The Kinds of Membrane Proteins

The Kinds of Membrane Proteins

How are proteins associated with the bilayer in membranes?

Proteins in a biological membrane can be associated with the lipid bilayer in either of two ways-as peripheral proteins on the surface of the membrane or as integral proteins within the lipid bilayer (Figure 8.16). Peripheral proteins are usually bound to the charged head groups of the lipid bilayer by polar interactions, electrostatic interactions, or both. They can be removed by such mild treatment as raising the ionic strength of the medium. The relatively numerous charged particles present in a medium of higher ionic strength undergo more electrostatic interactions with the lipid and with the protein, “swamping out” the comparatively fewer electrostatic interactions between the protein and the lipid.

Removing integral proteins from membranes is much more difficult. Harsh conditions, such as treatment with detergents or extensive sonication (expo-sure to ultrasonic vibrations), are usually required. Such measures frequently denature the protein, which often remains bound to lipids in spite of all efforts to obtain it in pure form. 

The denatured protein is of course inactive, whether or not it remains bound to lipids. Fortunately, nuclear magnetic resonance techniques enable researchers to study proteins of this sort in living tissue. The structural integrity of the whole membrane system appears to be necessary for the activities of most membrane proteins.

Proteins can be attached to the membrane in a variety of ways. When a protein completely spans the membrane, it is often in the form of an α-helix or β-sheet. These structures minimize contact of the polar parts of the peptide backbone with the nonpolar lipids in the interior of the bilayer (Figure 8.17). Proteins can also be anchored to the lipids via covalent bonds from cysteines or free amino groups on the protein to one of several lipid anchors. Myristoyl and palmitoyl groups are common anchors (Figure 8.17).

Membrane proteins have a variety of functions. Most, but not all, of the important functions of the membrane as a whole are those of the protein com-ponent. Transport proteins help move substances in and out of the cell, and receptor proteins are important in the transfer of extracellular signals, suchas those carried by hormones or neurotransmitters, into the cell. In addition, some enzymes are tightly bound to membranes; examples include many of the enzymes responsible for aerobic oxidation reactions, which are found in specific parts of mitochondrial membranes. Some of these enzymes are on the inner surface of the membrane, and some are on the outer surface. There is an uneven distribution of proteins of all types on the inner and outer layers of all cell membranes, just as there is an asymmetric distribution of lipids.

Summary

Proteins combine with lipid bilayers to form membranes. 

Peripheral proteins are loosely attached to one surface of the membrane by hydrogen bonds or electrostatic attractions.

Integral proteins are embedded more solidly in the membrane and, in some cases, may be covalently attached to lipid anchors.