ADENOSINE TRIPHOSPHATE REQUIREMENT OF NITROGENASE FROM AZOTOBACTER VINELANDII

Knowledge of the amount of adenosine triphosphate hydrolyzed during the transfer of reducing electrons in the reactions catalyzed by nitrogenase (the adenosine triphosphate:2e- ratio) is requisite to understanding the energetics of the reactions and perhaps also the function(s) of adenosine triphosphate. Most reported adenosine triphosphate:2e- ratios range between 2 and 5, but both higher and lower values have been indicated. An extensive examination of adenosine triphosphate hydrolysis in the H2 evolution and N2 or C2H2 reductions catalyzed by the nitrogenase complex from Azotobacter vinelandii gave ratios approaching 5 at 30°. This ratio was independent of substrate, substrate concentration, H2 inhibition, and pH but was temperature dependent. Adenosine triphosphate:2e- values increased with temperature from 4.3 at 20° to 5.8 at 40°. The discussion includes an attempt to explain variations in the values observed with the clostridial and azotobacter enzymes. It is suggested that adenosine triphosphate is hydrolyzed in two ways, only one of which leads to electron transfer and that the concept of single-reaction stoichiometry is not applicable to this system. © 1969, American Chemical Society. All rights reserved.