CcNir

Nitroxyl (HNO/NO ) heme-model complexes ( FeNO , according to the Enemark-Feltham notation) have received special attention due to the intermediacy of nitroxyl-heme adducts in a variety of catalytic processes related to the biogeochemical cycle of nitrogen (104). For example, for the six-electron reduction of nitrite to ammonia that is catalvzed by cytochrome c nitrite reductase (ccNir), a heme FeNO complex is proposed as an intermediate (Scheme 5) (105,106). This intermediate has also been suggested for the reduction of NO to N2O by P450nor (Scheme 6) (107). Then, the isolation of a suitable FeNO heme complex that allows structural and functional characterizations will help to imderstand the reaction mechanism of ccNir and other enz5mies. 

Scheme 5. Reduction of nitrite to ammonia by ccNiR. Proposed mechanism for the formation of the FeNO intermediate.

A very similar catalytic cycle has been proposed for the cytochrome c nitrite reductases, ccNiR, which serve in some anaerobes as terminal electron acceptors . The ccNiR has five type c hemes per monomer and a lysine-coordinated heme at the active site. Potentiometric titrations showed enzyme s unique lysine-coordinated heme, labeled heme 1, has an Fe reduction potential at —107 mV. The other hemes with bis-histidine coordination, termed hemes 2 through 5, have reduction potentials between -37 and —323 mV. Tentative assignments of each heme s potentials have been proposed using EPR titration data correlated with a recently determined structure . 

Figure 4.3. First-order derivatives of RDE voltammograms, demonstrating catalytic reduction of nitrite and hydroxylamine by the five-heme ccNiR, NrfA. (A) Catalytic current derivatives for nitrite concentrations from 1.7 to 265 /rM. (B) Same for hydroxylamine concentrations from 1.1 to 374 fM.. Experiments were performed in 2 mM CaC12, 50 mM Hepes, pH 7.0, at 20°C. Approximate potentials of the five hemes are marked on the x-axis (a) heme 2(b) heme 1 and 3 (c) heme 4 and 5.

Recently, several substrate-bound intermediates for ccNiR from Wolinella succinogenes, including the nitrite and hydroxylamine adducts, have been stmc-turally characterized by Kronek and coworkers. Their proposed mechanism points out an inherent problem associated with generating a ferrous nitroxyl intermediate, Fe -NO , by sequential electron transfers. Scheme 4.4. If transiently... 

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