Characterization of the kinetics and electron paramagnetic resonance spectroscopic properties of Acidithiobacillus ferrooxidans sulfide:quinone oxidoreductase (SQR)

Acidithiobacillus ferrooxidans sulfide:quinone oxidoreductase (SQR) catalyzes the oxidation of sulfide to polysulfide chains or elemental sulfur coupled to quinone reduction via a non-covalent FAD cofactor. We investigated the role of the FAD using kinetics and EPR spectroscopy. The properties of the enzyme were compared with alanine and/or serine variants of conserved cysteine residues (Cys128, Cys160, Cys356) structurally close to the FAD cofactor and histidine residues (His132, His198) implicated in function. When the pre-steady state reduction of FAD was monitored, variants of Cys128 and His132 had similar rates to wild-type enzyme confirming they do not participate in the reductive half reaction whereas variants of Cys160, Cys356 and His198 had greatly reduced activity. Using steady state kinetics of Na2S-dependent decylubiquinone (DUQ) reduction we measured a kcat of 6.5 s−1 and a Km (Na2S) of 3.0 μM and a Km (DUQ) of 3.4 μM. Variants of Cys160, Cys356 and His198 had greatly diminished DUQ reduction activity whereas variants of Cys128 and His132 were less affected. A neutral flavin semiquinone was observed in the EPR spectrum of SQR reduced with Na2S which was enhanced in the Cys160Ala variant suggesting the presence of a Cys356-Sγ-S-C4A-FAD adduct. Potentiometric titrations of the FAD semiquinone revealed an Em of −139 ± 4 mV at pH 7.0.