Despite the diversity of integral membrane proteins, most fall into two general classes. One of these includes proteins attached or anchored to the membrane by only a small hydrophobic segment, such that most of the protein extends out into the water solvent on one or both sides of the membrane. The other class includes those proteins that are more or less globular in shape and more totally embedded in the membrane, exposing only a small surface to the water solvent outside the membrane. In general, those structures of integral membrane protein within the nonpolar core of the lipid bilayer are dominated by a-helices or ^-sheets because these secondary structures neutralize the highly polar N—H and C=O functions of the peptide backbone through H-bond formation.
In the case of the proteins that are anchored by a small hydrophobic polypep-tide segment, that segment often takes the form of a single a-helix. One of the best examples of a membrane protein with such an a-helical structure is gly-cophorin. Most of glycophorin's mass is oriented on the outside surface of the cell, exposed to the aqueous milieu (Figure 9.14). A variety of hydrophilic oligosaccharide units are attached to this extracellular domain. These oligosaccharide groups constitute the ABO and MN blood group antigenic specifici
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