Reactions of Azotobacter vinelandii nitrogenase using Ti(III) as reductant

Nitrogenase-catalyzed reactions using Ti(III) were examined under a wide variety of conditions to determine the suitability of Ti(III) to serve as a general nitrogenase reductant. Solutions prepared from H2-reduced TiCl3, aluminum-reduced TiCl3, TiCl2, evaporated TiCl3 from an HCl, solution, and TiF3 were evaluated as reductants. Three general types of reactivity were observed. The first showed that, below Ti(III) concentrations of about 0.50 mM, nitrogenase catalysis utilized Ti(III) in a first-order reaction. The second showed that, above 0.50 mM, the rate of nitrogenase catalysis was zero order in Ti(III), indicating the enzyme was saturated with this reductant. Above 2.0-5.0 mM, nitrogenase catalysis was inhibited by Ti(III) depending on the titanium source used for solution preparation. This inhibition was investigated and found to be independent of the buffer type and pH, while high salt and citrate concentrations caused moderate inhibition. [Ti(IV)] above 2.0-3.0 mM and [Ti(III)] above about 5.0 mM were inhibitory. ATP/2e values were 4-5 for [Ti(III)] at or below 1.0-2.0 mM, 2.0 from 5.0 to 7.0 mM Ti(III) where nitrogenase is not inhibited, and 2.0 above 7.0 mM Ti(III) where severe inhibition occurs. For nitrogenase-catalyzed reactions using Ti(III) as reductant, the potential of the solution changes with time as the Ti(III)/Ti(IV) ratio changes. From the change in the rate of product formation (Ti(III) disappearance) with change in solution potential, the rate of nitrogenase catalysis was determined as a function of solution potential. From such experiments, a midpoint turnover potential of -480 mV was determined for nitrogenase catalysis with an associated n = 2 value.