Respiratory electron transport complexes are organized as individual entities or combined as large super-complexes (SC). The Gram-negative bacteria deploy a mitochondrial-like cytochrome (cyt) bc1 (Complex III, CIII2), and may have specific cbb3-type cyt c oxidases (Complex IV, CIV) instead of the canonical aa3-type CIV. Electron transfer between these complexes is mediated by soluble (c2) and membrane-anchored (cy) cyts. This study describes the cryo-EM structures of the bc1-cbb3 type SC (CIII2CIV, 5.2Å resolution), together with three conformers of native CIII2 (3.3Å resolution). The SC is active both in vivo and in vitro, contains all catalytic subunits and cofactors, and additional transmembrane helices attributed to cyt cy and the assembly factor CcoH. The cyt cy is integral to SC, its cyt domain is mobile and conveys electrons to CIV differently than cyt c2. The structure of this SC illustrates the characteristics of membrane-confined and membrane-external respiratory electron transport pathways of Gram-negative bacteria (Steimle et al., (2021).

Cryo-EM structures of the bc1-cbb3 type CIII2CIV super-complexes, their electron carrier cytochromes and electronic communication between the complexes

ACS Fall 2021

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