Electron transfer to the active site of the bacterial nitric oxide reductase is controlled by ligand binding to heme b3

The active site of the bacterial nitric oxide reductase from Paracoccus denitrificans contains a dinuclear centre comprising heme b3 and non heme iron (FeB). These metal centres are shown to be at isopotential with midpoint reduction potentials of Em ≈ + 80 mV. The midpoint reduction potentials of the other two metal centres in the enzyme, heme c and heme b, are greater than the dinuclear centre suggesting that they act as an electron receiving/storage module. Reduction of the low-spin heme b causes structural changes at the dinuclear centre which allow access to substrate molecules. In the presence of the substrate analogue, CO, the midpoint reduction potential of heme b3 is raised to a region similar to that of heme c and heme b. This leads us to suggest that reduction of the electron transfer hemes leads to an opening of the active site which allows substrate to bind and in turn raises the reduction potential of the active site such that electrons are only delivered to the active site following substrate binding.

Research highlights
► The potentials of the active site metal centres of NOR are lower than the electron receiving hemes.
► Reduction of the electron receiving hemes opens the active site to ligands.
► CO raises the potential of the active site heme to allow internal electron transfer.
► The resulting compound reveals the EPR spectrum of the cryptic FeB centre.