X-RAY-ABSORPTION SPECTROSCOPY OF THE MOLYBDENUM SITE OF ESCHERICHIA-COLI FORMATE DEHYDROGENASE

X-ray absorption spectroscopy at the molybdenum and selenium K-edges h as been used to probe the active site structure of Escherichia coli fo rmate dehydrogenase H. The active sites of both oxidized and reduced w ild-type protein, and of a variant containing cysteine instead of sele nocysteine, were studied. The oxidized and reduced enzymes were found to be very similar, both containing a novel des-oxo molybdenum site, w ith four Mo-S ligands at 2.35 Angstrom, (probably) one Mo-O at 2.1 Ang strom, and one Mo-Se ligand at 2.62 Angstrom being indicated from the Mo K-edge data. The selenium K-edge EXAFS not only is in good agreemen t with the Mo K-edge data but also indicates the unexpected presence o f Se-S ligation, with a bond length of 2.19 Angstrom. We suggest that the active site of Escherichia coli formate dehydrogenase H contains a novel selenosulfide ligand to molybdenum, where the selenium and sulf ur originate from selenocysteine and one of the pterin-cofactor dithio lenes, respectively.