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additional information
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comparison of reaction mechanisms of nitrogenase, EC 1.18.6.1, and multiheme cytochrome c nitrite reductase, ccNIR, EC 1.7.2.2, overview
evolution
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the enzyme is a member of the multiheme cytochrome c family. Members of the Hao subfamily, here called epilonHao proteins, have been predicted from the genomes of nitrate/nitrite-ammonifying bacteria that usually lack NrfA. Formation of a membrane-bound HaoCA assembly reminiscent of the menaquinol-oxidizing NrfHA complex. epsilonHao proteins form a subfamily of ammonifying cytochrome c nitrite reductases that represents a missing link in the evolution of NrfA, EC 1.7.2.2, and Hao, EC 1.7.99.1, enzymes, epsilonHao-type proteins are ancestors of different multiheme cytochrome c (MCC) subfamilies that catalyze either reductive (NrfA-type MCCs) or oxidative (Hao/Hdh-type MCCs) reactions. Comparison of the enzyme from Caminibacter mediatlanticus with NrfA from Wolinella succinogenes and Hao from Nitrosomonas europaea, overview
evolution
the enzyme is a member of the multiheme cytochrome c family. Members of the Hao subfamily, here called epilonHao proteins, have been predicted from the genomes of nitrate/nitrite-ammonifying bacteria that usually lack NrfA. Formation of a membrane-bound HaoCA assembly reminiscent of the menaquinol-oxidizing NrfHA complex. epsilonHao proteins form a subfamily of ammonifying cytochrome c nitrite reductases that represents a missing link in the evolution of NrfA, EC 1.7.2.2, and Hao, EC 1.7.99.1, enzymes, epsilonHao-type proteins are ancestors of different multiheme cytochrome c (MCC) subfamilies that catalyze either reductive (NrfA-type MCCs) or oxidative (Hao/Hdh-type MCCs) reactions. Comparison of the enzyme from Campylobacter curvus with NrfA from Wolinella succinogenes and Hao from Nitrosomonas europaea, overview
evolution
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the enzyme is a member of the multiheme cytochrome c family. Members of the Hao subfamily, here called epilonHao proteins, have been predicted from the genomes of nitrate/nitrite-ammonifying bacteria that usually lack NrfA. Formation of a membrane-bound HaoCA assembly reminiscent of the menaquinol-oxidizing NrfHA complex. epsilonHao proteins form a subfamily of ammonifying cytochrome c nitrite reductases that represents a missing link in the evolution of NrfA, EC 1.7.2.2, and Hao, EC 1.7.99.1, enzymes, epsilonHao-type proteins are ancestors of different multiheme cytochrome c (MCC) subfamilies that catalyze either reductive (NrfA-type MCCs) or oxidative (Hao/Hdh-type MCCs) reactions. Comparison of the enzyme from Campylobacter fetus with NrfA from Wolinella succinogenes and Hao from Nitrosomonas europaea, overview
evolution
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the enzyme is a member of the multiheme cytochrome c family. Members of the Hao subfamily, here called epilonHao proteins, have been predicted from the genomes of nitrate/nitrite-ammonifying bacteria that usually lack NrfA. Formation of a membrane-bound HaoCA assembly reminiscent of the menaquinol-oxidizing NrfHA complex. epsilonHao proteins form a subfamily of ammonifying cytochrome c nitrite reductases that represents a missing link in the evolution of NrfA, EC 1.7.2.2, and Hao, EC 1.7.99.1, enzymes, epsilonHao-type proteins are ancestors of different multiheme cytochrome c (MCC) subfamilies that catalyze either reductive (NrfA-type MCCs) or oxidative (Hao/Hdh-type MCCs) reactions. Comparison of the enzyme from Nautilia profundicola with NrfA from Wolinella succinogenes and Hao from Nitrosomonas europaea, overview
metabolism
nitrite-loaded ccNiR is reduced in a concerted two-electron step to generate an [FeNO]7 moiety at the active site, with an associated midpoint potential of +246 mV vs standard hydrogen electrode at pH 7. Cyanide-bound active site reduction is a one-electron process with a midpoint potential of +20 mV, and without a strong-field ligand the active site midpoint potential shifts 70 mV lower still. The [FeNO]7 moiety possesses an spectral signature, different from those normally observed for [FeNO]7 hemes, that may indicate magnetic interaction of the active site with nearby hemes. Catalytic nitrite reduction to ammonia by ccNiR requires an applied potential of at least -120 mV, well below the midpoint potential for [FeNO]7 formation
metabolism
reduction of nitrite-loaded ccNiR by N,N,N',N'-tetramethyl-pphenylenediamine generates a transient intermediate, identified as FeH1II(NO2-), where FeH1 represents the ccNiR active site. FeH1II(NO2-) accumulates rapidly and is then more slowly converted to the two-electron-reduced moiety [FeH1NO]7. ccNiR is not reduced beyond the [FeH1NO]7 state. The midpoint potentials for sequential reduction of FeH1III(NO2-) to FeH1II(NO2-) and then to [FeH1NO]7 are 130 and 370 mV versus the standard hydrogen electrode, respectively. With weak reductants, free NO radical is released from nitrite-loaded ccNiR
metabolism
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three-step nitrite reduction cycle on a dinuclear ruthenium platform [(HB(pyrazol-1-yl)3Ru)2(my-pz)] producing ammonia. The cycle comprises conversion of a nitrito ligand to a NO ligand using 2H+ and e-, subsequent reduction of the NO ligand to a nitrido and a H2O ligand by consumption of 2H+ and 5e-, and recovery of the parent nitrito ligand. Release of ammonia is detected
physiological function
periplasmic multiheme c-type cytochromes cytochrome c peroxidase and cytochrome c nitrite reductase NrfA mediate resistance to hydrogen peroxide. NrfA and a cytoplasmic flavodiiron protein Fdp are key components of nitric oxide detoxification. In addition, NrfA mediates resistance to hydroxylamine stress
physiological function
gene SO0265 encodes CcmI, an apocytochrome c chaperone, that is important and essential for maturation of c-type cytochromes with the canonical heme binding motif(s) (HBM and CX2CH) and nitrite reductase NrfA carrying a non-canonical CX2CK motif, respectively. The N-terminal transmembrane segment of CcmI, CcmI-1, is sufficient for maturation of the former but the entire protein is required for maturation of the latter. SirE is not required for maturation of NrfA
physiological function
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the NO2--mediated conversion of cardiolipin-cytc-Fe(II) to cardiolipin-cytc-Fe(II)-NO may play anti-apoptotic effects impairing lipid peroxidation and therefore the initiation of the cell death program by the release of pro-apoptotic factors in the cytoplasm
physiological function
Q9L2D3; Q9L2D1
ammonium-producing nitrite reductase NirBD is involved in regulating NO homeostasis. NirBD clears the accumulated nitrite from the medium. Nir deletion mutants show increased NO-dependent gene expression at later culture stages, whereas the wild-type M145 shows decreased expression. The nir deletion mutant produces more red-pigmented antibiotic than that produced by the wild-type
physiological function
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ammonium-producing nitrite reductase NirBD is involved in regulating NO homeostasis. NirBD clears the accumulated nitrite from the medium. Nir deletion mutants show increased NO-dependent gene expression at later culture stages, whereas the wild-type M145 shows decreased expression. The nir deletion mutant produces more red-pigmented antibiotic than that produced by the wild-type
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physiological function
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periplasmic multiheme c-type cytochromes cytochrome c peroxidase and cytochrome c nitrite reductase NrfA mediate resistance to hydrogen peroxide. NrfA and a cytoplasmic flavodiiron protein Fdp are key components of nitric oxide detoxification. In addition, NrfA mediates resistance to hydroxylamine stress
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