Glycoprotein - Structure, Functions, Examples and Glycolipids

Glycoprotein: Glycoproteins are proteins that have carbohydrates covalently attached to them. During the synthesis of these proteins, the carbohydrate components are added. In some cases, these carbohydrates are added as the protein continues to grow. Glycoproteins specifically have oligosaccharide chains covalently bonded to the amino acid side chains or the polypeptide backbone. This process, known as glycosylation, occurs in various organisms.

Glycoprotein Meaning

• Hexoses such as mannose, galactose, and glucose
• Deoxyhexoses such as fucose
• Sialic acids like N-acetyl neuraminic acid
• Amino hexoses such as N-acetyl glucosamine and N-acetyl galactosamine
• Pentoses like xylose

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Glycoprotein Structure

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• N-linked glycosylation: The glycan is attached to the nitrogen atom of an asparagine (Asn) amino acid in the protein sequence, typically occurring in a specific motif: Asn-X-Ser/Thr, where X can be any amino acid.
• O-linked glycosylation: The glycan is attached to the oxygen atom of a serine (Ser) or threonine (Thr) amino acid in the protein sequence. O-linked glycosylation can occur at various positions in the protein and is more diverse than N-linked glycosylation.

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Glycoprotein Functions

• Cell-Cell Recognition: The carbohydrate chains of glycoproteins act as identity tags, helping cells recognize and interact with each other. This is crucial for processes like immune response and embryonic development.
• Cell Adhesion: Glycoproteins assist cells in adhering to each other and to the extracellular matrix, a network of proteins and carbohydrates that supports and structures tissues.
• Signaling: Glycoproteins play a role in cell signaling, the process by which cells communicate. When a signaling molecule binds to a glycoprotein receptor on the cell surface, it can trigger a series of events leading to changes in the cell's behavior.
• Transport: Some glycoproteins function as channels or pumps that facilitate the movement of molecules across cell membranes.
• Protection: Glycoproteins help protect cells from damage. For example, mucins in mucus are glycoproteins that trap and eliminate pathogens.
• Immune Function: Antibodies, produced by B cells, are glycoproteins. Their carbohydrate chains aid in binding to specific antigens, which are foreign molecules that trigger an immune response.
• Structural Support: Some glycoproteins, like collagen, provide structural support to tissues.

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Glycoprotein Examples

1. Antibodies (Immunoglobulins): These are essential immune system components, particularly in the adaptive immune response. They bind to specific antigens (foreign substances) and help remove them from the body. The sugar chains on antibodies can affect their ability to bind to antigens and interact with other immune cells.
2. Enzymes: Many enzymes are glycoproteins. The sugars can influence the enzyme's activity, stability, and targeting within the cell. For example, lactase, which breaks down lactose in milk, is a glycoprotein.
3. Hormones: Some hormones, such as follicle-stimulating hormone (FSH) and erythropoietin (EPO), are glycoproteins. The sugar chains can affect their interaction with target cells and their lifespan in the bloodstream.
4. Blood Group Antigens: The ABO blood group system is determined by specific glycoproteins on the surface of red blood cells. These antigens determine blood type compatibility for transfusions.
5. Cell Adhesion Molecules: These glycoproteins are found on the cell surface and facilitate cell-to-cell interactions. They are essential for processes like tissue development, immune response, and cell migration.
6. Mucins: These glycoproteins are rich in sugars and form mucus, which acts as a lubricant and protective barrier in various tissues, such as the respiratory and digestive systems.
7. Viral Envelope Proteins: Many viruses, including HIV and influenza, have glycoproteins on their envelope. These glycoproteins are crucial for viral attachment to host cells and viral entry. The SARS-CoV-2 virus, which causes COVID-19, has a spike protein that is a glycoprotein.

Glycoprotein Hormones

• An alpha subunit, which is the same across all glycoprotein hormones.
• A beta subunit, which is unique to each hormone and determines its specific biological function.

• Luteinizing hormone (LH): Induces ovulation in females and stimulates testosterone production in males.
• Follicle-stimulating hormone (FSH): Promotes the development of ovarian follicles in females and sperm production in males.
• Thyroid-stimulating hormone (TSH): Encourages the thyroid gland to produce thyroid hormones.
• Human chorionic gonadotropin (hCG): Produced by the placenta during pregnancy, it supports the corpus luteum, which is crucial for progesterone production.

Glycoprotein in Cell Membrane

Glycoprotein and Glycolipid