Manganese(II) zero-field interaction in cambialistic and manganese superoxide dismutases and its relationship to the structure of the metal binding site

The Mn(II) high-magnetic-field electron paramagnetic resonance (HFEPR) spectra of five different superoxide dismutases (SODs) were measured at 190 and 285 GHz. The native E. coli manganese SOD was found to be distinct from the other SODS by virtue of its large zero-field E-value. The two wild-type cambialistic proteins from Porphyromonas gingivalis and Rhodobacter capsulatus were also distinct. However, the Gly155Thr mutant of the P. gingivalis SOD changed the Mn(II) spectrum so that it closely resembled the spectrum of manganese reconstituted E. coli iron SOD. This observation paralleled enzyme activity measurements that show that this mutation causes the loss of activity with manganese and enhanced activity with iron indicating a conversion from a cambialistic to an iron-specific protein. The Mn(II) magnetic parameters were determined by simultaneously fitting the multifrequency data. Simulations were carried out by numerically diagonalizing the spin Hamiltonian and explicitly calculating all possible transition probabilities. The relationship between the Mn(II) zero-field interaction and structure of the metal binding site is also discussed.