Density Functional Theory Analysis of Structure, Energetics, and Spectroscopy for the Mn-Fe Active Site of Chlamydia trachomatis Ribonucleotide Reductase in Four Oxidation States

Models for the Mn Fe active site structure of ribonucleotide reductase (RNR) from pathogenic bacteria Chlamydia trachomatis(Ct) in different oxidation states have been studied in this paper, using broken-symmetry density functional theory (DFT) incorporated with the conductor like screening (COSMO) solvation model and also with finite-difference Poisson Boltzmann self-consistent reaction field (PB-SORE) calculations. The detailed structures for the reduced Mn(II)-Fe(II), the met Mn(III)-Fe(III), the oxidized Mn(IV)-Fe(III) and the superoxidized Mn(IV)-Fe(IV) states are predicted. The calculated properties, including geometries, Fe-57 Mossbauer isomer shifts and quadrupole splittings, and Fe-57 and Mn-55 electron nuclear double resonance (ENDOR) hyperfine coupling constants, are compared with the available experimental data. The Mossbauer and energetic calculations show that the (mu-oxo, mu-hydroxo) models better represent the structure of the Mn(IV) Fe(III) state than the di-mu-oxo models. The predicted Mn(IV) Fe(III) distances (2.95 and 2.98 angstrom) in the (mu-oxo, mu-hydroxo) models are in agreement with the extended X-ray absorption fine structure (EXAFS) experimental value of 2.92 angstrom (Younker et al. J. Am. Chem. Soc. 2008, 130, 15022-15027). The effect of the protein and solvent environment on the assignment of the Mn metal position is examined by comparing the relative energies of alternative mono-Mn(II) active site structures. It is proposed that if the Mn(II) Fe(II) protein is prepared with prior addition of Mn(II) or with Mn(II) richer than Fe(II), Mn is likely positioned at metal site 2, which is further from Phe127.