Differential effects on N2 binding and reduction, HD Formation, and Azide reduction with α-195His- and α-191Gln-substituted MoFe proteins of Azotobacter vinelandii Nitrogenase

In contrast to the wild-type MoFe protein, neither the α-195Asn nor the α-191Lys MoFe protein catalyzed N2 reduction to NH3, when complemented with wild-type Fe protein. However, N2 was bound by the α-195Asn MoFe protein and inhibited the reduction of both protons and C2H2. The α-191Lys MoFe protein did not interact with N2. With the α-195Asn MoFe protein, the N2-induced inhibition of substrate reduction was reversed by removing the N2. Surprisingly, even though added H2 also relieved N2 inhibition of substrate reduction, the α-195Asn MoFe protein did not catalyze HD formation under a N2/D2 atmosphere. This observation is the first indication that these two reactions have different chemical origins, prompting a revision of the current hypothesis that these two reactions are consequences of the same nitrogenase chemistry. A rationale that accounts for the dichotomy of the two reactions is presented. The two altered MoFe proteins also responded quite differently to azide. It was a poor substrate for both bu...