Core subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I) which catalyzes electron transfer from NADH through the respiratory chain, using ubiquinone as an electron acceptor (PubMed:10852722, PubMed:18721790). Essential for catalysing the entry and efficient transfer of electrons within complex I (By similarity). Plays a key role in the assembly and stability of complex I and participates in the association of complex I with ubiquinol-cytochrome reductase complex (Complex III) to form supercomplexes (By similarity). a ubiquinone + 5 H(+)(in) + NADH = a ubiquinol + 4 H(+)(out) + NAD(+) Binds 1 [2Fe-2S] cluster per subunit. Binds 2 [4Fe-4S] clusters per subunit. Core subunit of respiratory chain NADH dehydrogenase (Complex I) which is composed of 45 different subunits (PubMed:10852722, PubMed:18721790, PubMed:25209663). This is the largest subunit of complex I and it is a component of the iron-sulfur (IP) fragment of the enzyme (PubMed:10852722, PubMed:25209663). Complex I associates with ubiquinol-cytochrome reductase complex (Complex III) to form supercomplexes (By similarity). Interacts with MDM2 and AKAP1 (By similarity). Belongs to the complex I 75 kDa subunit family.