Structure and Function of Complex I of Mitochondrial Respiratory Chain

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Mitochondrial NADH : ubiquinone oxidoreductase (complex I) is the most complicated system in the respiratory chain. It consists of many subunits, some of which hold iron-sulfur clusters, but structural information is still limited. Two hydrophilic subfraction of bovine heart complex I were systematically resolved into individual polypeptides by chromatography. Three polypeptides (51, 24, and 9kDa) were isolated from the flavoprotein fraction (FP) of complex I, and the complete amino acid sequence of the 9kDa polypeptide was determined. The 9kDa polypeptide was composed of 75 amino acids with a molecular weight of 8437. This protein exhibited no obvious sequence similarity to other proteins. The iron-sulfur protein fraction (IP) of complex I was separated into eight polypeptides, 75, 49, 30, 20, 18, 15, 13kDa-A, and13kDa-B. The 2OkDa polypeptide was recognized as a novel component of IP for the first time. The 13kDa-A polypeptide was composed of 96 amino acids with a molecular weight of 10536. The 13kDa-B polypeptide consisted of 114 amino acids and had an acetylated amino terminus. The molecular weight of this protein was calculated to be 13130 including the acetyl group. The 13kDa polypeptides might be iron-sulfur proteins because the characteristic arrangement of cysteine residues were present in the 13kDa-A polypeptide. The partial amino acid sequences of the 30 and 20kDa polypeptides were also determined. Comparison of sequences revealed significant sequence similarities of the 30 and 20kDa polypeptides to the products of the ORF158 and psbG gene encoded in the chloroplast genome, respectively. The conserved sequence in the 2OkDa and PsbG proteins was also found in the small subunit of the nickel-containing hydrogenases. The conserved region among these proteins contained two cysteine residues, which might be involved in the formation of an iron-sulfur cluster. These results suggest that complex I is related to other redox enzyme complexes.