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). May donate electrons to ubiquinone. a ubiquinone + 5 H(+)(in) + NADH = a ubiquinol + 4 H(+)(out) + NAD(+) Binds 2 [4Fe-4S] clusters per subunit. Complex I is composed of at least 49 different subunits. This is a component of the iron-sulfur (IP) fragment of the enzyme. Belongs to the complex I 23 kDa subunit family.