Nitric Oxide Reductase

Respiratory nitric oxide reductase (NOR) is a heterodimer with subunits of molecular masses of 54 (NORB) and 18 kDa (NORC) that catalyzes the conversion of nitric oxide to nitrous oxide. Nitric oxide reductase, a membrane enzyme contains a catalytic center composed by a non-heme iron coupled to a b type heme. Iron/heme determinations show that one heme c is present in the small subunit (NORC) and that approximately two heme b and one non-heme iron are associated with the large subunit (NORB). Mössbauer characterization of the as-purified, ascorbate-reduced and dithionite-reduced enzyme confirms the presence of three heme groups (the catalytic heme b₃, and the electron transfer heme b and heme c) and one redox-active non-heme Fe (Fe_B). Consistent with results obtained for other cNORs, heme c and heme b in Ps. nautica cNOR were found to be low-spin while Fe_B was found to be high-spin. Unexpectedly, as opposed to the presumed high-spin state for heme b₃, the Mössbauer data demonstrate unambiguously that heme b₃ is, in fact, low-spin in both ferric and ferrous states, suggesting that heme b₃ is six-coordinated regardless of its oxidation state. We suspect that the presence of a sixth ligand in the Fe²⁺ heme b₃ could weaken its affinity for NO and thus promote in a first catalytic step the binding of NO molecules at the Fe_B²⁺ site. At this point, the function of heme b₃ would then be to orient the Fe_B-bound NO molecules for the formation of the N-N bound and to provide reducing equivalents for NO reduction.

People Involved
Isabel Moura (PI)
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Publications
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Funding
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