Electron transfer from heme bL to the [3Fe-4S] cluster of Escherichia coli nitrate reductase A (NarGHI).
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(H56R) and NarGHI(H205Y)) that lack the highest potential heme of NarI (heme b(H)), a large negative DeltaE(m,7) 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(H), but lacking heme b(L) (NarGHI(H66Y)), there is no effect on the NarH [3Fe-4S] cluster redox properties. These results suggest a role for heme b(H) in electron transfer to the [3Fe-4S] cluster. Studies of the pH dependence of the [3Fe-4S] cluster, heme b(H), and heme b(L) E(m) values suggest that significant deprotonation is only observed during oxidation of the latter heme (a pH dependence of -36 mV pH(-1)). In NarI expressed in the absence of NarGH [NarI(DeltaGH)], apparent exposure of heme b(H) to the aqueous milieu results in both it and heme b(L) having E(m) values with pH dependencies of approximately -30 mV pH(-1). These results are consistent with heme b(H) 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(L) but do not inhibit oxidation of heme b(H). 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(P) site --> heme b(L) --> heme b(H) --> [3Fe-4S] cluster.
Rothery
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CIHR Group in the Molecular Biology of Membrane Proteins, Department of Biochemistry, University of Alberta, Edmonton, Alberta T6G 2H7, Canada.
Blasco
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Weiner
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D016227:Benzoquinones / Q000737:chemistry; D019281:Dimerization; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport / Q000235:genetics; D004791:Enzyme Inhibitors / Q000737:chemistry; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics; D006418:Heme / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D020035:Holoenzymes / Q000235:genetics / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D006873:Hydroquinones / Q000737:chemistry; D006912:Hydroxyquinolines / Q000737:chemistry; D007506:Iron-Sulfur Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D005810:Multigene Family; D016297:Mutagenesis, Site-Directed; D009565:Nitrate Reductases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D010084:Oxidation-Reduction / Q000187:drug effects; D011090:Polyenes / Q000737:chemistry; D011199:Potentiometry; D011485:Protein Binding; D019163:Reducing Agents / Q000737:chemistry; D013053:Spectrophotometry
D016227:Benzoquinones; D004791:Enzyme Inhibitors; D020035:Holoenzymes; D006873:Hydroquinones; D006912:Hydroxyquinolines; D007506:Iron-Sulfur Proteins; D011090:Polyenes; D019163:Reducing Agents; C001333:2-(n-heptyl)-4-hydroxyquinoline N-oxide; D006418:Heme; C041573:stigmatellin; D009565:Nitrate Reductases; C016421:duroquinol; C034257:duroquinone