Resonance Raman spectra of oxidized CODH at pH 7.2 and pH 10, CODH reduced with CO and then reoxidized in air at pH 7.2. Spectra were obtained at 30 K using 514.5 nm excitation. Resonance Raman spectra of FAD, desulfo-xanthine oxidase, and plant sulfite oxidase, which is known to possess a MoO2 core [26], are shown in the upper panel for comparison.