Reduction of azide by the N2-fixing enzyme system.

In 1937, Wilson and Umbreit1 reported that H2 inhibited N2 fixation in red clover plants and when this inhibition proved to be competitive, it implied that N2 and H2 were bound at the same site on the enzyme. The enzyme also binds CO2 and N20' competitively with N2. The first evidence that any compound other than N2 bound at the N2-fixing site could be reduced was supplied when Mozen and Burris4 demonstrated that N20 was reduced by Azotobacter vinelandii and soybean nodules. (The reduction of N20 by nodules was verified by Hoch, Schneider, and Burris;5 and recently the reaction was verified in A. vinelandii and extended to Clostridium pasteurianum by Hardy and Knight.6) The reduction of azide constituted the next evidence for the low specificity of reduction at the N2-binding site (report of Schollhorn's work at the 1965 conference on N2 fixation summarized by Delwiche'). Shortly thereafter, Schollhorn and Burris8 also reported the reduction of acetylene and its competitive inhibition of N2 fixation, and Dilworth9 entirely independently also observed acetylene reduction and established that the reduction yielded ethylene. As all of these reductions require the same conditions as does N2 fixation, i.e., a source of adenosine 5'-triphosphate (ATP) and a strong reducing agent, they have aroused interest because of the potential information they can supply regarding the active site of the enzyme and the nature of the binding of N2 at this site. The list of compounds reduced by the N2-fixing enzyme complex now has been extended to cyanide,10 isocyanide,"1 and a variety of analogues of these compounds.