Supramolecular Systems Seen in the Biological World

The second example of a biological supermolecule is a cell membrane. As described in Chap. 4, a cell membrane consists mainly of a fluidic lipid bilayer containing proteins (Fig. 6.2). The Hpids are self-assembled into the bilayer structure and the proteins float within the Hpid bilayer. The whole structure is formed through self-assembly processes. 

The membrane protein is stably buried in the Hpid bilayer due to the amphiphilic nature of the membrane protein. The surfaces of some parts of the protein have mainly hydrophobic amino acid residues, and hydrophihc residues are located on the other surfaces. The former parts are accommodated in the hydrophobic lipid bilayer and the latter protein regions are exposed to the surface of the water. Membrane proteins work as receptors, channels and so on. Specific interactions between these proteins lead to complex functions such as signal transduction and energy conversion. Many of the functions expressed by the cell membrane can be attributed to the functionality of the membrane proteins. 

The amounts and species of proteins in the cell membrane depend on the functions of the particular cell. The protein weight content of the myelinforming protein that covers nerve cells is only 25%. The inner membrane of a mitochondrion, which performs energy conversion, contains 75% by weight of proteins. 

The arrangement of the proteins buried in the lipid bilayer allows them to respond dynamically to external stimuli. These characteristics of the membrane proteins are related to their various functions. In the following sections, the excellent functions expressed by these membrane proteins are explained and their artificial mimics are introduced. 

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