Second-Coordination-Sphere\nEffects Reveal Electronic\nStructure Differences between the Mitochondrial Amidoxime Reducing\nComponent and Sulfite Oxidase

A combination of X-ray absorption\nand low-temperature\nelectronic\nabsorption spectroscopies has been used to probe the geometric and\nelectronic structures of the human mitochondrial amidoxime reducing\ncomponent enzyme (hmARC1) in the oxidized Mo(VI) and reduced Mo(IV)\nforms. Extended X-ray absorption fine structure analysis revealed\nthat oxidized enzyme possesses a 5-coordinate [MoO<sub>2</sub>(S<sub>Cys</sub>)(PDT)]<sup>−</sup> (PDT = pyranopterin dithiolene)\nactive site with a cysteine coordinated to Mo. A 5-coordinate geometry\nis retained in the reduced state, with the equatorial oxo being protonated.\nLow-temperature electronic absorption spectroscopy of hmARC1 reveals\na spectrum for the oxidized enzyme that is significantly different\nfrom what has been reported for sulfite oxidase family enzymes. Time-dependent\ndensity functional theory computations on oxidized and reduced hmARC1,\nand a small molecule analogue for hmARC1<sub>ox</sub>, have been used\nto assist us in making detailed band assignments and developing a\ngreater understanding of enzyme electronic structure contributions\nto reactivity. Our understanding of the hmARC<sub>red</sub> HOMO and\nthe LUMO of the benzamidoxime substrate reveal a potential π-bonding\ninteraction between these redox orbitals, with two-electron occupation\nof the substrate LUMO along the reaction coordinate activating the\nO–N bond for cleavage and promoting oxygen atom transfer to\nthe Mo site.