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 the catalytic activity of complex I (By similarity). 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:10852722, PubMed:18721790, PubMed:25209663). This is a component of the iron-sulfur (IP) fragment of the enzyme (PubMed:25209663). Hydroxylated ar Arg-114 by NDUFAF5 early in the pathway of assembly of complex I, before the formation of the juncture between peripheral and membrane arms. Belongs to the complex I 20 kDa subunit family.