The properties and catalytic reactions of the enzyme nitrogenase purified from Klebsiella pneumoniae were studied by electron-paramagnetic-resonance (e.p.r.) spectroscopy at temperatures down to 8 degrees K. The two protein fractions, Kp1 (the iron-molybdenum protein) and Kp2 (the iron protein), were examined alone and in steady-state mixtures and also in pre-steady-state experiments, by using the rapid-freezing method. Kp1 protein in dithionite solution shows a rhombic type of spectrum with g(1) 4.32, g(2) 3.63, g(3) 2.009 at pH6.8 (0 degrees C). Small changes in the spectrum produced by protons (pK=8.7 at 0 degrees C) or by acetylene indicate binding of these oxidizing substrates to this protein fraction. Kp2 protein shows a rhombic spectrum with g(1) 2.053, g(2) 1.942, g(3) 1.865, which integrates to about 0.45 electron/molecule. Binding of ATP, with a dissociation constant of 4x10(-4)m, changes the spectrum to an axial form with g( parallel) 2.036, g( perpendicular) 1.929, thus indicating a conformation change of Kp2 protein. The Kp2 protein spectrum disappears reversibly on cautious oxidation. The signals of both proteins are diminished in their steady-state mixtures, obtained in the presence of ATP and dithionite (with an ATP-generating system and Mg(2+) ions) and with protons, N(2) or acetylene as oxidizing substrate. At the same time as dithionite is consumed in such reactions, the Kp1 protein signal is gradually restored and the Kp2 protein signal diminishes to zero. In rapid-freezing experiments the signals from the two proteins decreased at indistinguishable rates (t((1/2)) about 10ms), then they remained constant. Results are interpreted in terms of a scheme in which reducing equivalents pass from dithionite to Kp2 protein, then, in an ATP-dependent reaction to Kp1 protein, this being finally reoxidized by N(2) or another oxidizing substrate. In this scheme Kp1 protein cycles between its signal-giving state and a very highly reduced signal-free state.