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Electron transfer from heme b to the [3Fe-4S] cluster of Escherichia coli nitrate reductase A (NarGHI)

Biochemistry, ISSN: 0006-2960, Vol: 40, Issue: 17, Page: 5260-5268
2001
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Article Description

We have investigated the functional relationship between three of the prosthetic groups of Escherichia coli nitrate reductase A (NarGHI): the two hemes of the membrane anchor subunit (NarI) and the [3Fe-4S] cluster of the electron-transfer subunit (NarH). In two site-directed mutants (NarGHI and NarGHI) that lack the highest potential heine of NarI (heine b), a large negative ΔE is elicited on the NarH [3Fe-4S] cluster, suggesting a close juxtaposition of these two centers in the holoenzyme. In a mutant retaining heme b, but lacking heme b (NarGHI), there is no effect on the NarH [3Fe-4S] cluster redox properties. These results suggest a role for heme b in electron transfer to the [3Fe-4S] cluster. Studies of the pH dependence of the [3Fe-4S] cluster, heine b, and heine b E values suggest that significant deprotonation is only observed during oxidation of the latter heine (a pH dependence of -36 mV pH). In NarI expressed in the absence of NarGH [NarI(ΔGH)], apparent exposure of heme b to the aqueous milieu results in both it and heme b having E values with pH dependencies of approximately -30 mV pH. These results are consistent with heme b being isolated from the aqueous milieu and pH effects in the holoenzyme. Optical spectroscopy indicates that inhibitors such as HOQNO and stigmatellin bind and inhibit oxidation of heme b but do not inhibit oxidation of heme b. Fluorescence quench titrations indicate that HOQNO binds with higher affinity to the reduced form of NarGHI than to the oxidized form. Overall, the data support the following model for electron transfer through the NarI region of NarGHI: Q site → heme b → heme b → [3Fe-4S] cluster.

Bibliographic Details

Richard A. Rothery; Francis Blasco; Joel H. Weiner

American Chemical Society (ACS)

Biochemistry, Genetics and Molecular Biology

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