Elucidating the behavior of an enzyme

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IET in the SDH(R55Q) variant is inhibited by sulfate in laser flash photolysis experiments, a behavior that differs from that of SDH(WT), but which also occurs in HSO. A new analysis of the possible mechanistic pathways for sulfite-oxidizing enzymes is presented and related to available kinetic and EPR results for these enzymes.

All reported sulfite oxidizing enzymes have a conserved arginine in their active site which hydrogen bonds to the equatorial oxygen ligand on the Mo atom.

The authors declare no competing financial interests.

The p H profiles of SDH(WT), SDH(R55M), and SDH(R55Q) show that the arginine substitution also alters the behavior of the Mo-heme IET equilibrium (K(eq)) and rate constants (k(et)) of both variants with respect to the SDH(WT) enzyme.

SDH(WT) has a k(et) that is independent of p H and a K(eq) that increases as p H decreases; on the other hand, both SDH(R55M) and SDH(R55Q) have a k(et) that increases as p H decreases, and SDH(R55M) has a K(eq) that is p H-independent.

Unexpectedly, the uncharged SDH variants (R55Q and R55M) showed increased IET rate constants relative to the wildtype (3–4 fold) when studied by laser flash photolysis.

The gain in function observed in SDH is not due to a required role of this residue in the IET pathway itself, but to the fact that it plays an important role in heme orientation during the inter-domain movement necessary for IET in HSO (as seen in viscosity experiments).

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