Revisiting the Mössbauer Isomer Shifts of the FeMoco Cluster of Nitrogenase and the Cofactor Charge

Abstract

Despite decades of research, the structure-activity relationship of nitrogenase is still not understood. Only recently was the full molecular structure of the FeMo cofactor (FeMoco) revealed, but the charge and metal oxidation states of FeMoco have been controversial. With the recent identification of the interstitial atom as a carbide and the more recent revised oxidation-state assignment of the molybdenum atom as Mo(III), here we revisit the Mössbauer properties of FeMoco. By a detailed error analysis of density functional theory-computed isomer shifts and computing isomer shifts relative to the P-cluster, we find that only the charge of [MoFeSC] fits the experimental data. In view of the recent Mo(III) identification, the charge of [MoFeSC] corresponds to a formal oxidation-state assignment of Mo(III)3Fe(II)4Fe(III), although due to spin delocalization, the physical oxidation state distribution might also be interpreted as Mo(III)1Fe(II)4Fe(2.5)2Fe(III), according to a localized orbital analysis of the M = 3/2 broken symmetry solution. These results can be reconciled with the recent spatially resolved anomalous dispersion study by Einsle et al. that suggests the Mo(III)3Fe(II)4Fe(III) distribution, if some spin localization (either through interactions with the protein environment or through vibronic coupling) were to take place.

QM/MM Study of the Nitrogenase MoFe Protein Resting State: Broken-Symmetry States, Protonation States, and QM Region Convergence in the FeMoco Active Site

“…These relative energies (all at the TPSSh level) are similar to what has been found before 32 but due to the small differences we do not think these energies help us deduce the correct BS solution. After all, there is nothing to indicate that our calculations come close to chemical accuracy (~ 1 kcal/mol).…”

“…The charge on FeMoco has been controversial in nitrogenase research as discussed in a recent article by one of us 32 . Early spectroscopic work 73 It is nonetheless of interest to use all available experimental data and structural data is both fundamental, accessible from computations and a vital test for a computational model.…”

“…Due to resolution limitations and the fact that homocitrate was severely truncated and approximated as a glycolate group in their study, the validity of this assignment was unclear. In previous studies 28,29,30,31,32 by one of us (RB) we have assigned the alkoxide group of the homocitrate as protonated but have not until now given a complete justification for this as the question of how well the protein environment is described surrounding the homocitrate remained unclear. The only exception to this is the 4ND8 crystal structure, recently published by Rees & Howard et al 85 which is a crystal structure solved under basic (pH 9.5) conditions, where the O-O distance is on average 2.77 Å as shown in Figure 6d.…”

“…Recently Adamo 26 et al used QM/QM' calculations to propose new mechanisms and Ryde et al studied protonation states by QM/MM 27 . In previous articles by one of us (RB) we utilized large 225-atom cluster models in our studies of the spectroscopic properties of the FeMo cofactor 28,29,30,31,32 . Large cluster models become difficult to work with and require many constraints to keep the correct active site geometry and may not describe correctly the structural flexibility of the cofactor in the enzyme active site pocket.…”

Density Functional Calculations for Prediction of 57Fe Mössbauer Isomer Shifts and Quadrupole Splittings in β-Diketiminate Complexes

“…Others have also noted that the isomer shift correlations in the literature often need to be “tuned” to different kinds of environments, for example iron-sulfur clusters. 12 …”