The MCD and EPR of the Heme Centers of Nitric Oxide Reductase from Pseudomonas stutzeri: Evidence That the Enzyme Is Structurally Related to the Heme-Copper Oxidases

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Abstract
EPR spectra at liquid helium temperatures and MCD spectra at room temperature and 4.2 K are presented for fully oxidized nitric oxide reductase (NOR) from Pseudomonas stutzeri. The MCD spectra show that the enzyme contains three heme groups at equivalent concentrations but distinctive in their axial coordination. Two, in the low-spin ferric state at all temperatures, give rise to infrared charge-transfer transitions which show the hemes to have bis-histidine and histidine−methionine ligation, respectively. The EPR spectra show them to be magnetically isolated. The third heme has an unusual temperature-dependent spin state and spectroscopic features which are consistent with histidine−hydroxide coordination. No EPR signals have been detected from this heme. Together with its unusual near-infrared MCD, this suggests a spin−spin interaction between this heme and another paramagnet. The three hemes account for only 75% of the iron content, and it is concluded that the additional paramagnet is a mononuclear ferric ion. These results provide further evidence that NOR is indeed structurally related to heme-copper oxidases and that it contains a heme/non-heme iron spin-coupled pair at the active site.