Nanotechnology-Enabled Sensors

7.4 Proteins in Nanotechnology Enabled Sensors 429
Fig. 7.49 A cross section of a membrane showing the phospholipid bilayer.
A group of the transmembrane proteins consist of polypeptide chains
which cross the bilayer and usually have an α-helix form. In many transmembrane
proteins the polypeptide chain crosses the membrane only once.
This type of protein can be a receptor for the extracellular signals. Other
trans-membrane proteins form pores that allow water-soluble molecules to
cross the membrane. Such pores cannot be formed by proteins with a single,
uniformly hydrophobic transmembrane α-helix. Instead, these proteins
generally have α-helices that cross the bilayer a few times. Some of these
α-helices have both hydrophobic and hydrophilic side chains. The hydrophobic
chains are close to the bilayer lipids and hydrophilic chains are located
inside the cell. These pores function as selective transporters of small
water-soluble molecules across the membrane. The subunits made of such
helices are denoted as β, γ, and so on.