A glycoprotein is a protein that has a sugar bonded to it, a process known as glycosylation. Most commonly these sugars, or oligosaccharides, are attached to either the amine or the carboxyl groups of the amino acids in the polypeptide chain. Glycoproteins are often found in the cell membrane or secreted into the extracellular environment, though they are sometimes found in the cytosol or nucleus. When incorporated into the cell membrane, these protein molecules often have their oligosaccharide chains on the external side of the cell membrane for cell-cell communication purposes. Some common glycoproteins are mucins, transferrin, collagens, immunoglobulins and hormones.
Watch this video to find out more about Glycoproteins:
Found in skin cells, mucins are large proteins that tend to be large because they are densely glycosylated. These proteins are found in the gel-like secretions of endothelial cells. They are stored in vesicles until released by the cell. Mucins are one type of molecule that makes up the mucus our bodies produce. One normal function of mucus is to coat the mucous membranes of the gastrointestinal, respiratory, and other systems of the body. They also serve to protect against viruses and other infectious substances. Mucus can sometimes increase in volume or change color, when fighting off disease. These changes occur as a result of bacterial isolation or immunological changes.
Transferrin is a glycoprotein that is found in the blood. It is able to reversibly bind iron molecules, thus controlling the body’s amount of free iron. This is a transport molecule that can pick up iron and transport it to another part of the body. Once the molecule locates a cell that possesses a transferrin receptor, it releases the iron to that cell using receptor-mediated endocytosis. Most transferrin is produced in the liver and immune system and helps the body control cellular iron concentrations. Levels of transferrin production are increased by the body in anemia, when the body needs the ability to transport more iron, and decreased during diseases involving iron overload.
How Transferrin binds Iron in our system and transports it help make red blood cells:
The body’s structural needs are met by collagen, a glycoprotein found in connective tissue such as bone and cartilage. It can also be found in the fibrous tissues of muscles and tendons. Collagen is found in the heart because it provides both the structural support needed by the valves and chambers and the flexibility that the muscle requires so it can pump blood throughout the body. Collagen synthesis requires a ready supply of ascorbic acid, vitamin C, so a lack of this vital nutrient can result in scurvy, which is characterized by weak connective tissue and tooth loss.
Immunoglobulins, or antibodies, are used by the body’s immune system to fight disease. They are mostly produced by white blood cells. They contain antigen binding sites which allow them to target and remove specific pathogens from the body. This means that they are a targeted immune response. The body needs to have encountered a pathogen before to be able to target and remove it later. This is the concept behind immunizations, which introduce a weakened form of a disease into the body. The presence of immunoglobulins in the body can also be used to indicate the presence of some diseases, such as cirrhosis, Lyme disease, and even some autoimmune disorders.
Most glycoprotein hormones are produced in the pituitary gland and can be separated into gonadotropins and thyroid stimulating hormone (TSH). Because these proteins are water soluble, due to the sugars attached, they can be transported in the blood. This is how hormones send chemical signals from one part of the body to another. Gonadotropins, or reproductive hormones, regulate reproduction and growth, including sexual development. TSH, as the name implies, stimulates the thyroid to produce thyroxine and triiodothyronine, which regulate the body’s metabolism. A shortage of these hormones can result in infertility or hypothyroidism.
Due to the amphiphilic nature of glycoproteins, meaning they are both hydrophilic and lipophilic, they can perform many functions within the body. They are involved in both normal and abnormal bodily functions. This makes them a useful and essential biological molecule.
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