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Heme iron nitrosyl complex of MauG reveals an efficient redox equilibrium between hemes with only one heme exclusively binding exogenous ligands.

ABSTRACT: MauG is a diheme enzyme that oxidizes two protein-bound tryptophan residues to generate a catalytic tryptophan tryptophylquinone cofactor within methylamine dehydrogenase. Upon the two-electron oxidation of bis-ferric MauG, the two c-type hemes exist as a spin-uncoupled bis-Fe(IV) species with only one binding oxygen, which is chemically equivalent to a single ferryl heme plus a pi porphyrin cation radical ( Li , X. et al. ( 2008 ) Proc. Natl. Acad. Sci. U.S.A. 105 , 8597 - 8600 ). The EPR spectrum of the nitrosyl complex of fully reduced MauG shows a single six-coordinate Fe(II)-NO species, which is characteristic of a histidine-ligated Fe(II)-NO moiety in the heme environment. Exposure of partially reduced MauG to NO reveals a redox equilibrium with facile electron transfer between hemes but with only one binding nitric oxide. Thus, the second heme is able to stabilize all three redox states of iron (Fe(II), Fe(III), and Fe(IV)) in a six-coordinate protein-bound heme without binding exogenous ligands. This is unprecedented behavior for a protein-bound heme for which each of these redox states is relevant to the overall catalytic mechanism. The results also illustrate the electronic communication between the two iron centers, which function as a diheme unit rather than independent heme cofactors.

SUBMITTER: Fu R

PROVIDER: S-EPMC2801551 | biostudies-literature | 2009 Dec

REPOSITORIES: biostudies-literature

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Heme iron nitrosyl complex of MauG reveals an efficient redox equilibrium between hemes with only one heme exclusively binding exogenous ligands.

Fu Rong R Liu Fange F Davidson Victor L VL Liu Aimin A

Biochemistry 20091201 49

MauG is a diheme enzyme that oxidizes two protein-bound tryptophan residues to generate a catalytic tryptophan tryptophylquinone cofactor within methylamine dehydrogenase. Upon the two-electron oxidation of bis-ferric MauG, the two c-type hemes exist as a spin-uncoupled bis-Fe(IV) species with only one binding oxygen, which is chemically equivalent to a single ferryl heme plus a pi porphyrin cation radical ( Li , X. et al. ( 2008 ) Proc. Natl. Acad. Sci. U.S.A. 105 , 8597 - 8600 ). The EPR spect ...[more]

PMID: 19911786

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Project description:A series of tetranuclear iron complexes displaying a site-differentiated metal center was synthesized. Three of the metal centers are coordinated to our previously reported ligand, based on a 1,3,5-triarylbenzene motif with nitrogen and oxygen donors. The fourth (apical) iron center is coordinatively unsaturated and appended to the trinuclear core through three bridging pyrazolates and an interstitial μ4-oxide moiety. Electrochemical studies of complex [LFe3(PhPz)3OFe][OTf]2 revealed three reversible redox events assigned to the Fe(II)4/Fe(II)3Fe(III) (-1.733 V), Fe(II)3Fe(III)/Fe(II)2Fe(III)2 (-0.727 V), and Fe(II)2Fe(III)2/Fe(II)Fe(III)3 (0.018 V) redox couples. Combined Mössbauer and crystallographic studies indicate that the change in oxidation state is exclusively localized at the triiron core, without changing the oxidation state of the apical metal center. This phenomenon is assigned to differences in the coordination environment of the two metal sites and provides a strategy for storing electron and hole equivalents without affecting the oxidation state of the coordinatively unsaturated metal. The presence of a ligand-binding site allowed the effect of redox modulation on nitric oxide activation by an Fe(II) metal center to be studied. Treatment of the clusters with nitric oxide resulted in binding of NO to the apical iron center, generating a {FeNO}(7) moiety. As with the NO-free precursors, the three reversible redox events are localized at the iron centers distal from the NO ligand. Altering the redox state of the triiron core resulted in significant change in the NO stretching frequency, by as much as 100 cm(-1). The increased activation of NO is attributed to structural changes within the clusters, in particular, those related to the interaction of the metal centers with the interstitial atom. The differences in NO activation were further shown to lead to differential reactivity, with NO disproportionation and N2O formation performed by the more electron-rich cluster.

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Heme iron nitrosyl complex of MauG reveals an efficient redox equilibrium between hemes with only one heme exclusively binding exogenous ligands.

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Heme iron nitrosyl complex of MauG reveals an efficient redox equilibrium between hemes with only one heme exclusively binding exogenous ligands.

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