Assignment of haem ligands and detection of electronic absorption bands of molybdenum in the di‐haem periplasmic nitrate reductase of Paracoccus pantotrophus

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Abstract
The periplasmic nitrate reductase (NAP) from Paracoccus pantotrophus is a soluble two‐subunit enzyme (NapAB) that binds two c‐type haems, a [4Fe–4S] cluster and a bis‐molybdopterin guanine dinucleotide cofactor that catalyses the reduction of nitrate to nitrite. In the present work the NapAB complex has been studied by magneto‐optical spectroscopy to probe co‐ordination of both the NapB haems and the NapA active site Mo. The absorption spectrum of the NapAB complex is dominated by features from the NapB c‐type cytochromes. Using a combination of electron paramagnetic resonance spectroscopy and magnetic circular dichroism it was demonstrated that both haems are low‐spin with bis‐histidine axial ligation. In addition, a window between 600 and 800 nm was identified in which weak absorption features that may arise from Mo could be detected. The low‐temperature MCD spectrum shows oppositely signed bands in this region (peak 648 nm, trough 714 nm) which have been assigned to S‐to‐Mo(V) charge transfer transitions.