A Low-Redox Potential Heme in the Dinuclear Center of Bacterial Nitric Oxide Reductase: Implications for the Evolution of Energy-Conserving Heme−Copper Oxidases

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Abstract
Bacterial nitric oxide reductase (NOR) catalyzes the two-electron reduction of nitric oxide to nitrous oxide. It is a highly diverged member of the superfamily of heme−copper oxidases. The main feature by which NOR is distinguished from the heme−copper oxidases is the elemental composition of the active site, a dinuclear center comprised of heme b3 and non-heme iron (FeB). The visible region electronic absorption spectrum of reduced NOR exhibits a maximum at 551 nm with a distinct shoulder at 560 nm; these are attributed to Fe(II) heme c (Em = 310 mV) and Fe(II) heme b (Em = 345 mV), respectively. The electronic absorption spectrum of oxidized NOR exhibits a characteristic shoulder around 595 nm that exhibits complex behavior in equilibrium redox titrations. The first phase of reduction is characterized by an apparent shift of the shoulder to 604 nm and a decrease in intensity. This is due to reduction of FeB (Em = 320 mV), while the subsequent bleaching of the 604 nm band represents reduction of heme b3 (Em = 60 mV). This separation of redox potentials (>200 mV) allows the enzyme to be poised in the three-electron reduced state for detailed spectroscopic examination of the Fe(III) heme b3 center. The low midpoint potential of heme b3 represents a thermodynamic barrier to the complete (two-electron) reduction of the dinuclear center. This may avoid formation of a stable Fe(II) heme b3−NO species during turnover, which may be an inhibited state of the enzyme. It would also appear that the evolution of significant oxygen reducing activity by heme−copper oxidases was not simply a matter of the substitution of copper for non-heme iron in the dinuclear center. Changes in the protein environment that modulate the midpoint redox potential of heme b3 to facilitate both complete reduction of the dinuclear center (a prerequisite for oxygen binding) and rapid heme−heme electron transfer were also necessary.

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