The Catalytic Subunit of Escherichia coli Nitrate Reductase A Contains a Novel [4Fe-4S] Cluster with a High-Spin Ground State,

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Abstract
We have used EPR spectroscopy, redox potentiometry, and protein crystallography to characterize the [4Fe-4S] cluster (FS0) of the Escherichia coli nitrate reductase A (NarGHI) catalytic subunit (NarG). FS0 is clearly visible in the crystal structure of NarGHI [Bertero, M. G., et al. (2003) Nat. Struct. Biol. 10, 681−687] but has novel coordination comprising one His residue and three Cys residues. At low temperatures (g = 5.023 and g = 5.556. We have assigned these features to a [4Fe-4S]+ cluster with an S = 3/2 ground state, with the g = 5.023 and g = 5.556 peaks corresponding to subpopulations exhibiting ΔS = 1/2 and ΔS = 3/2 transitions, respectively. Both peaks exhibit midpoint potentials of approximately −55 mV at pH 8.0 and are eliminated in the EPR spectrum of apomolybdo-NarGHI. The structure of apomolybdo-NarGHI reveals that FS0 is still present but that there is significant conformational disorder in a segment of residues that includes one of the Cys ligands. On the basis of these observations, we have assigned the high-spin EPR features of reduced NarGHI to FS0.

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