Friday, February 26, 2010
This is another rant about poor teaching and false text book information. I have been trying to summarize some of the key points of genetics and cell biology that I think should shape the minds of young biologists. To make my point, I want to outline where the common ABO blood group antigens are made and displayed.
ABO Antigens Are Oligosaccharides, Not Proteins
Red blood cells that are type A will clump together with antibodies that bind to A antigens. At this point many instructors leap ahead and say that the expression of the A gene results in the production of the A protein, that is displayed on the surface of RBCs. Wrong. The antigens are carbohydrates, short chains of sugars called oligosaccharides, attached to a lipid embedded in the RBC membrane.
Glycosyl Transferases - Enzymes that Assemble ABO Oligosaccharides
Oligosaccharides are synthesized by adding one sugar at a time onto a growing chain. Information-rich oligosaccharides, such as the ABO antigens, are made of several different sugars and are linked in a variety of ways to other sugars, so each sugar is added by a different enzyme. The sugar-adding-enzymes are called glycosyl transferases. Similar to other macromolecular polymerizations, e.g. protein and nucleic acid synthesis, the monomer sugars are first activated by bonding to a nucleotide phosphate (typically UDP) and the sugar is transferred from this activated intermediate to the growing sugar chain, in this case the H antigen.
The glycosyl transferase coded by A allele transfers an N-acetylgalactosamine to the end of the H antigen oligosaccharide, whereas the glycosyl transferase coded by the B allele transfers a galactose residue.
Dominance Means Functional Enzyme, Recessive Means Dysfunctional
The ABO blood group system provides a simple molecular interpretation of genetic dominance. An allele is dominant if it produces a functional enzyme that gives the phenotype, in this case a particular glycolipid attached to an RBC. A or B alleles that have mutated to yield dysfunctional proteins that no longer function as glycosyl transferases are recessive. Recessive alleles don’t impact phenotype. O antigen is nothing more than the original unmodified H oligossacharide.
ABO Is Bizarre because A and B Code for Two Alternative Functional Enzymes - Codominance
Multiple mutations converted an allele that coded for a glycosyl transferse into a second allele that transferred a different sugar. A and B alleles code for two different glycosyl transferases that transfer two different sugars. The result is codominance. AB individual have both enzymes and produce both A and B antigens on their RBCs. This is very unusual and inconsistent with Mendelian genetics.
AB Glycosyl Transferases Are in the Golgi to Produce Oligosaccharides for Export
Part of the peculiarity of the ABO system derives from the action of the A and B glycosyl transferases in the Golgi. Proteins destined for secretion have a hydrophobic group of amino acids, the signal peptide, that is synthesized first as a messenger RNA is translated into protein on a ribosome. The signal peptide orchestrates binding of the ribosome to the endoplasmic reticulum (ER) and the growing polypeptide is extruded into the ER. A series of vesicle budding and fusing events transfers proteins to the lamina of the Golgi apparatus and a final fusion event with the cytoplasmic membrane results in secretion.
Specific enzymes retained in the ER and the Golgi recognize particular amino acid sequences and attach sugars to exposed hydroxyl of amino groups (O or N glycosylations). The ABO antigens are a little unusual in that the oligosaccharide is attached to a lipid anchor, so that the A and B glycosyl transferases localized in the Golgi attach terminal sugars and the glycolipid remains bound to the cytoplasmic membrane and is displayed on the surface of the RBC.
RBCs Don’t Have Nuclei, ER or Golgi, but Normoblasts Do
Mammalian RBC’s don’t have nuclei, because the nuclei were expelled during RBC development. Since the ER is an extension of the outer membrane of the nucleus and the Golgi is derived from the ER, it follows that RBCs don’t have any of these structures. RBCs are just collapsed bags of ribosomes, hemoglobin mRNA, cytoplasmic enzymes, a few mitochondria, lots of hemoglobin and stiffened membranes displaying ABO oligosaccharide antigens. The ABO antigens were assembled and displayed on the membrane before the loss of the nucleus.
ABO Antigens Are Also on von Willebrand Factor of Endothelial Cells and Platelets
I have never heard a good explanation of why a transfusion from a universal, type O donor to recipients with type A, B or AB blood doesn’t cause clumping of RBCs. The galactose and galactosamine sugars that decorate B and A RBCs also commonly decorate the surfaces of bacteria. The immune system is prevented from making antibodies against its own antigens and so it only produces antibodies against A or B antigens found in bacteria, if they are not own self RBCs. Type O individuals lack A and B antigens of their own, so they produce anti-A and Anti-B against bacterial oligosaccharides. So why don’t the anti-A and anti-B antibodies in type O blood serum clump type A or type B blood?
The answer is that a serum protein, von Willebrand Factor (vWF) is also decorated with the ABO oligosaccharides. A type A person has type A vWF and a type B person has type A vWF. Thus, the antibodies against A and B oligosaccharide antigens that are present in type O blood will be inactivated by binding to the A or B oligosaccharides of the recipients vWF.
vWF is synthesized by endothelial cells that line veins and arteries. It is present in the secretory Weibel-Palade bodies of endothelial cells. It is also present for secretion by platelets.
So, the ABO blood group antigens are very strange examples that are not protein, not Mendelian and are not even predominantly found on RBCs. The AB oligosaccharides do provide good examples of the use of activated intermediates in macromolecular synthesis, the origin of organelles, secretion, erythropoiesis and immunology.
at 2:33 PM