Chemistry:Types and Function of Oligosaccharides

Types of Oligosaccharides Disaccharides These are the sugars having two monomeric units, and thus it is called di_saccharide. Some examples are maltose, sucrose, and lactose. Maltose is the action of an enzyme and it gives glucose +glucose; Sucrose (or cane sugar) in the action of invertase produces fructose + glucose, and Lactose(milk sugar) in the action of enzyme lactase produces galactose + glucose. Trisaccharides These contain three monomers like raffinose. Tetrasaccharides These contain four monomeric units like stachyose. Functions Cell Recognition All the cells are coated either in glycolipids or glycoproteins, both of which help determine the cell types. Proteins or lectins that bind carbohydrates may recognize the particular oligosaccharides and provide some useful information for cell recognition depending on the oligosaccharide binding. An important example of oligosaccharide cell recognition is given as the role of glycolipids in blood type determining. Various blood types are distinguished by the modification of glycan that is present on the surface of blood cells. These may be visualized using mass spectrometry. The oligosaccharides that are found on the A, B, and H antigens take place on the non-reducing ends of the oligosaccharide. The H antigen (that indicates an O blood type) serves as a precursor for both the A and B antigens. Thus, a person with blood type A will have both the A antigen and H antigen present on the glycolipids of the membrane of red blood cell plasma. A person with blood type B will have both the B and H antigens present. And a person with blood type AB will have the three antigens A, B, and H. And finally, a person having blood type O will have only the H antigen. This means that all the blood types contain the H antigen which explains why the blood type O is called the "universal donor." Cell Adhesion Several cells produce particular carbohydrate-binding proteins called lectins that mediate cell adhesion with the oligosaccharides. Selectins, which are a family of lectins, mediate certain cell-cell adhesion processes, including those of leukocytes to the endothelial cells. In the immune response, endothelial cells may express certain selectins transiently in response to the injury or damage to the cells. Also, in response, a reciprocal selectin–oligosaccharide interaction will take place between the two molecules that allow the white blood cell to help to eliminate the damage or infection. Often, Protein-Carbohydrate bonding is mediated by the van der Waals forces and hydrogen bonding.