X-ray Absorption Spectroscopy of Selenate Reductase

The metal sites of selenate reductase from Thauera selenatis have been characterized by Mo, Se, and Fe K-edge X-ray absorption spectroscopy. The Mo site of the oxidized enzyme has 3 to 4 sulfur ligands at 2.33 Å from two molybdopterin cofactors, one MoO group at 1.68 Å and one Mo−O with an intermediate bond length of 1.81 Å. The reduced enzyme has a des-oxo active site, again with about four Mo−S ligands (at 2.32 Å) and possibly one oxygen ligand at 2.22 Å. The enzyme was found to contain Se in a reduced form (probably organic) although the sequence does not indicate the presence of selenocysteine. The Se is coordinated to both a metal (probably Fe) and a lighter scatterer such as carbon.