Question

the structure and composition of glycoproteins determines the A.B and O blood groups in humans. explain how this happens and draw the structure of carbohydrates attached to the blood antigen O in the Haworth Projetion, showing the correct glycoside linkages

Answer 1

Glycoproteins are the proteins conjugated with carbohydrates. There are three classes of glycoproteins namely simple glycoproteins, proteoglycans and mucoproteins. Most of the glycoproteins are present on the structures of cell membranes. In simple glycoproteins, protein constituent gives major weight and they act in binding of sperm to egg and cell adhesion.

The surface of blood cells is covered with glycoproteins that project out into the blood stream, so that antibodies can recognise them.

The proteins on the erythrocyte cell surface contain a wide variety of different oligosaccharides that are attached in various ways to the protein. However, it spite of this variation, there are a few structures that are very common. One of the most common “core” structures is something called H-antigen. It is composed of many different sugars but the outside end of the H-antigen structure always consists of a fucose (Fuc) residue, a galactose residue (Gal), and an N-acetylglucosamine (GlcNAc) residue.

The core oligosaccharide is modified by adding an N-acetylgalactosamine (GalNAc) residue in the presence of N-acetylaminogalactosyltransferase enzyme to form a branched structure at the end of the oligosaccharide. This enzyme is coded by a gene on chromosome 9.  If the RBC's contain GalNAc on its surface, then the blood type is A. If the gene is defective for making GalNAc enzyme, then the blood group is O.

People with blood type A will not have antibodies to H antigen since this is the normal precursor to A antigen and there will always be some on the cell surface. In other words, the H antigen will be recognized as self.Normal red blood cells are recognized as “self” so we don’t have antibodies against our own cells. However, we will have antibodies against the red blood cells of other people’s blood if their cell surface carbohydrates are different from ours. This is the basis of ABO blood group and it’s why we have to match blood types in a blood transfusion.

The ABO blood group was discovered over one hundred years ago by Karl Landsteiner (Nobel Laureate: Karl Landsteiner). The biochemical basis was only elucidated in the 1970’s when the technology for examining the carbohydrate structure of glycoproteins was worked out.

If the small number of mutations present on allele of the GalNAc enzyme switches it to code for galactose (Gal instead of GalNAc), . The variant enzyme is called B enzyme (galactosyltransferase) and the B antigen structure has a terminal galactose (Gal) instead of a terminal GalNAc. If a person contains homozygous allels for B on chromosome 9,  the surface of red blood cells have the B antigen oligosaccharide. No antibodies are synthesised by the body against these cells, because thy are self, but antibodes will be synthesised against others blood groups. So, we must check for compatibility between blood groups during blood transfusion. The following structure gives the linkage of carbohydrate chains to the blood cells.

AB N ac Fucose Legend Red blood cel N ac Galactose