EPR spectra of FixABCX from A. vinelandii prepared in 50 mM HEPES pH 7.5, 150 mM NaCl, and 5% glycerol. Black: As-prepared FixABCX (100 μM); Blue: FixABCX (100 μM) reduced with NADH (1 mM); Red: FixABCX (100 μM) reduced with sodium dithionite (10 mM). Simulations of spectra of NADH- and Na-dithionite-reduced FixABCX are shown in lighter colored traces. Microwave frequency, 9.38 GHz; microwave power, 1 mW; modulation frequency, 100 kHz; modulation amplitude, 10.0 G; sample temperature, 10 K.