Effect of Outer-Sphere Side Chain Substitutions on the Fate of the trans Iron-Nitrosyl Dimer in Heme/Nonheme Engineered Myoglobins (Fe_BMbs): Insights into the Mechanism of Denitrifying NO Reductases

Denitrifying NO reductases are transmembrane protein complexes that utilize a heme/nonheme diiron center at their active sites to reduce two NO molecules to the innocuous gas N₂O. Fe_BMb proteins, with their nonheme iron sites engineered into the heme distal pocket of sperm whale myoglobin, are attractive models for studying the molecular details of the NO reduction reaction. Spectroscopic and structural studies of Fe_BMb constructs have confirmed that they reproduce the metal coordination spheres observed at the active site of the cytochrome c-dependent NO reductase from Pseudomonas aeruginosa. Exposure of Fe_BMb to excess NO, as examined by analytical and spectroscopic techniques, results primarily in the formation of a five-coordinate heme-nitrosyl complex without N₂O production. However, substitution of the outer-sphere residue Ile107 with a glutamic acid (i.e., I107E) decreases the formation rate of the five-coordinate heme-nitrosyl complex and allows for the substoichiometric production of N₂O. Here, we aim to better characterize the formation of the five-coordinate heme-nitrosyl complex and to explain why the level of N₂O production increases with the I107E substitution. We follow the formation of the five-coordinate heme-nitrosyl inhibitory complex through the sequential exposure of Fe_BMb to different NO isotopomers using rapid-freeze-quench resonance Raman spectroscopy. The data show that the complex is formed by the displacement of the proximal histidine by a new NO molecule after the weakening of the Fe(II)-His bond in the intermediate six-coordinate low-spin (6cLS) heme-nitrosyl complex. These results lead us to explore diatomic migration within the scaffold of myoglobin and whether substitutions at residue 107 can be sufficient to control access to the proximal heme cavities. Results on a new Fe_BMb construct with an I107F substitution (Fe_BMb3) show an increased rate for the formation of the five-coordinate low-spin heme-nitrosyl complex without N₂O production. Taken together, our results suggest that production of N₂O from the [6cLS heme {FeNO}⁷/{Fe_BNO}⁷] trans iron-nitrosyl dimer intermediate requires a proton transfer event facilitated by an outer-sphere residue such as E107 in Fe_BMb2 and E280 in P. aeruginosa cNOR.