When located in the cell membrane, glycoproteins help to identify, adhere to and communicate with the cell, among other functions.
In this lesson, we’ll discuss how glycoprotein completes these functions in the cell membrane.
What are Glycoproteins?
Glycoproteins are proteins that have sugar molecules attached to them. These sugar molecules are actually gathered into short chains, or oligosaccharides. The sugar molecules that make up oligosaccharides aren’t just limited to the type of sugar you add to your coffee–there are actually many different types of simple sugar molecules. Just like the different letters of the alphabet can be joined together to create thousands of different words, these sugars can join together in different patterns to create thousands of different oligosaccharides.
And, just like we use words to communicate, cells use oligosaccharides on the outside of their plasma membranes to communicate with other cells.The cell uses glycoproteins embedded in the plasma membrane to get the oligosaccharides on the outside of the cell. Embedded proteins typically have a portion of the protein outside the cell, which is typically decorated with different oligosaccharides, depending on what message is being sent. Let’s discuss a few examples of the types of communication that occur using glycoproteins in cell membranes.
Glycoproteins help cells recognize each other. The unique patterns of oligosaccharides on the cell surface can be read by corresponding glycoproteins on another cell. However, this process is different from how we read words.
It’s more like fitting a key into a lock; glycoproteins will often bind together if they’re a match. This binding process communicates to the cell that it has found its match. This is what helps our immune system to recognize foreign invaders and then defend our body against them. The reverse is true, too. Viruses use glycoproteins to bind to and infect a host cell. Glycoprotein recognition also helps in reproduction.
A sperm cell has special glycoproteins in its membrane that recognize special glycoproteins on the membrane of the egg, thereby allowing the sperm cell to bind to the egg and then fertilize it.Another important example of glycoprotein recognition occurs in our blood. If you know your blood type, you know what special glycoproteins are present in your red blood cells to identify them.
In addition to identifying other cells, there are many cases in which the cells need to stick together. All of our tissues are actually formed by rows and rows of cells that are stuck together. In many cases, glycoproteins are like the glue that holds the cells together–actually, they act more like Velcro than glue. These glycoproteins are often referred to as cell adhesion molecules, or CAMs.
Many CAMs are present on the surface of cells, and they stick to CAMs on neighboring cells like two pieces of Velcro stick together. CAMs also adhere the cell to the surrounding extracellular matrix, which is the substance that gives our tissues their shape. When bound to other CAMs or to the extracellular matrix, the CAM can communicate to the cell what its environment is like.
Receptor-Mediated Cell Signaling
Many receptors on the cell surface are also glycoproteins. In fact, in the examples already discussed, the glycoproteins often acted like receptors of other glycoproteins. When a specific molecule binds to its receptor, it triggers a chain reaction inside the cell.
This chain reaction will generate a desired effect. This type of cell signaling is the main type of communication in cells.The glycoprotein receptors are the yellow T-shaped objects in the cell membrane. Glycoproteins don’t just bind other glycoproteins; they also bind other types of molecules. For example, viruses use glycoproteins to bind to a target host cell.
Once bound to the glycoprotein receptor, the receptor triggers a signal that causes the cell to engulf the virus. Once engulfed, the virus will infect the cell.
Glycoproteins are special proteins that have oligosaccharides attached to them. Since there are millions of different oligosaccharides, they can be used in different forms of communication between cells. In particular, glycoproteins in the cell membrane are very important for cell-to-cell recognition and adhesion, as well as serving as receptors for other types of molecules.
With a strong understanding of the above facts, you could be able to do the following:
- Illustrate the structure of glycoproteins
- Provide examples of cell-to-cell recognition
- Express knowledge of the way in which glycoproteins facilitate cell adhesion
- Discuss the role of glycoproteins as receptors