Electrochemical behaviour of bacterial nitric oxide reductase-Evidence of low redox potential non-heme Fe-B gives new perspectives on the catalytic mechanism

Nitric oxide reductase (NOR) is a membrane bound enzyme involved in the metabolic denitrification pathway, reducing nitric oxide (NO) to nitrous oxide (N2O), subsequently promoting the formation of the N-N bond. Three types of bacterial NOR are known, namely cNOR, qNOR and qCuNOR, that differ on the physiological electron donor. cNOR has been purified as a two subunit complex, the NorC, anchored to the cytoplasmic membrane, with a low-spin heme c, and the NorB subunit showing high structural homology with the HCuO subunit I, comprising a bis-histidine low-spin heme b and a binuclear iron centre. The binuclear iron centre is the catalytic site and it is formed by a heme b(3) coupled to a non-heme iron (Fe-B) through a mu-oxo bridge. The catalytic mechanism is still under discussion and three hypotheses have been proposed: the trans-mechanism, the cis-Fe-B and the cis-heme b(3) mechanisms. In the present work, the Pseudomonas nautica cNOR electrochemical behaviour was studied by cyclic voltammetry (CV), using a pyrolytic graphite electrode modified with the immobilised protein. The protein redox centres were observed and the formal redox potentials were determined. The binuclear iron centre presents the lowest redox potential value, and discrimination between the heme b(3) and Fe-B redox processes was attained. Also, the number of electrons involved and correspondent surface electronic transfer rate constants were estimated. The pH dependence of the observed redox processes was determined and some new insights on the NOR catalytic mechanism are discussed.