It has been shown that treatment of bovine mitochondrial complex I (NADH-ubiquinone oxidoreductase) with NADH or NADPH, but not with NAD or NADP, increases the susceptibility of a number of subunits to tryptic degradation. This increased susceptibility involved subunits that contain electron carriers, such as FMN and iron-sulfur clusters, as well as subunits that lack electron carriers. Results shown elsewhere on changes in the cross-linking pattern of complex I subunits when the enzyme was pretreated with NADH or NADPH (Belogrudov, G., and Hatefi, Y. (1994) Biochemistry 33, 4571-4576) also indicated that complex I undergoes extensive conformation changes when reduced by substrate. Furthermore, we had previously shown that in submitochondrial particles the affinity of complex I for NAD increases by >20-fold in electron transfer from succinate to NAD when the particles are energized by ATP hydrolysis. Together, these results suggest that energy coupling in complex I may involve protein conformation changes as a key step.In addition, it has been shown here that treatment of complex I with trypsin in the presence of NADPH, but not NADH or NAD(P), produced from the 39-kDa subunit a 33-kDa degradation product that resisted further hydrolysis. Like the 39-kDa subunit, the 33-kDa product bound to a NADP-agarose affinity column, and could be eluted with a buffer containing NADPH. It is possible that together with the acyl carrier protein of complex I the NADP(H)-binding 39-kDa subunit is involved in intramitochondrial fatty acid synthesis.Among the enzyme complexes of the mitochondrial electron transport/oxidative phosphorylation system, complex I (NADH-ubiquinone oxidoreductase) has the most complicated structure and mechanism of action. The enzyme is composed of Ն43 subunits (1, 2), and contains one FMN and 5 EPR-visible iron-sulfur clusters, designated centers N1a, N1b, N2, N3, and N4 (3). Chemical analysis of iron and sulfide suggested the possible presence of 8 iron-sulfur clusters per FMN (4). These results agreed with more recent data on the amino acid sequences of complex I subunits, in which 8 cysteine motifs were found for ligating 6 tetranuclear and 2 binuclear iron-sulfur clusters (1, 2).Early studies showed that bovine complex I could be resolved into three domains, designated as flavoprotein (FP), 1 iron-sulfur protein (IP), and hydrophobic protein (HP) (5, 6). This tridomain architecture appears to be the structural design of complex I from Neurospora crassa mitochondria (Ն35 subunits) (7), Escherichia coli plasma membrane (13 subunits) (8, 9), and possibly Paracoccus denitrificans (14 subunits) (10). Bovine FP is water-soluble; it is composed of three subunits with molecular masses of 51, 24, and 9 kDa, and it contains per mol 1 FMN, a tetranuclear iron-sulfur cluster, and a binuclear iron-sulfur cluster. Data from the bovine (11), the Paracoccus (12), and the E. coli (13) enzymes have indicated that the 51-kDa subunit binds NAD(H) and contains FMN and a tetranuclear cluster (center N3), and the 24-kDa s...