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We have used Escherichia coli cytoplasmic membrane preparations enriched in wild-type and mutant (NarH-C16A and NarH-C263A) nitrate reductase (NarGHI) to study the role of the [Fe-S] clusters of this enzyme in electron transfer from quinol to nitrate. The spectrum of dithionite-reduced membrane bound NarGHI has major features comprising peaks at g = 2.04 and g = 1.98, a peak-trough at g = 1.95, and a trough at g = 1.87. The oxidized spectrum of NarGHI in membranes comprises an axial [3Fe-4S] cluster spectrum with a peak at g = 2.02 (g(z)) and a peak-trough at g = 1.99 (g(xy)). We have shown that in two site-directed mutants of NarGHI which lack the highest potential [4Fe-4S] cluster (B. Guigliarelli, A. Magalon, P. Asso, P. Bertrand, C. Frixon, G. Giordano, and F. Blasco, Biochemistry 35:4828-4836, 1996), NarH-C16A and NarH-C263A, oxidation of the NarH [Fe-S] clusters is inhibited compared to the wild type. During enzyme turnover in the mutant enzymes, a distinct 2-n-heptyl-4-hydroxyquinoline-N-oxide-sensitive semiquinone radical species which may be located between the hemes of NarI and the [Fe-S] clusters of NarH is observed. Overall, these studies indicate (i) the importance of the highest-potential [4Fe-4S] cluster in electron transfer from NarH to the molybdenum cofactor of NarG and (ii) that a semiquinone radical species is an important intermediate in electron transfer from quinol to nitrate.

journal of bacteriology

Characterization by electron paramagnetic resonance of the role of the Escherichia coli nitrate reductase (NarGHI) iron-sulfur clusters in electron transfer to nitrate and identification of a semiquinone radical intermediate