A model for the structural differences between antigens constructed by the A1 and A2 subgroup transferases.14
A1 and A2 subgroup α(1,3)N-acetylgalactosaminyltransferases can form the type 2 A moieties (at left) characteristic of the A2 phenotype. These molecules may then serve as precursors for the action of a β(1,3) galactosyltransferase (β1,3Gal transferase) that synthesizes a type 3 precursor (type 3 Gal A) also characteristic of the A2 phenotype. The H locus α(1,2)fucosyltransferase (H transferase) then modifies this precursor, yielding a type 3 H antigen. Type 3 H determinants are then efficiently utilized as substrates by the A1 transferase (but not by the A2 transferase) to form type 3 A molecules that maintain repetitive A-reactive units, and that are proposed to be responsible for the A1 phenotype. R indicates the glycoconjugate substructure that consists of N-linked, O-linked, or lipid-linked glycoconjugates.
