Membrane Carbohydrate Overview
- Individual cells and cellular compartments are divided by the selective barriers found in cell membranes.
- Membranes are collections of lipids, proteins, and carbohydrates bound collectively by adhesive forces.
- As a crucial component of cell membranes and covalently attached to proteins or lipids as glycoproteins or glycolipids, carbohydrates serve as the adhesion and address sites for cells.
- The fluid lipid bilayer with floaty proteins and carbohydrates is how the fluid mosaic model characterises membranes.
- Membrane carbohydrates are chemically related to glycolipids and glycoproteins.
- The amino acid chain of certain membrane polysaccharides, which are components of proteoglycans, are inserted amid the lipid fatty acids.
- Although some carbohydrates are associated with inner membranes, most are located on the outside monolayer of the plasma membrane, which is exposed to the surrounding environment.
Membrane Carbohydrate Types
Only the outer layer of the plasma membrane contains carbohydrate groups, which are joined to lipids or proteins to create glycolipids or glycoproteins.
- The majority of the glycoproteins, or proteins related to carbohydrates, are located in membranes.
- The majority of membrane proteins include carbs.
- Since oligosaccharides are connected to proteins in glycoproteins, the majority of the molecule is made up of proteins. Because these oligosaccharides are often branched and lack serial repetitions, they are rich in information and can serve as highly specialised binding sites for other proteins.
- The ER and Golgi apparatus lumens are filled with sugar residues, similar to how glycolipids are processed. Because of this, the non-cytosolic side of the membrane always contains oligosaccharide chains.
- The endoplasmic reticulum, which creates N-linked sugars, and the Golgi apparatus, which creates O-linked sugars, are the two parts of the cell where sugars may be joined to proteins. A sugar is joined to an atom of nitrogen in N-linked glycoproteins and an atom of oxygen in O-linked glycoproteins. Because of their differing structures, N-and O-linked sugars have distinct activities.
- Membrane-bound glycoproteins play a role in several biological processes, including cell identification and cell surface antigenicity.
- Membrane lipids called glycolipids have oligosaccharide head groups that are hydrophilic.
- Membranes include three different kinds of glycolipids: glycophosphatidylinositol, glycoglycerolipids, and glycosphingolipids, which are found in animal cells in the greatest amounts. Plant cell plasma membranes contain more glycoglycerolipids than animal cells do.
- Membrane lipids only make up 5% of glycolipids.
- Similar to glycoproteins, glycolipids serve as distinct sites for recognition for proteins that bind carbohydrates.
- Proteoglycan molecules are polysaccharide chains that form part of an integral membrane.
- Proteoglycans are mostly found outside of the cell as a component of the extracellular matrix. They are made up of lengthy polysaccharide chains that are bonded covalently to a protein core.
- In contrast, some proteoglycans have a protein core that either spans the lipid bilayer or is attached to it through a glycosylphosphotidylinositol (GPI) anchor.
Membrane Carbohydrate Structure
- The plasma membrane contains carbohydrates in the form of short, occasionally branched chains of sugars, which are linked to peripheral proteins (creating glycoproteins) or the polar ends of phospholipid molecules in the outermost lipid layer (forming glycolipids).
- Straight or branched, carbohydrate chains can include between 2 and 60 monosaccharide units.
- The six main sugars are D-galactose, D-mannose, L-fucose, N-acetylneuraminic acid (sometimes referred to as sialic acid). N-acetyl-D-glucosamine and N-acetyl-D-galactosamine are combined in different ways to produce the oligosaccharide chains of membrane glycoproteins and glycolipids. These might all be produced from glucose.
- The arrangement of sugars in the oligosaccharide side chains of glycoproteins and glycolipids varies greatly.
- Although they typically include less than 15 sugars, in contrast to the amino acids in a polypeptide chain that are all linked by identical peptide bonds, they are capable of forming a variety of covalent connections with one another. They are also frequently branched.
- There are hundreds of distinct trisaccharides that may be created by combining even three sugars.
- In theory, the oligosaccharides on the cell surface are particularly well-suited to a role in particular cell-recognition processes due to their variety and exposed location.
Functions of Membrane Carbohydrates
- In addition to serving as a structural barrier, membrane carbohydrates help in cell identification and adherence via cell-cell communication or interactions with pathogens.
- Erythrocytes’ cell surface carbohydrates, which may potentially elicit immune reactions, are responsible for determining blood types.
- After an infection, selectin-containing proteins are made visible in the plasma membranes of endothelial cells close to the wounded tissue. They are able to identify and bind the carbohydrates found on lymphocytes that go through the circulation. Through this process, lymphocytes are able to adhere to blood vessel walls, pass through the endothelium, and go to the infection’s focal point.
- Additionally, crucial to embryonic development are carbohydrates acting as recognition molecules.
- Carbohydrates in the plasma membrane play an important role in pathogen identification and binding during infection.
- In humans, the glycocalyx serves crucial purposes as well. It enables blood vessel cells to tolerate the forceful liquid flow over their surfaces.
- By keeping digestive enzymes in its coat, the glycocalyx aids in the digestion of food for the absorption of nutrients and protects the microvilli in the stomach which absorb nutrients.
- Glycoproteins include hormones, enzymes, and specific plasma transport proteins. The physiological function of these molecules depends on the presence of carbohydrates.
- Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular biology of the cell. New York: Garland Science.