Lawrence Berkeley National Laboratory

Serial femtosecond crystallography (SFX) and continuous serial electron diffraction (c-SerialED) both enable high-resolution structure determination from protein microcrystals with minimal radiation damage, making it ideal for studying redox-active metalloenzymes. Here, c-SerialED and SFX were used to solve structures of the class Ia ribonucleotide reductase R2 subunit in oxidized (Fe^III-Fe^III), reduced (Fe^II-Fe^II), and re-oxidized states at ∼1.8 Å resolution, capturing three points in a redox reaction. These results demonstrate that c-SerialED can track reversible changes at the redox-site, enabling future time-resolved studies. Comparison between c-SerialED structures and SFX diffraction and emission data confirmed minimal radiation damage. Furthermore, previously reported structures use mercury in the crystallization condition and show mercury-induced conformational changes. Here, we use mercury-free crystallization conditions and reveal a water molecule in the redox center of the reduced state, absent in the previous structures, making these structures more representative of the physiological state.