Answer:
The blood group A antigen is synthesized by an N-acetylgalactosaminyltransferase that uses a UDP-α-N-acetylgalactosamine (UDP-GalNAc) as sugar donor substrate, while the B antigen is synthesized by a galactosyltransferase that uses a UDP-α-galactose (UDP-Gal) as a sugar donor substrate.
