EXAFS Comparison of the Dimanganese Core Structures of Manganese Catalase, Arginase, and Manganese-Substituted Ribonucleotide Reductase and Hemerythrin

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Abstract
The solution structures of the binuclear Mn centers in arginase, Mn catalase, and the Mn-substituted forms of the Fe enzymes ribonucleotide reductase and hemerythrin have been determined using X-ray absorption spectroscopy (XAS). X-ray absorption near edge structure (XANES) spectra for these proteins were compared to those obtained for Mn(II) models. The Mn model spectra show an inverse correlation between the XANES peak maximum and the root-mean-square (RMS) deviation in metal−ligand bond lengths. For these complexes, the XANES maxima appear to be more effective than the 1s → 3d areas as an indicator of metal-site symmetry. Arginase and Mn-substituted ribonucleotide reductase have symmetric nearest neighbor environments with low RMS deviation in bond length, while Mn catalase and Mn-substituted hemerythrin appear to have a larger RMS bond length deviation. The 1s → 3d areas for arginase and Mn-substituted ribonucleotide reductase are consistent with six coordinate Mn, while the 1s → 3d areas for Mn catalase and Mn-substituted hemerythrin are larger, suggesting that one or both of the Mn ions are five-coordinate in these proteins. Extended x-ray absorption fine structure (EXAFS) spectra were used to determine the Mn2 core structure for the four proteins. In order to quantitate the number of histidine residues bound to the Mn2 centers, EXAFS data for the crystallographically characterized model hexakis−imidazole Mn(II) dichloride tetrahydrate were used to calibrate the Mn−imidazole multiple scattering interactions. These calibrated parameters allowed the outer shell EXAFS to be fit to give a lower limit on the number of bound histidine residues. The EXAFS spectra for Mn-substituted ribonucleotide reductase and arginase are nearly identical, with symmetric Mn−nearest neighbor environments and outer shell scattering consistent with a lower limit of one histidine per Mn2 core. In contrast, the EXAFS data for Mn catalase and Mn-substituted hemerythrin show two distinct Mn−nearest neighbor shells, modeled as Mn−O at ca. 2.1 Å and Mn−N at ca. 2.3 Å, and outer shell carbon scattering consistent with a lower limit of ca. 2−3 His residues per Mn2 core. Only Mn catalase shows clear evidence for Mn···Mn scattering. The observed Mn···Mn distance is 3.53 Å, which is significantly longer than the ∼3.3 Å distances that are typically observed for Mn(II)2 cores with two single atom bridges, but which is typical of the distances seen in Mn(II)2 cores having one single atom bridge (e.g., aqua or hydroxo) together with one or two carboxylate bridges. The absence of EXAFS-detectable Mn···Mn interactions for the other three proteins suggests either that there are no single atom bridges in these cases or that the Mn···Mn interactions are more disordered.

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