Towards serial femtosecond crystallography of metalloproteins with sub-ångström details

To understand the chemical properties of the metalloproteins and their dynamics it is essential to obtain fine structural information at resolutions higher than 1.2 Å and more importantly with extremely low to radiation-damage free models, thereby enabling independent atomic refinement without restraints that are conventionally required at lower resolutions. Here we applied megahertz serial femtosecond crystallography at ambient temperature using ultra-short X-ray free electron laser pulses to obtain ultra-high-resolution structures (0.97-1.05Å) of high-potential iron sulfur protein (HiPIP), a model metalloprotein with a single Fe4S4 cluster (Fig.1). HiPIP is a small bacterial photosynthetic protein that mediates the photoinduced electron transfer between cytochrome bc1 and light-harvesting reaction center (LH1-RC), thereby enabling the re-reduction of the photo-oxidized LH1-RC in bacterial photosynthesis. Our ultra-high-resolution MHz-SFX structure allowed us to unambiguously resolve the Fe4S4 cluster as well as the hydrogen atoms of key residues. Additionally, we resolved a cryogenic synchrotron structure of HiPIP at a resolution of ~0.94 Å and compared it with the MHz-SFX structure. The results reveal key differences between these structures. We found that most of the intra-atomic distances of the Fe4S4 cluster are shortened by 0.03 to 0.1 Å in comparison with the cryogenic structure, indicating that the MHz-SFX model are less affected by radiation-damage. Secondly, by comparing our structure with a neutron diffraction data of the active state, we also observed key differences in the amide protons of key residues within the cluster vicinity which may have an impact on the understanding of HiPIP electron transfer mechanism in bacterial photosynthesis. In our presentation we will highlight these differences between MHz-SFX and cryogenic HiPIP structures.