Structure of Acinetobacter baumannii cytochrome bo3 ubiquinol oxidase

Abstract Heme-copper oxidases (HCOs) are terminal oxidases that couple oxygen reduction to proton pumping for ATP synthesis. Although our previous work has elucidated the structure and proton transfer mechanism of the Escherichia coli cytochrome bo₃ ubiquinol oxidase (cyt bo₃), the quinone dynamics and structural diversity across HCOs remain unclear. Here, we report the high-resolution cryo-EM structures of cyt bo₃ from the pathogen Acinetobacter baumannii. We captured four distinct conformational states of its native ubiquinone-8 substrate within the binding pocket. Comparative analysis revealed that conformational transitions of the substrate are directly coupled to movements of the TM0 helix. Notably, in the locked state, the substrate headgroup is stabilized by specific hydrogen bonds and adopts a distinct depth and orientation. Additionally, a unique hairpin-like loop was identified in subunit II, a specific feature absent in the homologs. Our observations not only provide structural details of a pathogenic respiratory terminal oxidase but also reveal a dynamic substrate catalytic mechanism, highlighting potential avenues for targeting bacterial energy metabolism.