Fig. 6: Structure of the cytoplasmic input and output subunits.

a RnfB. This subunit is built up of the Fd segments FdI (brown), FdII (red), and FdIII (salmon). Each of the latter forming the globular FeII+III domain carries two [4Fe-4S] clusters (Fe and S as brown and yellow spheres). One extra [4Fe-4S] cluster (4) is packed between the segment linking FdII and FdIII (light green) and the RnfC interface segment (gold). The Fd(I) segment is too mobile for reliable model building. Therefore, the FdI structure was calculated by Alphafold2. The position of the [4Fe-4S] clusters was derived from four clustered cysteines but also from a related Fd domain of a corrinoid iron-sulfur protein³⁷. b RnfC. The subunit is architecturally composed of an N-terminal α,β module built up of three subdomains and a C-terminal Fd domain (orange) carrying two [4Fe-4S] clusters. The ubiquinone-like subdomain (magenta) consists of two three-stranded antiparallel β-strands (30–101) packing against each other and the NuoF-like subdomains (sky blue and blue) of a four-stranded parallel β-sheet surrounded by three helices (102–282) and a five-stranded antiparallel β-sheet (283–352), respectively. The non-covalently bound FMN is embedded at the center of the subunit and accessible for NAD⁺ via a wide cleft (see arrow).