Structure of the molybdenum site in YedY, a sulfite oxidase homologue from Escherichia coli.

YedY from Escherichia coli is a new member of the sulfite oxidase family of molybdenum cofactor (Moco)-containing oxidoreductases. We investigated the atomic structure of the molybdenum site in YedY by X-ray absorption spectroscopy, in comparison to human sulfite oxidase (hSO) and to a MoIV model complex. The K-edge energy was indicative of MoV in YedY, in agreement with X- and Q-band electron paramagnetic resonance results, whereas the hSO protein contained MoVI. In YedY and hSO, molybdenum is coordinated by two sulfur ligands from the molybdopterin ligand of the Moco, one thiolate sulfur of a cysteine (average Mo−S bond length of ∼2.4 A), and one (axial) oxo ligand (Mo═O, ∼1.7 A). hSO contained a second oxo group at Mo as expected, but in YedY, two species in about a 1:1 ratio were found at the active site, corresponding to an equatorial Mo−OH bond (∼2.1 A) or possibly to a shorter Mo−O− bond. Yet another oxygen (or nitrogen) at a ∼2.6 A distance to Mo in YedY was identified, which could originate from ...