The cell membrane is composed of a phospholipid bilayer that regulates what comes into the cell and what goes out. Many molecules and ions travel through the cell membrane with the help of proteins. There are two basic types of membrane proteins: peripheral and integral.
Peripheral Membrane Proteins
Peripheral membrane proteins are bound to the cell membrane via other molecules, usually other proteins or lipids. The bonding is relatively weak and usually consists of either electrostatic interactions or hydrogen bonds. These proteins are easily detached from the cell membrane through pH changes or from a lack of calcium ions.
Integral Membrane Proteins
The second type of membrane protein is the integral protein. These proteins are much harder to remove from the cell membrane because they usually traverse deep into the cell membrane or completely through it. Integral proteins are bound to the lipids in the cell membrane through strong hydrophobic interactions. The only way to remove these proteins from the membrane is through detergents which break the bonds between the protein and lipid.
Six Types of Integral Proteins
There are six types of integral membrane proteins. They are distinguished by structure and location of protein domains.
Type 1 – Consists of one transmembrane helix, with the amino end outside the cell.
Type 2 – Consists of one transmembrane helix, with the amino end inside the cell.
Type 3 – A single polypeptide that traverses the membrane multiple times (multiple transmembrane helices).
Type 4 – Several polypeptides that traverse the membrane once, creating a channel ( individual transmembrane helices).
Type 5 – The protein is covalently linked to membrane lipids.
Type 6 – The protein traverses the membrane and is linked covalently linked to the lipids.
Reference:
Nelson, David L. Lehninger Principles of Biochemistry 4th ed. W.H. Freeman and Company. 2005