The cell’s membrane is made of a variety of lipids and proteins. In this lesson, we will focus on the roles the peripheral proteins play in the cell and in human diseases.
What Are Peripheral Proteins?
Most molecules cannot cross cell membranes alone because it is impenetrable to them. The role of the plasma membrane is to form a barrier and protect the cell from the environment. The problem is, the cell membrane is a bit overzealous at its job. If it were up to the cell membrane, hardly anything would get in or out of the cell. This is counterproductive if the cell is going to stay alive since the cell membrane blocks vital molecules from crossing. Think of the plasma membrane as a dumb dictator whom you cannot reason with. For this reason, the cell membrane is full of proteins that help keep the plasma membrane and other membranes in the cell in check.
There are two types of membrane proteins: integral and peripheral. Think of proteins as diplomats who can reason with cell membranes and form ways to bring molecules into the cell. There are two ways a protein can attach to the plasma membrane as depicted in Figure 2 (see video).
Peripheral proteins don’t cross the membrane. They kind of hang out on either side of cell membranes. They are loosely attached to other proteins or the membrane itself through hydrogen bonds. They’re called ‘peripheral proteins’ because they sit on the outside of the membrane and do not integrate into the membrane.
Multifunction Membrane Proteins
Even though peripheral membranes don’t form entryways to the cell since they don’t penetrate the membrane, they’re really vital to the function of the cell. Think of them as the wingmen of the membrane proteins since they play vital roles for the cell. They do this by acting as partners with other proteins. Peripheral membrane proteins serve multiple roles including:
Peripheral membranes allow many molecules to be carried around the cell. Some peripheral membrane proteins carry molecules between other proteins. As shown in Figure 3 (see video), the function of cytochrome c is to carry electrons from one protein to the next. If cytochrome c was not present, the proteins that aid in generating energy would not obtain the electrons necessary to generate energy in the cell’s mitochondria.
You couldn’t exist if your cells didn’t have this protein on the membranes of the mitochondria in your cells. Cytochrome c helps transport electrons to generate energy in the mitochondria. The other proteins that produce the energy couldn’t do their jobs without cytochrome c acting as a wingman to bring electrons to them in order to produce energy.
Cytochrome c is released from the membranes of the mitochondria when a cell becomes damaged and can’t be repaired. Released cytochrome c signals a cascade of proteins that lead to cell death. When this does not happen, cells with damaged DNA are allowed to replicate, which leads to cancer.
When a cell gets old or its DNA becomes damaged, cytochrome c is released from the membrane and signals to the cell that it needs to die as shown in Figure 4. A good wingman knows when to tell their partner to give up. In some cancer cells, this does not happen, and cells with damaged DNA are allowed to grow out of control and form tumors. So cytochrome c is a peripheral protein, that is essential for life and for health, but we never even think about it.
Peripheral membrane proteins are used to anchor the plasma membrane to the cytoskeleton. Spectrin and ankyrin are examples of proteins that anchor membranes to the cytoskeleton. Normal red blood cells are spherical. As shown in Figure 5 (see video), people who are born with defective genes for making ankyrin or spectrin have abnormal shaped red blood cells because the cell membrane is not properly attached to the cytoskeleton. This condition is called hereditary spherocytosis. In addition to the red blood cells being abnormal in shape, they are weaker than normal blood cells and are easily lysed or popped, leading to anemia. Even though ankyrin and spectrin are not the proteins that determine the shape of the cytoskeleton, they do help the cytoskeleton shape the cell by attaching it to the cell membrane.
Many proteins located on the periphery of the cell membrane are used to initiate cell signaling events in the cell. Cell signaling is the way cells talk to each other or to themselves. The phospholipases are a family of peripheral membrane proteins that release fatty acids from the membrane. In releasing the fatty acids from the membrane, it sends a signal to other proteins in the cell. This is similar to the release of cytochrome c discussed previously.
High levels of phospholipase A2 can lead to diseases ranging from Alzheimer’s disease to restriction of blood flow that could lead to a heart attack or stroke. This is because phospholipase A triggers inflammation and other stressful events in the cell. In fact, many snake venoms belong to the phospholipase A2 family and have the same effect as your cells making too much phospholipase A2. This may not be the best example of a good wingman, but nothing is perfect.
The roles discussed are only a short list of the things that peripheral proteins do in your cells and, in turn, for you.
Let’s review. Membrane proteins do the work of the plasma membrane. Peripheral membrane proteins do not cross the membrane, but they can be attached to either side of the membrane or other proteins in the membrane. Peripheral membrane proteins have multiple functions including _transportation to various locations in the cell, signaling, and maintaining the cell shape and structure.