文摘
X-ray absorption spectroscopy at the molybdenum and seleniumK-edges has been used to probethe active site structure of Escherichia coli formatedehydrogenase H. The active sites of both oxidizedandreduced wild-type protein, and of a variant containing cysteine insteadof selenocysteine, were studied. Theoxidized and reduced enzymes were found to be very similar, bothcontaining a novel des-oxo molybdenumsite, with four Mo-S ligands at 2.35 Å, (probably) one Mo-O at 2.1Å, and one Mo-Se ligand at 2.62 Åbeing indicated from the Mo K-edge data. The selenium K-edge EXAFSnot only is in good agreement withthe Mo K-edge data but also indicates the unexpected presence of Se-Sligation, with a bond length of 2.19Å. We suggest that the active site of Escherichia coliformate dehydrogenase H contains a novel seleno-sulfide ligand to molybdenum, where the selenium and sulfur originatefrom selenocysteine and one of thepterin-cofactor dithiolenes, respectively.
