What Is a Glycoprotein?

A glycoprotein is a molecule that contains both a protein portion and at least one carbohydrate portion. Glycoproteins are common in biology and perform a range of functions. Some examples of their individual functions are as structural cell components, enzymes, or hormones.

Carbohydrates are a class of molecule in organic chemistry and biology. The class contains many molecules, but they all contain only carbon, hydrogen, and oxygen atoms. Proteins, on the other hand, are made up of different building blocks called amino acids. Amino acids contain nitrogen, which distinguishes them from carbohydrates.

Each glycoprotein contains a protein and one or more carbohydrate additions. Different glycoproteins have different ratios of carbohydrate to protein, with the mass of the carbohydrates taking up under 1 percent to about 80 percent of the finished product. The placement of the carbohydrates also allows scientists to split the glycoproteins into two groups. One group is the O-linked glycans, where the carbohydrate is stuck to the amino acids threonine or serine on the protein. The other is the N-linked glycans, where the carbohydrate is attached to an asparagine amino acid.

Human cells initially build a protein base inside the cell in a structure called the endoplasmic reticulum. After the protein is made, it releases from the endoplasmic reticulum and moves through the cell to another structure known as the Golgi apparatus. The cell machinery adds various carbohydrate portions onto the basic protein structure during this journey and at the Golgi apparatus.

Once the glycoprotein is complete, it can perform its function. One subset of glycoproteins protrudes from the cell wall and acts as receptors for other molecules. They can also help stick cells together to form a strong tissue, such as cartilage. Human blood types A, B, and O also depend on the presence of certain glycoproteins on the outside of the red blood cells.

Other forms of glycoproteins range around the body. One such example is the hormone set of glycoproteins, including human chorionic gonadotropin, which functions during reproduction, and erythropoietin, which helps control red blood cell levels. Various glycoproteins can also work to carry useful molecules around the body, like vitamins.

Performing certain reactions is another function of members of the glycoprotein group, and the three enzyme groups hydrolases, transferases, and oxidoreductases are glycoproteins. Certain glycoproteins can also inhibit other molecules whose function is to break down protein. Uses of glycoproteins in nonhuman animals also include an antifreeze effect in certain fish who live in the Antarctic waters. Beetle species can also use glycoprotein as a disinfectant layer on the outside of the beetle body.