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Electrostatic control of enzyme reactions effect of ionic strength on the pk a of an essential acidic group on glucose oxidase ec

, : Electrostatic control of enzyme reactions effect of ionic strength on the pk a of an essential acidic group on glucose oxidase ec Biochemistry 20(25): 7182-7185

The dissociation constant of an essential acidic group on the reduced form of glucose oxidase from Aspergillus niger (K4) was extremely sensitive to ionic strength. Increasing the ionic strength from 0.025-0.225 causes a decrease in pK4,obsd of 0.9 pH unit, from 8.2-7.3. Analysis of the ionic strength dependence of pK4,obsd, making the assumption that the enzyme is a homogeneously charged impenetrable sphere (Edsall, J. T., and Wyman, J. 1958) predicts that the intrinsic pKa of the acidic group is 6.7 and that the charge on the protein is -78. The enzyme was titrated from its isoelectric point (pH 4.05) to pH 7.7, the pH at which the ionic strength dependence was determined. It has an actual charge at that pH of -77, in remarkable agreement with the theoretical prediction. Thus, glucose oxidase exerts electrostatic control on pK4,obsd as though it were a uniformly charged sphere. The group responsible for pK4,obsd has not been identified. However, its measured .DELTA.Hoobsd of 8.0 kcal mol-1 and .DELTA.Soobsd of -6.1 cal mol-1K-1, together with its pKa of 6.7, are consistent with the group being a histidine residue.

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