Accessory subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I), that is believed not to be involved in 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. Mammalian complex I is composed of 45 different subunits. This is a component of the iron-sulfur (IP) fragment of the enzyme. Contains two C-X9-C motifs that are predicted to form a helix-coil-helix structure, permitting the formation of intramolecular disulfide bonds. Belongs to the complex I NDUFS5 subunit family.