What are the enzymes encoded by allele A and allele B in the human ABO blood group? What chemical reaction each do enzyme catalyze?

The enzyme encode by ABO blood group is Histo-blood group ABO system transferase.

Allele A for ABO produces α-1,3-N-acetylgalactosamine transferase (A-transferase), it aids in the production of antigen in A and AB individulas while

Allele B produces α-1,3-galactosyl transferase (B-transferase) it aids the production of B antigen in B and AB individuals.

A transferase catalyzes the transfer from the Uridine diphosphate - GalNAc donor nucleotide residues to Gal residues of the acceptor H antigen, converting the H antigen into antigen A.

B Transferase catalyzes the reaction by transferring Gal residues from the Uridine diphosphate to acceptor H antigen

which then convert the H antigen into B antigen.