Heme enzymes catalyze the reduction of nitrite into different nitrogen-containing compounds via the nitrite reductase (NiR) mechanism. These systems contain a histidine, cysteine, or tyrosine residue proximal to the heme, with the latter system being less studied. This work focuses on the electrochemical reduction and spectroscopic studies of human serum albumin reconstituted with heme (HSA-heme), an artificial system that contains a weak Fe-O(tyrosine) interaction and demonstrates NiR activity. Cyclic voltammetry shows that HSA-heme in a surfactant film of dimethyldidodecylammonium bromide (ddab) behaves in a similar fashion to myoglobin/ddab. Both films exhibit fast electron transfer kinetics for the FeIII/II and FeII/I redox couples as well as catalytic reduction of nitrite and nitric oxide (NO). In short, while both reduction processes suggest similar paths and products at HSA-heme, it was determined that NO initially binds to FeIII, which is then chemically reduced to FeII, while nitrite binds to FeII after it is electrochemically reduced from FeIII. This is similar to what is exhibited by myoglobin/ddab; however there appears to be some differences in the NO reduction mechanism between myoglobin and HSA-heme that is evident at faster scan rates. Importantly, the second-sphere coordination in HSA-heme (e.g. proximal ligand TYR161; and ILE142, LEU115, and TYR138 that display long- range interactions) plays an important role in NOx electrocatalysis, as demonstrated through control experiments in the absence of the heme cofactor. Lastly, molecular docking studies demonstrate a slightly positive free binding energy of NOx on both heme systems. This may be correlated with a lower nitrite formation constant and a faster reaction rate between nitrite and the heme center, as observed in previous studies.
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