Hysteretic enzyme adaptation to environmental pH: change in storage pH of alkaline phosphatase leads to a pH-optimum in the opposite direction to theapplied change

The activity of alkaline phosphatase (AP) shows a change in optimum pH in t he opposite direction to the applied change in storage pH. Typically, a cha nge in storage pH from 9.8 to 8.5 results in a (reversible) change of the p H-optimum from 10.0 to 10.8. Protein fluorescence analysis shows that this response is probably due to conformational changes induced by the different storage conditions. As storage pH increases, a more 'open' or less 'compac t' conformation is attained. Analysis of the diprotic model (a model which describes possible pH-responses of enzymes) indicates, that, as the AP conf ormation is getting more 'open', an increase in the dissociation of activit y-regulating protons of AP occurs. This leads to a decrease in pH-optimum, precisely as found in the experiment. The prerequisite for such a response, however, is that the conformational adaptation to environmental assay pH i s slow (hysteretic) when compared with assay time (400 s). The relaxation t ime of this adaptation was found to be in the order of 2 h. (C) 1999 Elsevi er Science B.V. All rights reserved.