Human Biochemistry, Volume 2 - Murray R. 1993

Special Topics
Glycoproteins and Proteoglycans
Glycoproteins — ABO Locus

The ABO locus encodes two specific transferases whose function is to transfer specific Gal components to the oligosaccharide precursor Fuc-a 1,2-Gal-ß—R, which is formed through the action of the fucosyltransferase encoded by the H or Se alleles. The A-specific transferase catalyzes the reaction

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The B-specific transferase catalyzes the reaction

Table 54.8. Expression of A and B Antigens

Accordingly, in individuals with the A allele in their genotype, the GalNAc component is attached to the precursor generated by 1,2 Fuc-transferase, whereas in individuals carrying the B allele, the Gal component is transferred to the same precursor. Individuals possessing both alleles in their genotype are capable of synthesizing both Oligosaccharides: one with GalNAc residues and the other with Gal residues at the non-reducing end. Individuals lacking both A and B alleles (OO homozygotes) fail to attach either GalNAc or Gal to the precursor. Anti-A antiserum, first described by Landsteiner, recognizes the specific oligosaccharide containing GalNAc at the non-reducing end. Anti-B antiserum recognizes a closely related oligosaccharide containing a Gal residue at the non-reducing end. If both residues (both GalNAc and Gal) are absent from the non-reducing end of the oligosaccharide, it is not recognized by either of these antisera, and the Blood group antigen is classified as type O. Obviously, individuals with the hh genotype, which prevents the attachment of the Fuc residue to the corresponding Gal-ß—R oligosaccharide, are unable to express A or B antigenic determinants and also have blood group O.