Sequential Interchange of Four Amino Acids from Blood Group B to Blood Group A Glycosyltransferase Boosts Catalytic Activity and Progressively Modifies Substrate Recognition in Human Recombinant Enzymes

The human blood group A and B glycosyltransferase enzymes are highly homologous and the alteration of four critical amino acid residues (Arg-176 --> Gly, Gly-235 --> Ser, Leu-266 --> Met, and Gly-268 --> Ala) is sufficient to change the enzyme specificity from a blood group A to a blood group B glycosyltransferase. To carry out a systematic study, a synthetic gene strategy was employed to obtain their genes and to allow facile mutagenesis. Soluble forms of a recombinant glycosyltransferase A and a set of hybrid glycosyltransferase A and B mutants were expressed in Escherichia coli in high yields, which allowed them to be kinetically characterized extensively for the first time. A functional hybrid A/B mutant enzyme was able to catalyze both A and B reactions, with the kcat being 5-fold higher for the A donor. Surprisingly, even a single amino acid replacement in glycosyltransferase A with the corresponding residue from glycosyltransferase B (Arg-176 --> Gly) produced enzymes with glycosyltransferase A activity only, but with very large (11-fold) increases in the kcat and increased specificity. The increases observed in kcat are among the largest obtained for a single amino acid change and are advantageous for the preparative scale synthesis of blood group antigens.