How Do Glycoproteins Impact Cell Communication and Immunity?
Polymer proteins that are attached covalently to carbohydrates are glycoproteins. During protein synthesis, the carbohydrate component of the protein is added. Sometimes, however, protein is combined with carbohydrate components as the protein continues to grow. It is therefore clear that glycoproteins are considered to have a major biological function involving carbohydrates. Glycoproteins contain carbohydrates called oligosaccharides, which are polymers composed of 3 to 10 monosaccharides. Liposaccharides are rarely found free in cells. The N- and O-linked amino acids are usually attached to the proteins. Oligosaccharides are formed by several types of sugar in humans. This includes:
Glucose, galactose, and mannose are hexoses.
Fucoses are deoxyhexose.
N-acetyl neuraminic acid, similar to sialic acid.
Glucosamine and galactosamine are amino hexoses.
Xylose-like pentoses.
Glycoprotein Structure
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Glycoprotein structures have been described by their name - a sugar portion (glycol) attached to a protein. They are covalently bonded together.
Carbohydrates (sugar) are commonly referred to as glycans in the glycoprotein structures. A glycoprotein can have between one and eighty percent oligosaccharides, known as glycan. Sugar molecules that are glycosylated before binding to proteins are called glycosyl groups.
Having the hydroxyl group (-OH) removed from a monosaccharide will result in a glycosyl group with an empty spot, which will make it unstable. A total of thirteen monosaccharides are currently attached to eight different amino acids. To make an unstable monosaccharide stable again, these attachments are needed. Glycosyl groups are replaced by hydroxyl groups in amino acids associated with them.
In glycosylation, glycosyl groups are added to peptide chains, proteins, or lipids. Human body proteins are glycosylated to more than half. Cellular glycosylation takes place in the Golgi and endoplasmic reticulum of eukaryotic cells.
Glycoprotein Function
Glycoproteins work to structure and generate cells, reproduce, regulate the immune system, produce hormones, and protect organisms.
Membrane lipid bilayers are coated with glycoproteins. They can function in the aqueous environment because of their hydrophilic nature, which enables them to act in the recognition of cell-cell and binding of other molecules. A tissue's stability and strength are enhanced by crosslinking cell surface glycoproteins (e.g., collagen). A plant's ability to stand upright against gravity is due to glycoproteins found in its cells.
Intercellular communication is not the only important function of glycosylated proteins. The proteins also facilitate communication between organ systems. Gray matter in the brain contains glycoproteins, which work in conjunction with axons and synaptosomes.
Glycoprotein Examples
There are too many examples of glycoprotein functions to list them all. Different species of glycoproteins have distinct characteristics; rat glycoproteins are very different from those in mice. The following article discusses just the glycoprotein function of the most important human glycoproteins.
Serum Glycoprotein: The serum glycoproteome is composed of hundreds of glycoproteins that exist in our blood plasma. A simple blood test can be used to determine one or more of these glycoproteins.
Zona Pellucida Glycoprotein: We are unable to reproduce without glycoproteins. A sperm cell's ability to bind to and move into released, unfertilized eggs is controlled by the zona pellucida that surrounds human egg cells.
Cartilage Glycoprotein: Excess levels of human cartilage glycoprotein YKL-40 cause cartilage remodelling to fail during osteoarthritis.
Mucin-Type Glycoprotein: Mucin is an O-linked oligosaccharide found in airways, digestive systems, sweat glands, and cancer cells, among others. The mucin substance surrounds tissues and cells with a protective barrier.
Glycoprotein Hormones: These three hormone glycoproteins are follicle-stimulating hormone, luteinizing hormone, and thyroid-stimulating hormone, just to name a few.
Immune Glycoprotein: Galectins, C-type lectins, and singles are some of the receptors our immune system uses to recognize surface membrane glycoproteins.
Glycoprotein IIb IIIa Inhibitors: Inhibitors of glycoprotein IIb IIIa are also related to the blood clotting process. As a result of their blocking glycoprotein IIb and IIIa receptors, platelets cannot join together to form a clot. Some types of coronary artery procedures use medicines based on this principle to prevent blood clots.
Glycoprotein Hormones
The name of the hormones are:
Follicle-stimulating hormone
Luteinizing hormone
Thyroid-stimulating hormone
Human chorionic gona
Glycoprotein and Glycolipid
The main difference between glycoprotein and glycolipid is that glycolipids are lipids that are attached to carbohydrate molecules, whereas glycoproteins are proteins that are attached to carbohydrate molecules. Additionally, glycolipids serve as cell markers or antigens that the immune system recognizes as self or non-self, while glycoproteins serve as receptors for chemical signals and play a role in adhesion.
Glycoprotein and glycolipids are two types of molecules mainly found in the cell membrane. Glycolipids and glycoproteins play several important roles in the cell.
Do You Know?
The followings are functions of Glycolipids:
Cells receive energy from it.
Cell membranes are made up of phospholipids.
It helps determine a person's blood type.
They serve as receptors on the surface of red blood cells.
The immune system, the last function of Glycolipids, is also assisted by destroying and eliminating pathogens from the body.
Some Types of Glycolipids Examples Are:
Glyceroglycolipids
Galactolipids
Sulfolipids
Cerebrosides
Glucocerebrosides
Sulfatides
Globosides
FAQs on Glycoprotein: Structure, Functions, and Biological Significance
1. What is a glycoprotein and what are its primary functions?
A glycoprotein is a type of conjugated protein where one or more carbohydrate chains, known as oligosaccharides, are covalently bonded to a protein core. These molecules are vital for many biological processes. Their primary functions include:
- Cell-Cell Recognition: Acting as surface markers for cells to identify and interact with each other.
- Immune Response: Antibodies are glycoproteins that identify and neutralise foreign invaders like bacteria and viruses.
- Hormonal Regulation: Many hormones, such as Follicle-Stimulating Hormone (FSH), are glycoproteins.
- Structural Support: They are key components of the extracellular matrix, providing structural integrity to tissues.
2. Where are glycoproteins primarily found in an organism?
Glycoproteins are found throughout the body in various locations. Their most prominent location is on the outer surface of the cell surface membrane, where their carbohydrate chains extend into the extracellular space, forming the glycocalyx. They are also found as secreted proteins in bodily fluids (e.g., mucus, blood plasma) and within cellular organelles like the endoplasmic reticulum and Golgi apparatus, where they are synthesized and modified.
3. What are some key examples of glycoproteins in the human body?
Several crucial molecules in the human body are glycoproteins. Some important examples include:
- Antibodies (Immunoglobulins): Critical proteins of the immune system that target pathogens.
- Major Histocompatibility Complex (MHC) molecules: Proteins on the cell surface essential for the immune system to recognize foreign molecules.
- Hormones: Including Human Chorionic Gonadotropin (hCG) and Thyroid-Stimulating Hormone (TSH).
- Collagen: The most abundant protein in the body, which is a glycoprotein and provides structural support to tissues.
- Mucins: The main components of mucus, which protect epithelial linings.
4. How does a glycoprotein differ from a proteoglycan?
The primary difference between a glycoprotein and a proteoglycan lies in their composition and structure. A glycoprotein is predominantly a protein with short, often branched, carbohydrate chains (oligosaccharides) attached. In contrast, a proteoglycan is predominantly a carbohydrate, consisting of a small protein core attached to very long, unbranched carbohydrate chains called glycosaminoglycans (GAGs). Essentially, glycoproteins are proteins with some sugar, while proteoglycans are sugars with some protein.
5. What is the fundamental difference between glycosylation and glycation?
The fundamental difference is that glycosylation is a highly specific, controlled enzymatic process, while glycation is a random, non-enzymatic reaction. Glycosylation is a form of post-translational modification that occurs in the endoplasmic reticulum and Golgi apparatus to produce functional glycoproteins. Glycation, on the other hand, is the accidental attachment of free sugars (like glucose) to proteins in the bloodstream, a process that can impair protein function and is associated with aging and diabetic complications.
6. Are glycoproteins considered proteins or carbohydrates?
Glycoproteins are fundamentally classified as proteins. They are a type of conjugated protein, meaning they are protein molecules that have been chemically modified by the attachment of one or more carbohydrate units. While they contain a carbohydrate component, the protein part typically forms the larger and structurally dominant part of the molecule, defining its primary identity and function.
7. What specific roles do glycoproteins play in the cell membrane?
In the cell membrane, glycoproteins perform several critical roles essential for cell survival and communication. These include:
- Receptors: They act as specific receptors for hormones, neurotransmitters, and even viruses, which must bind to these glycoproteins to enter the cell.
- Cell Adhesion: They help cells stick to each other to form tissues and also attach to the extracellular matrix.
- Cell-Cell Recognition: The unique carbohydrate patterns on glycoproteins act like cellular 'fingerprints', allowing cells to distinguish between 'self' and 'non-self' cells, which is vital for the immune system.
- Forming the Glycocalyx: Collectively, they form a carbohydrate layer called the glycocalyx that protects the cell from mechanical and chemical damage.
8. How do antibodies, as glycoproteins, use their carbohydrate parts to fight infection?
While the protein part of an antibody (the variable region) is responsible for recognizing and binding to specific antigens, the carbohydrate portion attached to the constant region (Fc) plays a crucial role in its effector functions. This 'glyco' part helps stabilise the antibody structure and mediates communication with other parts of the immune system. For example, it can influence the antibody's ability to activate the complement system (a cascade of proteins that helps clear pathogens) or to bind to Fc receptors on other immune cells like macrophages, signaling them to destroy the captured pathogen.
