Molybdenum cofactor properties and [Fe-S] cluster coordination in Escherichia coli nitrate reductase A: investigation by site-directed mutagenesis of the conserved his-50 residue in the NarG subunit.

Most of the molybdoenzymes contain, in the amino-terminal region of their catalytic subunits, a conserved Cys group that in some cases binds an [Fe-S] cluster. In dissimilatory nitrate reductases, the first Cys residue of this motif is replaced by a conserved His residue. Site-directed mutagenesis of this residue (His-50) was performed on the NarG subunit from Escherichia coli nitrate reductase A. The results obtained by EPR spectroscopy enable us to exclude the implication of this residue in [Fe-S] binding. Additionally, we showed that the His-50 residue does not coordinate the molybdenum atom, but its substitution by Cys or Ser introduces a perturbation of the hydrogen bonding network around the molybdenum cofactor. From potentiometric studies, it is proposed that the high-pH and the low-pH forms of the Mo(V) are both involved during the redox turnover of the enzyme. Perturbation of the Mo(V) pKV value might be responsible for the low activity reported in the His-50-Cys mutant enzyme. A catalytic model is proposed in which the protonation/deprotonation of the Mo(V) species is an essential step. Thus, one of the two protons involved in the catalytic cycle could be the one coupled to the molybdenum atom in the dissimilatory nitrate reductase of E. coli.