1. Redox properties of lysine- and methionine-coordinated hemes ensure downhill electron transfer in NrfH2A4 nitrite reductase.
- Author
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Todorovic S, Rodrigues ML, Matos D, and Pereira IA
- Subjects
- Electrons, Hydroxyquinolines chemistry, Models, Molecular, Oxidation-Reduction, Protein Conformation, Solutions, Solvents chemistry, Spectrum Analysis, Spectrum Analysis, Raman, Thermodynamics, Bacterial Proteins chemistry, Cytochromes a1 chemistry, Cytochromes c1 chemistry, Desulfovibrio vulgaris enzymology, Heme chemistry, Nitrate Reductases chemistry
- Abstract
The multiheme NrfHA nitrite reductase is a menaquinol:nitrite oxidoreductase that catalyzes the 6-electron reduction of nitrite to ammonia in a reaction that involves eight protons. X-ray crystallography of the enzyme from Desulfovibrio vulgaris revealed that the biological unit, NrfH2A4, houses 28 c-type heme groups, 22 of them with low spin and 6 with pentacoordinated high spin configuration. The high spin hemes, which are the electron entry and exit points of the complex, carry a highly unusual coordination for c-type hemes, lysine and methionine as proximal ligands in NrfA and NrfH, respectively. Employing redox titrations followed by X-band EPR spectroscopy and surface-enhanced resonance Raman spectroelectrochemistry, we provide the first experimental evidence for the midpoint redox potential of the NrfH menaquinol-interacting methionine-coordinated heme (-270 ± 10 mV, z = 0.96), identified by the use of the inhibitor HQNO, a structural analogue of the physiological electron donor. The redox potential of the catalytic lysine-coordinated high spin heme of NrfA is -50 ± 10 mV, z = 0.9. These values determined for the integral NrfH2A4 complex indicate that a driving force for a downhill electron transfer is ensured in this complex.
- Published
- 2012
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