The activation of beta-galactosidase (E. coli) by Mg(2+) at lower pH values

The activation of beta-galactosidase (E. coli) by Mg(2+) at pH values below 7.6 was studied. If the Mg(2+) concentration was high enough, the k(cat) values at pH values down to 5.0 remained at the same high level as at pH 7 and 7.6 (600-620 s-(1) with o-nitrophenyl-beta-D-galactopyranoside as the substrate). Very high concentrations of Mg(2+) (greater than 100 mM at pH 5) were, however, needed to saturate the Mg(2+) site at lower pH values. The Km values at low levels of Mg(2+) were high at every pH but they decreased and approached the same low value at every pH (about 0.13 mM) as the [Mg(2+)] was increased. These data indicate that it is difficult to bind Mg(2+) at lower pH values, but the k(cat) and K(m) values of the enzyme, and therefore the rates of galactosylation (k(2)), degalactosylation (k(3)), and binding (K(s)), do not change substantially as a function of pH provided that a Mg(2+) is bound to the enzyme. The data also showed that Mg(2+) and protons compete for the same site. Analysis by plotting log [Mg(2+)](mid) vs. pH showed that the binding of Mg(2+) to the free enzyme involves two groups with pK(a) values in the vicinity of 7 and one group with a pK(a) value near 5.5. (The values referred to as [Mg(2+)](mid) are the Mg(2+) concentrations that resulted in k(cat) values midway between basal and maximum.) The "apparent" pK(a) values of the groups when a Mg(2+) was bound (at saturating [Mg(2+)]) all appeared to be below 5.0.