Effects on Substrate Reduction of Substitution of Histidine-195 by Glutamine in the α-Subunit of the MoFe Protein of Azotobacter vinelandii Nitrogenase

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Abstract
Studies of the substrate-reducing capabilities of an altered nitrogenase MoFe protein (α-195Gln instead of α-195His) from a mutant of Azotobacter vinelandii show, contrary to an earlier report [Kim, C.-H., Newton, W. E., and Dean, D. R. (1995) Biochemistry 34, 2798−2808], that the α-195Gln MoFe protein can reduce N2 to NH3 but at a rate that is 2 by this altered MoFe protein, as monitored by the inhibition of H2 evolution, is markedly increased as temperature is lowered but virtually eliminated at 45 °C. This inhibition of H2 evolution results in an increase in the ATP:2e- ratio, i.e., the number of molecules of MgATP hydrolyzed for each electron pair transferred to substrate, from ca. 5 (the wild-type level) at 45 °C to nearly 25 at 13 °C. Like wild-type nitrogenase, the N2 inhibition of H2 evolution reaches a maximum at an Fe protein:MoFe protein molar ratio of ca. 2.5, suggesting that a highly reduced enzyme may not be necessary for N2 binding. N2 binding to the α-195Gln MoFe protein retains a hallmark of the wild type by producing HD under a mixed N2/D2 atmosphere. The rate of HD production and the fraction of total electron flow allocated to HD are similar to those for wild-type nitrogenase under the same conditions. However, the electrons forming HD do not come from those normally producing NH3 (as occurs in the wild type) but are equivalent to those whose evolution as H2 had been inhibited by N2. N2 also inhibits C2H2 reduction catalyzed by the α-195Gln nitrogenase. This inhibition is relieved by added H2, resulting in a lowering of the elevated ATP:2e- ratio to that found under Ar. With solutions of NaCN, which contain both the substrate, HCN, and the inhibitor, CN-, reduction of HCN is not impaired with the α-195Gln nitrogenase, but the inhibition by CN- of total electron flow to substrate, which is observed with the wild-type MoFe protein, is completely absent. Unlike that of the catalyzed reduction of H+, HCN, or C2H2, the extent of azide reduction to either N2 or N2H4 is markedly decreased (to 5−7% of that of the wild type) with the α-195Gln nitrogenase. Azide, like N2, inhibits H2 evolution and increases the ATP:2e- ratio. Both effects are freely reversible and abolished by CO. Added D2 does not relieve either effect, implying that N2 produced from N3- is not the inhibitory species. The correlation between the extremely low rates of reduction for both N2 and azide by the α-195Gln nitrogenase and their common ability to inhibit H2 evolution suggests that α-histidine-195 may be an important proton conductor to the FeMo cofactor center and specifically required for reduction of these two substrates.