3.D.10.1.9
Succinate dehydrogenase with 5 subunits, Sdh2, SdhABCDF. The cryo-EM structure of trimeric Mycobacterium smegmatis succinate dehydrogenase with a membrane-anchor, SdhF, has been determined (Gong et al. 2020).  Diheme-containing succinate:menaquinone oxidoreductases (Sdh) are widespread in Gram-positive bacteria. Gong et al. 2020 presented the 2.8 Å cryo-EM structure of Sdh, which forms a trimer with a membrane-anchored SdhF as a subunit of the complex (PDB 6LUM). The 3 kDa SdhF forms a single transmembrane helix, and this helix plays a role in blocking the canonically proximal quinone-binding site. The authors also identified two distal quinone-binding sites with bound quinones. One distal binding site is formed by neighboring subunits of the complex, and the electron/proton transfer pathway for succinate oxidation by menaquinone was revealed. The structure provides insight into the physiological significance of a trimeric respiratory complex II. The structure of the menaquinone binding site could provide a framework for the development of Sdh-selective anti-mycobacterial drugs (Gong et al. 2020). The architecture of SdhABC (type F), with a membrane-embedded Rieske FeS cluster, has been solved to 2.5 Å resolution (Zhou et al. 2021). A quinone-binding site and a rarely observed Rieske-type [2Fe-2S] cluster, the latter being embedded in the transmembrane region, were identified, and an electron transfer pathway that connects the substrate-binding and quinone-binding sites was identified (Zhou et al. 2021).