Electrochemical titration of the S = 32 and S = 12 states of the iron protein of nitrogenase

AbstractThe iron protein of nitrogenase delivers electrons and ATP to the iron-molybdenum protein, which in turn reduces dinitrogen to ammonia. The iron protein contains a single four-iron, four-sulfur prosthetic group, detectable by ESR spectroscopy m its reduced form. Until recently, spin quantitations suggested that only a portion of the reduced iron protein was being detected by ESR, but recent work in several laboratories has shown that there is a spin = 32 signal near g = 5 in addition to the well characterized spin = 12 signal near g = 1.94. In this paper we characterize the redox properties of both states in the presence and absence of ATP and ADP, and find that the new spin state has identical redox properties to those previously determined for the spin = 12 state