Molecular structure and function of bacterial nitric oxide reductase

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… understand the molecular mechanism of the catalytic N 2 O for- mation on the binuclear center of cNOR, we followed the reaction of the fully reduced P.a. enzyme with NO using rapid mixing freeze- quenching techniques [19]. The sample produced at 1 ms after the mixing of the enzyme with NO gave an ESR spectrum, in which [35], and the latter is assigned to the Fe B (II)-NO complex (S = 3/2). On the basis of the EPR data, we proposed that two NO molecules are shared with two irons in the binuclear center, and that this species might be the reaction intermediate, which is consis- tent with the trans-mechanism (see Fig. 7). Low redox potential of the heme b 3 iron (60 mV) in Pa.d. cNOR [21], as compared with the other redox centers (N300 mV), indicates that the heme b 3 iron is difficult to be reduced (Scheme 1). However, a recent study involving the redox titration in Pa.d. cNOR showed that the presence of an exoge- nous ligand, CO, results in the elevation of the redox potential of heme b 3 to a range similar to that of the other redox centers [56]. Therefore, the binding of NO to heme b 3 iron could facilitate the re- duction of the heme b 3 iron (Scheme 1). In the trans-mechanism, the electron transfer from both Fe(II) to the two NO ligands, eventu- ally the Fe(III)-NO ? state, would promote N\N bond formation through a disproportionation-type reaction, and two protons then would facilitate N\O bond cleavage to produce N 2 O and H 2 O (Scheme 1). This proposed mechanism was if good agreement with the model study reported by Collman and co-workers, in which NO binds to both heme and non-heme Fe(II) and one N 2 O molecule is produced from two equivalents of NO [57]. However, another NOR model with a NO complex is thermally stable and does not react fur- ther to give N 2 O upon the addition of the proton source [58]. In addi- tion, it is important to bear in mind that the presence of two paramagnetic Fe(II)-NO species in the single binuclear center can lead to spin-coupling and the production of an ESR-silent species [59]. By contrast, two other mechanisms, so-called cis-mechanisms, have been also proposed, in which two NO molecules bind to either the heme b 3 iron or Fe B (see Fig. 7). Thomson and co-workers favor the cis-Fe B mech- anism because this model has a vacant heme b 3 site, and a rather stable ferrous heme nitrosyl {FeNO} 7 species, a potential “dead-end” product, is therefore not formed during the turnover [21,[60][61][62][63]. In the cis- heme b 3 mechanism, the first NO molecule binds to heme b 3 to form a {FeNO} 7 species which is then electrophilically attacked by the second NO molecule [64,65]. The mechanism appears to be analogous to that proposed for fungal P450-type nitric oxide reductase, P450nor [66][67][68]. However, in the case of P450nor, the key step is the formation of the short-lived intermediate {FeNO} 8 by a two-electron (H ? ) reduction of the ferric heme-NO complex, while the one-electron reduced form {FeNO} 7 of P450nor never reacts with another …

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Context 2
… understand the molecular mechanism of the catalytic N 2 O for- mation on the binuclear center of cNOR, we followed the reaction of the fully reduced P.a. enzyme with NO using rapid mixing freeze- quenching techniques [19]. The sample produced at 1 ms after the mixing of the enzyme with NO gave an ESR spectrum, in which [35], and the latter is assigned to the Fe B (II)-NO complex (S = 3/2). On the basis of the EPR data, we proposed that two NO molecules are shared with two irons in the binuclear center, and that this species might be the reaction intermediate, which is consis- tent with the trans-mechanism (see Fig. 7). Low redox potential of the heme b 3 iron (60 mV) in Pa.d. cNOR [21], as compared with the other redox centers (N300 mV), indicates that the heme b 3 iron is difficult to be reduced (Scheme 1). However, a recent study involving the redox titration in Pa.d. cNOR showed that the presence of an exoge- nous ligand, CO, results in the elevation of the redox potential of heme b 3 to a range similar to that of the other redox centers [56]. Therefore, the binding of NO to heme b 3 iron could facilitate the re- duction of the heme b 3 iron (Scheme 1). In the trans-mechanism, the electron transfer from both Fe(II) to the two NO ligands, eventu- ally the Fe(III)-NO ? state, would promote N\N bond formation through a disproportionation-type reaction, and two protons then would facilitate N\O bond cleavage to produce N 2 O and H 2 O (Scheme 1). This proposed mechanism was if good agreement with the model study reported by Collman and co-workers, in which NO binds to both heme and non-heme Fe(II) and one N 2 O molecule is produced from two equivalents of NO [57]. However, another NOR model with a NO complex is thermally stable and does not react fur- ther to give N 2 O upon the addition of the proton source [58]. In addi- tion, it is important to bear in mind that the presence of two paramagnetic Fe(II)-NO species in the single binuclear center can lead to spin-coupling and the production of an ESR-silent species [59]. By contrast, two other mechanisms, so-called cis-mechanisms, have been also proposed, in which two NO molecules bind to either the heme b 3 iron or Fe B (see Fig. 7). Thomson and co-workers favor the cis-Fe B mech- anism because this model has a vacant heme b 3 site, and a rather stable ferrous heme nitrosyl {FeNO} 7 species, a potential “dead-end” product, is therefore not formed during the turnover [21,[60][61][62][63]. In the cis- heme b 3 mechanism, the first NO molecule binds to heme b 3 to form a {FeNO} 7 species which is then electrophilically attacked by the second NO molecule [64,65]. The mechanism appears to be analogous to that proposed for fungal P450-type nitric oxide reductase, P450nor [66][67][68]. However, in the case of P450nor, the key step is the formation of the short-lived intermediate {FeNO} 8 by a two-electron (H ? ) reduction of the ferric heme-NO complex, while the one-electron reduced form {FeNO} 7 of P450nor never reacts with another …

Author: Sandeep

Sandeep is currently working as a freelance writer, as he prepares for his next big adventure abroad to learn more about the Health, Fitness and Diet articles

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