Characterization of an Escherichia coli sulfite oxidase homologue reveals the role of a conserved active site cysteine in assembly and function

We report the biochemical and biophysical characterization of YedYZ, a sulfite oxidase homologue from Escherichia coli. YedY is a soluble catalytic subunit with a twin arginine leader sequence for export to the periplasm by the Tat translocation system. YedY is the only molybdoenzyme so far isolated from E. coli with the Mo-MPT form of the molybdenum cofactor. The electron paramagnetic resonance (EPR) signal of the YedY molybdenum is similar to that of known Mo-MPT containing enzymes, with the exception that only the Mo(IV) --> Mo(V) transition is observed, with a midpoint potential of 132 mV. YedZ is a membrane-intrinsic cytochrome b with six putative transmembrane helices. The single heme b of YedZ has a midpoint potential of -8 mV, determined by EPR spectroscopy of YedZ-enriched membrane preparations. YedY does not associate strongly with YedZ on the cytoplasmic membrane. However, mutation of the YedY active site Cys102 to Ser results in very efficient targeting of YedY to YedZ in the membrane, demonstrating a clear role for YedZ as the membrane anchor for YedY. Together, YedYZ comprise a well-conserved bacterial heme-molybdoenzyme found in a variety of bacteria that can be assigned to the sulfite oxidase class of enzyme.