Title

Variable-temperature, variable-field magnetic circular dichroism spectroscopic study of NifEN-bound precursor and "FeMoco"

Document Type

Article

Publication Date

2-1-2011

Abstract

NifEN plays a key role in the biosynthesis of the iron-molybdenum cofactor (FeMoco) of nitrogenase. A scaffold protein that hosts the conversion of a FeMoco precursor to a mature cofactor, NifEN can assume three conformations during the process of FeMoco maturation. One, designated ΔnifB NifEN, contains only two permanent [Fe(4)S(4)]-like clusters. The second, designated NifEN(Precursor), contains the permanent clusters and a precursor form of FeMoco. The third, designated NifEN("FeMoco"), contains the permanent [Fe(4)S(4)]-like clusters and a fully complemented, "FeMoco"-like structure. Here, we report a variable-temperature, variable-field magnetic circular dichroism spectroscopic investigation of the electronic structure of the metal clusters in the three forms of dithionite-reduced NifEN. Our data indicate that the permanent [Fe(4)S(4)]-like clusters are structurally and electronically conserved in all three NifEN species and exhibit spectral features of classic [Fe(4)S(4)](+) clusters; however, they are present in a mixed spin state with a small contribution from the S > ½ spin state. Our results also suggest that both the precursor and "FeMoco" have a conserved Fe/S electronic structure that is similar to the electronic structure of FeMoco in the MoFe protein, and that the "FeMoco" in NifEN("FeMoco") exists, predominantly, in an S = 3/2 spin state with spectral parameters identical to those of FeMoco in the MoFe protein. These observations provide strong support to the outcome of our previous EPR and X-ray absorption spectroscopy/extended X-ray absorption fine structure analysis of the three NifEN species while providing significant new insights into the unique electronic properties of the precursor and "FeMoco" in NifEN.

Publication Source (Journal or Book title)

Journal of biological inorganic chemistry : JBIC : a publication of the Society of Biological Inorganic Chemistry

First Page

325

Last Page

32

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