Intramolecular electron transfer in sulfite-oxidizing enzymes: probing the role of aromatic amino acids

Sulfite oxidase (SO) is a molybdoheme enzyme that is important in sulfur catabolism, and mutations in the active site region are known to cause SO deficiency disorder in humans. This investigation probes the effects that mutating aromatic residues (Y273, W338, and H337) in the molybdenum-containing domain of human SO have on both the intramolecular electron transfer (IET) rate between the molybdenum and iron centers using laser flash photolysis and on catalytic turnover via steady-state kinetic analysis. The W338 and H337 mutants show large decreases in their IET rate constants (k _ET) relative to the wild-type values, suggesting the importance of these residues for rapid IET. In contrast, these mutants are catalytically competent and exhibit higher k _cat values than their corresponding k _ET, implying that these two processes involve different conformational states of the protein. Redox potential investigations using spectroelectrochemistry revealed that these aromatic residues close to the molybdenum center affect the potential of the presumably distant heme center in the resting state (as shown by the crystal structure of chicken SO), suggesting that the heme may be interacting with these residues during IET and/or catalytic turnover. These combined results suggest that in solution human SO may adopt different conformations for IET and for catalysis in the presence of the substrate. For IET the H337/W338 surface residues may serve as an alternative-docking site for the heme domain. The similarities between the mutant and wild-type EPR spectra indicate that the active site geometry around the Mo(V) center is not changed by the mutations studied here.