X-Band EPR spectra of Co(II)-substituted forms of EcMetAP-I: (A) [Co_(H79A EcMetAPI)], (B) [CoCo(H79A EcMetAP-I)], and (C) [CoCo(WT EcMetAP-I)]. Spectra A and B were recorded at 8 K with a 0.2 mW microwave power; spectrum C was recorded at 9 K with a 0.2 mW microwave power. The amplitude of trace A is shown multiplied by 1.88 compared to spectrum B. The inset shows the spectra of [CoCo(H79A EcMetAP-I)] (D) and [CoCo(WT EcMetAP-I)] (E) overlaid. All the spectra were recorded at a field modulation amplitude of 1.2 mT, a modulation frequency of 100 kHz, and a sweep rate of 10.2 mT s−1, and the samples were in 25 mM HEPES buffer and 150 mM KCl (pH 7.5).