Membrane Tetraheme Cytochrome cm552 of the Ammonia-Oxidizing Nitrosomonas europaea: A Ubiquinone Reductase

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Abstract
Cytochrome cm552 (cyt cm552) from the ammonia-oxidizing Nitrosomonas europaea is encoded by the cycB gene, which is preceded in a gene cluster by three genes encoding proteins involved in the oxidation of hydroxylamine: hao, hydroxylamine oxidoreductase; orf2, a putative membrane protein; cycA, cyt c554. By amino acid sequence alignment of the core tetraheme domain, cyt cm552 belongs to the NapC/TorC family of tetra- or pentaheme cytochrome c species involved in electron transport from membrane quinols to a variety of periplasmic electron shuttles leading to terminal reductases. However, cyt cm552 is thought to reduce quinone with electrons originating from HAO. In this work, the tetrahemic 27 kDa cyt cm552 from N. europaea was purified after extraction from membranes using Triton X-100 with subsequent exchange into n-dodecyl β-d-maltoside. The cytochrome had a propensity to form strong SDS-resistant dimers likely mediated by a conserved GXXXG motif present in the putative transmembrane segment. Optical spectra of the ferric protein contained a broad ligand−metal charge transfer band at ∼625 nm indicative of a high-spin heme. Mössbauer spectroscopy of the reduced 57Fe-enriched protein revealed the presence of high-spin and low-spin hemes in a 1:3 ratio. Multimode EPR spectroscopy of the native state showed signals from an electronically interacting high-spin/low-spin pair of hemes. Upon partial reduction, a typical high-spin heme EPR signal was observed. No EPR signals were observed from the other two low-spin hemes, indicating an electronic interaction between these hemes as well. UV−vis absorption data indicate that CO (ferrous enzyme) or CN− (ferric or ferrous enzyme) bound to more than one and possibly all hemes. Other anionic ligands did not bind. The four ferrous hemes of the cytochrome were rapidly oxidized in the presence of oxygen. Comparative modeling, based on the crystal structure and conserved residues of the homologous NrfH protein from Desulfovibrio of cyt cm552, predicted some structural elements, including a Met-ligated high-spin heme in a quinone-binding pocket, and likely axial ligands to all four hemes.

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