STRUCTURE-FUNCTION RELATIONSHIPS FOR SOME METALLOALKALINE PHOSPHATASES OF E COLI

The alkaline phosphatase of Escherichia coli is a zinc metalloprotein with a metal to enzyme ratio of 4. Inorganic phosphate protects the four zinc atoms against removal by EDTA. Cadmium and manganese can replace zinc on its four binding sites. Samples of phosphatase with different proportions of zinc and cadmium or of zinc and manganese have been prepared and tested for their enzymatic activity. These results together with others suggest a model which involves two functional and two structural zinc atoms. The zinc atoms necessary for the activity would have different apparent binding constants for the apoenzyme. This conclusion is in agreement with Cohen and Wilson's scheme postulated in 1966. Dissociation constants for Me2+‐apophosphatase complexes were determined (Me2+= Zn2+, Ni2+, Cd2+, Co2+, Mn2+). These constants follow the order Ni2+ > Co2+ > Zn2+ > Cd2+= Mn2+. The optical rotatory dispersion spectra of these different metallophosphatases are very similar between 25° and 55° suggesting little change in protein structure. However, only Co2+ and Zn2+ alkaline phosphatases are active enzymes. Copyright © 1969, Wiley Blackwell. All rights reserved