Glycoproteins are molecules containing both protein and carbohydrate moieties, performing various biological functions. They are divided into O-linked and N-linked glycans, and are built in the endoplasmic reticulum and Golgi apparatus. Glycoproteins act as receptors, form tissue, and transport molecules, while some are enzymes or hormones. They also have uses in nonhuman animals, such as an antifreeze effect in fish and a disinfectant layer in beetles.
A glycoprotein is a molecule that contains both a protein moiety and at least a carbohydrate moiety. Glycoproteins are common in biology and perform a number of functions. Some examples of their individual functions are as structural components of cells, enzymes or hormones.
Carbohydrates are a class of molecules in organic chemistry and biology. The class contains many molecules, but all of them 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 carbohydrate-to-protein ratios, with the mass of the carbohydrates occupying less than 1 percent up to about 80 percent of the finished product. The placement of carbohydrates also allows scientists to divide glycoproteins into two groups. One group is the O-linked glycans, where the carbohydrate is attached to the amino acids threonine or serine on the protein. The other is N-linked glycans, where the carbohydrate is attached to an amino acid asparagine.
Human cells initially build a protein base within the cell in a structure called the endoplasmic reticulum. After the protein is made, it is released from the endoplasmic reticulum and moves through the cell to another structure known as the Golgi apparatus. The cellular machinery adds various portions of carbohydrates to the basic protein structure during this journey and to the Golgi apparatus.
Once the glycoprotein is complete, it can perform its function. A subset of glycoproteins protrude from the cell wall and act as receptors for other molecules. They can also help bring cells together to form strong tissue, such as cartilage. Human blood types A, B, and O also depend on the presence of certain glycoproteins on the outside of red blood cells.
Other forms of glycoproteins are found throughout the body. One example is the hormone complex of glycoproteins, including human chorionic gonadotropin, which works during reproduction, and erythropoietin, which helps control red blood cell levels. Various glycoproteins can also work to transport useful molecules throughout the body, such as vitamins.
Carrying out certain reactions is another function of glycoprotein group members, and the three enzyme groups hydrolases, transferases and oxidoreductases are glycoproteins. Some glycoproteins can also inhibit other molecules whose function is to break down proteins. Uses of glycoproteins in nonhuman animals also include an antifreeze effect in some fish that live in Antarctic waters. Beetle species can also use the glycoprotein as a disinfectant layer on the outside of the beetle’s body.
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