Fig. 5: Superimposed RnfADEG and NqrEBDC core structures.

RnfG (brown)/NqrC (gold) are primarily built up of a β fold consisting of a central five-stranded antiparallel β-sheet flanked by one long N- and one C-terminal helix on both sides and some elongated linkers between β-strands. The extracellular RnfG is either attached to RnfD (hotpink) as in the cryo-EM structure or to RnfAE (NqrDE) as observed for NqrC in the Nqr crystal structure. Thus, FMN_RnfG/NqrC is either in a productive ET distance to FMN_RnfD or to the central Fe ion of RnfAE (NqrDE). Mobility of RnfG is rationalized by small respective contact areas. Different orientations between RnfG and NqrC are highlighted by transparent brown and gold rectangles. The exposed covalently attached FMN_RnfG is fixed by three protruding loops (marked by 1, 2, 3); the first (91–94) links two β-strands, the second (114–128) a β-strand and a small helical segment parallel to the β-sheet and the third (155–162) precedes the C-terminal helix. The orientation of RnfG substantially influences the conformation of the five-helix linker (green, yellow), in particular, the occlusion loop (black), which is transformed in Nqr to a prolonged helix 188:201 (VI) (gray arrow). The reason for the different orientations of RnfG and NqrC is unknown. Structural differences as e.g., the extracellular insertion 75–124 in NqrB and the deviating orientations of helix 21:42 (I) of RnfD have to be considered but also the high Na⁺ concentration in the buffer (200 mM NaCl) of Rnf and crystallographic effects of Nqr might play a role. Na⁺-dependent conformational changes are reported for NqrC⁶¹.