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:17275378). Essential for the catalytic activity of complex I (PubMed:17275378). a ubiquinone + 5 H(+)(in) + NADH = a ubiquinol + 4 H(+)(out) + NAD(+) Binds 1 [4Fe-4S] cluster. Core subunit of respiratory chain NADH dehydrogenase (Complex I) which is composed of 45 different subunits (PubMed:12611891). This is a component of the iron-sulfur (IP) fragment of the enzyme (By similarity). Hydroxylated ar Arg-111 by NDUFAF5 early in the pathway of assembly of complex I, before the formation of the juncture between peripheral and membrane arms. The disease is caused by variants affecting the gene represented in this entry. Belongs to the complex I 20 kDa subunit family.