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Vanadium nitrogenase: a two-hit wonder?

ABSTRACT: Nitrogenase catalyzes the biological conversion of atmospheric dinitrogen to bioavailable ammonia. The molybdenum (Mo)- and vanadium (V)-dependent nitrogenases are two homologous members of this metalloenzyme family. However, despite their similarities in structure and function, the characterization of V-nitrogenase has taken a much longer and more winding path than that of its Mo-counterpart. From the initial discovery of this nitrogen-fixing system, to the recent finding of its CO-reducing capacity, V-nitrogenase has proven to be a two-hit wonder in the over-a-century-long research of nitrogen fixation. This perspective provides a brief account of the catalytic function and structural basis of V-nitrogenase, as well as a short discussion of the theoretical and practical potentials of this unique metalloenzyme.

SUBMITTER: Hu Y

PROVIDER: S-EPMC3823560 | biostudies-literature | 2012 Jan

REPOSITORIES: biostudies-literature

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Vanadium nitrogenase: a two-hit wonder?

Hu Yilin Y Lee Chi Chung CC Ribbe Markus W MW

Dalton transactions (Cambridge, England : 2003) 20111118 4

Nitrogenase catalyzes the biological conversion of atmospheric dinitrogen to bioavailable ammonia. The molybdenum (Mo)- and vanadium (V)-dependent nitrogenases are two homologous members of this metalloenzyme family. However, despite their similarities in structure and function, the characterization of V-nitrogenase has taken a much longer and more winding path than that of its Mo-counterpart. From the initial discovery of this nitrogen-fixing system, to the recent finding of its CO-reducing cap ...[more]

PMID: 22101422

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Project description:The nitrogenase enzymes are responsible for all biological nitrogen reduction. How this is accomplished at the atomic level, however, has still not been established. The molybdenum-dependent nitrogenase has been extensively studied and is the most active catalyst for dinitrogen reduction of the nitrogenase enzymes. The vanadium-dependent form, on the other hand, displays different reactivity, being capable of CO and CO2 reduction to hydrocarbons. Only recently did a crystal structure of the VFe protein of vanadium nitrogenase become available, paving the way for detailed theoretical studies of the iron-vanadium cofactor (FeVco) within the protein matrix. The crystal structure revealed a bridging 4-atom ligand between two Fe atoms, proposed to be either a CO32- or NO3- ligand. Using a quantum mechanics/molecular mechanics model of the VFe protein, starting from the 1.35 Å crystal structure, we have systematically explored multiple computational models for FeVco, considering either a CO32- or NO3- ligand, three different redox states, and multiple broken-symmetry states. We find that only a [VFe7S8C(CO3)]2- model for FeVco reproduces the crystal structure of FeVco well, as seen in a comparison of the Fe-Fe and V-Fe distances in the computed models. Furthermore, a broken-symmetry solution with Fe2, Fe3, and Fe5 spin-down (BS7-235) is energetically preferred. The electronic structure of the [VFe7S8C(CO3)]2- BS7-235 model is compared to our [MoFe7S9C]- BS7-235 model of FeMoco via localized orbital analysis and is discussed in terms of local oxidation states and different degrees of delocalization. As previously found from Fe X-ray absorption spectroscopy studies, the Fe part of FeVco is reduced compared to FeMoco, and the calculations reveal Fe5 as locally ferrous. This suggests resting-state FeVco to be analogous to an unprotonated E1 state of FeMoco. Furthermore, V-Fe interactions in FeVco are not as strong compared to Mo-Fe interactions in FeMoco. These clear differences in the electronic structures of otherwise similar cofactors suggest an explanation for distinct differences in reactivity.

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Vanadium nitrogenase: a two-hit wonder?

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Vanadium nitrogenase: a two-hit wonder?

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