The vertebrate H⁺-translocating NADH dehydrogenase (NDH) complex (45 subunits) (Cardol et al., 2004).  The 3-d structure of the 44 subunit complex (14 core subunits present in bacteria, plus 30 additional subunits) with a molecular weight of 1 MDa, has been solved at 5 Å resolution by single particle electron cryo-microscopy (Vinothkumar et al. 2014).The core subunits contain eight iron-sulphur clusters and 60 transmembrane helices. The structures of many of the supernumerary subunits were determined or modeled. The structure provided insight into the roles of the supernumerary subunits in regulation, assembly and homeostasis.  One such subunit, GRIM-19 or NDUFA13, (Q9P0J0 of the human homologue) is essential for membrane potential formation and NADH assembly (Lu and Cao 2008).  Stroud et al. 2016 showed that 25 of the 31 accessory subunits in the 45 subunit human NADH dehydrogenase complex are required for assembly of a functional complex, and 1 subunit is essential for cell viability. Quantitative proteomic analysis revealed that loss of each subunit affects the stability of other subunits residing in the same structural module (Stroud et al. 2016). Leber's hereditary optic neuropathy (LHON) in humans is associated with combinations of individually non-pathogenic missense mitochondrial DNA (mtDNA) variants, affecting the MT-ND4, MT-ND4L and MT-ND6 subunit genes of Complex I (Caporali et al. 2018). It has been proposed that the quinone reaction cavity is indeed from the matrix-side region covered by the ND3 TMH1-2 loop (Masuya et al. 2021). Resting mitochondrial complex I from Drosophila melanogaster adopts a helix-locked state (Padavannil et al. 2023).  Mutations in the ND5 gene are associated with Leber Hereditary Optic Neuropathy (Pandya et al. 2024).