Core subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I) that is believed to belong to the minimal assembly required for catalysis. Complex I functions in the transfer of electrons from NADH to the respiratory chain. The immediate electron acceptor for the enzyme is believed to be ubiquinone (By similarity). This is the largest subunit of complex I and it is a component of the iron-sulfur (IP) fragment of the enzyme. It may form part of the active site crevice where NADH is oxidized (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. Complex I is composed of about 30 different subunits. Matrix and cytoplasmic side of the mitochondrial inner membrane. Belongs to the complex I 75 kDa subunit family.