What types of membrane proteins are there?

Membrane proteins can be classified into two broad categories – integral and peripheral - based on the nature of the interactions between the membrane and the proteins. Most membranes contain both types of membrane proteins. 

Integral or intrinsic membrane proteins

Integral membrane proteins are located between the inner and outer membrane and constitute 70% of the total membrane proteins. They are permanently anchored to the membrane and penetrate through the phospholipid bilayer. They contain both hydrophilic and hydrophobic regions.

Parts of integral proteins that project beyond the surface of the lipid bilayer are rich in amino acids with hydrophilic side chains. Parts of the protein that are buried in the hydrophobic portion of the lipid bilayer are rich in amino acids with hydrophobic side chains.

All transmembrane proteins that penetrate the lipid bilayer completely are integral proteins but all integral proteins are not transmembrane proteins. Some integral proteins only partially penetrate the lipid bilayer.

Integral proteins serve as channel proteins, carrier proteins, and enzymes. Examples include rhodopsin, glycophorin, and NADH dehydrogenase.

Peripheral or extrinsic membrane proteins

Peripheral membrane proteins are located on either the inner or outer membrane but not both. They constitute 30% of the total membrane proteins and are rich in amino acids with hydrophilic side chains that permit interaction with the surrounding water and with the polar surface of the lipid bilayer.

Peripheral proteins are temporarily and loosely attached to the membrane, and do not penetrate the hydrophobic core of the phospholipid bilayer. The looser attachment between makes the peripheral proteins easier to remove as compared to integral proteins. This removal does not interfere with the hydrophobic interactions of the lipid bilayer.

Peripheral proteins serve as receptors and surface antigens. Examples include erythrocyte spectrin and mitochondrial cytochrome c.