
highly relevant biological consequences of these interactions. The Singer-Nicolson fluid
mosaic model for biomembranes (1972), described a system where lipids were little else
than a matrix where proteins were dispersed. The picture currently accepted is much
more complex. Not only protein-protein interactions can be responsible for mediation of
cellular functions, but also protein-lipid interactions are much more complex and
important than originally thought. Detailed biophysical studies such as the ones
described here, allow gathering of valuable information regarding the character of such
interactions.
Biomembranes delineate the boundaries of cellular life. They allow the
differentiation of molecular environments through the control of permeability, creating
the conditions for specialization within the cell. This task demands the expenditure of a
massive amount of energy, emphasizing the dramatic importance of biomembranes.
The foremost structural framework of biomembranes is the lipid bilayer, but lipids
are far from being a homogeneous class of molecules. When the energies of interaction
between lipid molecules are significantly different, large scale phase separation of lipid
molecules in the membrane can occur, and even for small differences in lipid-lipid
interaction energies, short-range heterogeneities are expected. Therefore, the lateral
distribution of a multicomponent lipid bilayer is often heterogeneous.
This portrait of biomembranes is made even more complex by the notion that lipids
are not the only components found there. In fact, biomembranes are a mixture of
different classes of molecules with different functions. Apart from lipids, the most
notable component are proteins, which are highly concentrated in biomembranes and
are largely responsible for the cellular functions operated there.
Interactions between these quite different molecular species are very complex.
Hydrogen bonds, electrostatic, and van der Waals forces, all operate simultaneously in
dictating the equilibrium packing of proteins and lipids in bilayers. Notably, proteins
and lipids will arrange in such a manner that their hydrophobic domains are shielded
from the aqueous environment in order to prevent the entropy loss resulting from
interaction with water molecules. As a consequence, membrane proteins and lipids will
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