Manganese(II) Zero-Field Interaction in Cambialistic and Manganese Superoxide Dismutases and Its Relationship to the Structure of the Metal Binding Site

Abstract

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.

Multifrequency pulsed EPR studies of biologically relevant manganese(II) complexes

“…Results from the 190-285 GHz CW EPR studies by Un and coworkers showed that the value of Mn(II) ZFS parameters could be correlated to activity. For example, mutation of a conserved glycine residue 1.1 nm from the Mn-binding site in cambialistic SOD resulted in a loss of Mn-dependent activity accompanied by ZFS values that are more similar to those found for Mn-(Fe)SOD [42]. This change in the electronic structure coincided with increased activity of the Fe-reconstituted form of the cambialistic SOD.…”

“…The five half-filled 3d-orbitals make all ligand-field excited states spin-forbidden and thus preclude spin-orbit coupling (SOC) of orbital angular momentum into the ground electronic state. Furthermore, metal-to-ligand charge transfer excited states for Mn(II) complexes are normally too high in energy and the SOC constant for the corresponding ligand atom is too small to significantly contribute to changes in g. Nonetheless, with increasing magnitude of the applied magnetic field -as in a high-frequency EPR experiment -even small anisotropy (<200 ppm) in g can be resolved [42]. Not affected, however, by the higher field is the hyperfine A interaction between the unpaired electrons and nuclear spins.…”

“…The EPR at multiple frequencies (MF EPR) approach is well illustrated by recent high-field studies that precisely determined ZFS parameters for MnSODs from a variety of organisms [42,63,64]. Both Mn-and Fe-containing SODs possess nearly identical first coordination spheres, yet Fe-substituted MnSOD and Mn-substituted FeSOD (Mn-(Fe)SOD) are both inactive.…”

“…The ZFS parameters derived from spectral simulations are significantly different from those previously determined [110] and are noticeably larger than those for any other system studied in this repon. Usually such large values imply that the Mn(II) center is no longer in a pseudo-octahedral ligand field and is likely pentacoordinate [42,44,87] or some highly distorted tetrahedron [111]. However, NMR and X-ray crystallographic results clearly indicate that Mn(II)EDTA is seven-coordinate with a water ligand occupying an apical position [112,113].…”

Responses of Mn2+speciation inDeinococcus radioduransandEscherichia colito γ-radiation by advanced paramagnetic resonance methods

“…The ESE-EPR spectrum of Mn 2+ in a solution of N-methylimidazole (NMeIm) (Fig. S1), chosen as an intensity standard for ESEEM measurements (21) shows lower symmetry and much larger ZFS (23,24), leading to more pronounced wings associated with the satellite transitions. This is illustrated by the sample enzyme spectra in Fig.…”