Coordination of Selenium to Molybdenum in Formate Dehydrogenase H From Escherichia coli

Formate dehydrogenase H from Escherichia coli contains multiple redox centers, which include a molybdopterin cofactor, an iron-sulfur center, and a selenocysteine residue (SeCys-140 in the polypeptide chain) that is essential for catalytic activity. Here we show that addition of formate to the native enzyme induces a signal typical of Mo(V) species. This signal is detected by electron paramagnetic resonance (EPR) spectroscopy. Substitution of77Se for natural isotope abundance Se leads to transformation of this signal, indicating a direct coordination of Se with Mo. Mutant enzyme with cysteine substituted at position 140 for the selenocysteine residue has decreased catalytic activity and exhibits a different EPR signal. Since determination of the Se content of wild-type enzyme indicates ≈1 gram atom per mol, we conclude that it is the Se atom of the SeCys-140 residue in the protein that is coordinated directly with Mo. The amino acid sequence flanking the selenocysteine residue in formate dehydrogenase H is similar to a conserved sequence found in several other prokaryotic molybdopterin-dependent enzymes. In most of these other enzymes a cysteine residue, or in a few cases a serine or a selenocysteine residue, occurs in the position corresponding to SeCys-140 of formate dehydrogenase H. By analogy with formate dehydrogenase H in these other enzymes, at least one of the ligands to Mo should be provided by an amino acid residue of the protein. This ligand could be the Se of a selenocysteine residue, sulfur of a cysteine residue, or, in the case of a serine residue, oxygen.