Escherichia coli Co(II) alkaline phosphatase. Absorption, circular dichroism, and magnetic circular dichroism of the d-d electronic transitions.

The magnetic circular dichroism (MCD) associated with the visible d-d electronic transitions of Co(I1) alkaline phosphatase between 400 and 650 nm are similar in the acid form, the alkaline form, and the phosphate and arsenate complexes of the enzyme. Thus similar coordination geometries must exist in all forms of the enzyme. The MCD suggests that this geometry is that of a distorted tetrahedron. The natural circular dichroism (CD) of the unliganded enzyme shows little change with pH; however, the CD of the phosphate complex undergoes very large changes between pH 5 and 9 correlating with the second pK, of phosphate. In marked contrast, the MCD of the phosphate complex is not significantly pH dependent. The phosphate and arsenate complexes of Co(I1) alkaline phosphatase show natural CD bands of opposite sign, while the MCD bands are of the same sign. These features suggest that the natural CD is induced by the potential field of the protein external to the Co(I1) chromophore. MCD which is not sensitive to the external field reflects only the intrinsic geometry of the ligand field surrounding the Co(I1). The significance of the changes in external field to the mechanism of action is discussed.