Insights into the recognition and electron transfer steps in nitric oxide reductase from Marinobacter hydrocarbonoclasticus

Marinobacter hydrocarbonoclasticus nitric oxide reductase, cNOR, is an integral membrane protein composed of two subunits with different roles, NorC (electron transfer) and NorB (catalytic) that receives electrons from the soluble cytochrome c₅₅₂ and reduces nitric oxide to nitrous oxide in the denitrification pathway. The solvent-exposed domain of NorC, harboring a c-type heme was heterologously produced, along with its physiological electron donor, cytochrome c₅₅₂. These two proteins were spectroscopically characterized and shown to be similar to the native proteins, both being low-spin and Met-His coordinated, with the soluble domain of NorC presenting some additional features of a high-spin heme, which is consistent with the higher solvent accessibility of its heme and weaker coordination of the methionine axial ligand. The electron transfer complex between the two proteins has a 1:1 stoichiometry, and an upper limit for the dissociation constant was estimated by ¹H NMR titration to be 1.2 ± 0.4 μM. Electrochemical techniques were used to characterize the interaction between the proteins, and a model structure of the complex was obtained by molecular docking. The electrochemical observations point to the modulation of the NorC reduction potential by the presence of NorB, tuning its ability to receive electrons from cytochrome c₅₅₂.