Site directed substitutions suggest that His-418 of beta-galactosidase (E. coli) is a ligand to Mg2+.

Site directed mutagenesis was used to replace His-418 of beta-galactosidase with Phe (H418F) or Glu (H418E). Kinetic analysis revealed that H418F beta-galactosidase was not significantly affected by the presence of Mg2+ whereas H418E beta-galactosidase retained its sensitivity to Mg2+. H418F had a kcat similar to that of Mg(2+)-free wild type beta-galactosidase. Its pH profile was shifted 1.0 pH unit lower on the alkaline side as compared to wild type beta-galactosidase (with Mg2+). This was similar to the shifting of the wild type beta-galactosidase pH profile when Mg2+ was absent. H418E beta-galactosidase was inactivated (rather than activated) by Mg2+ binding. Equilibrium dialysis studies indicated that H418E and wild type beta-galactosidase bind Mg2+ tightly whereas H418F does not. The results indicate that His-418 is probably a ligand to Mg2+.