Electron paramagnetic resonance studies on nitrogenase. II. Interaction of adenosine 5′-triphosphate with azoferredoxin

The interaction of ATP with both iron-sulfur proteins of nitrogenase from Clostridium pasteurianum, azoferredoxin and molybdoferredoxin, has been studied by low-temperature EPR spectroscopy. ATP in the presence of Mg²⁺ changes the rhombic EPR signal of azoferredoxin with g-values of 2.06, 1.94 and 1.87 to an axial signal, with g values of 2.04 and 1.93. The binding of two molecules of ATP per azoferredoxin dimer (mol. wt 55 000) is suggested. Comparative data with other purine and pyrimidine nucleotides and ATP analogues demonstrate the involvement of structural elements of the substrate in the conversion of the EPR signal of azoferredoxin. A similar effect is induced by 5 M urea, which suggests that ATP causes a conformation change of the protein. In contrast, no effect of ATP was observed on the EPR signal of molybdoferredoxin.