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. a ubiquinone + 5 H(+)(in) + NADH = a ubiquinol + 4 H(+)(out) + NAD(+) Binds 1 FMN. 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 flavoprotein-sulfur (FP) fragment of the enzyme (PubMed:12611891). Interacts with RAB5IF (PubMed:31536960). The disease is caused by variants affecting the gene represented in this entry. Belongs to the complex I 51 kDa subunit family.