Role of Calcium in Metalloenzymes: Effects of Calcium Removal on the Axial Ligation Geometry and Magnetic Properties of the Catalytic Diheme Center in MauG

MauG is a diheme enzyme possessing a five-coordinate high-spin heme with an axial His ligand and a six-coordinate low-spin heme with His-Tyr axial ligation. A Ca2+ ion is linked to the two hemes via hydrogen bond networks, and the enzyme activity depends on its presence. Removal of Ca2+ altered the electron paramagnetic resonance (EPR) signals of each ferric heme such that the intensity of the high-spin heme was decreased and the low-spin heme was significantly broadened. Addition of Ca2+ back to the sample restored the original EPR signals and enzyme activity. The molecular basis for this Ca2+-dependent behavior was studied by magnetic resonance and Mossbauer spectroscopy. The results show that in the Ca2+-depleted MauG the high-spin heme was converted to a low-spin heme and the original low-spin heme exhibited a change in the relative orientations of its two axial ligands. The properties of these two hemes are each different than those of the heme in native MauG and are now similar to each other. The EPR spectrum of Ca2+-free MauG appears to describe one set of low-spin ferric heme signals with a large g(max) and g anisotropy and a greatly altered spin relaxation property. Both EPR and Mossbauer spectroscopic results show that the two hemes are present as unusual highly rhombic low-spin hemes in Ca2+-depleted MauG, with a smaller orientation angle between the two axial ligand planes. These findings provide insight into the correlation of enzyme activity with the orientation of axial heme ligands and describe a role for the calcium ion in maintaining this structural orientation that is required for activity.