Abstract
Mammalian mitochondrial complex I is a multisubunit membrane-bound assembly with a molecular mass approaching 1 MDa. By comprehensive analyses of the bovine complex and its constituent subcomplexes, 45 different subunits have been characterized previously. The presence of a 46th subunit was suspected from the consistent detection of a molecular mass of 10,566 by electrospray ionization mass spectrometry of subunits fractionated by reverse-phase high pressure liquid chromatography. The component was found associated with both the intact complex and subcomplex Ibeta, which represents most of the membrane arm of the complex, and it could not be resolved chromatographically from subunit SGDH (the subunit of bovine complex I with the N-terminal sequence Ser-Gly-Asp-His). It has now been characterized by tandem mass spectrometry of intact protein ions and shown to be a C-terminal fragment of subunit SGDH arising from a specific peptide bond cleavage between Ile-55 and Pro-56 during the electrospray ionization process. Thus, the subunit composition of bovine complex I has been established. It is a complex of 45 different proteins plus non-covalently bound FMN and eight iron-sulfur clusters.
Highlights
Complex I (NADH ubiquinone oxidoreductase) catalyzes the first stages of oxidative phosphorylation in mitochondria by oxidizing NADH in the matrix and transferring the electrons via flavin mononucleotide and a series of iron-sulfur centers to ubiquinone in the inner membrane [1, 2]
The component was found associated with both the intact complex and subcomplex I, which represents most of the membrane arm of the complex, and it could not be resolved chromatographically from subunit SGDH
The passage of two electrons, one at a time, from NADH to ubiquinone is coupled to the translocation of four protons from the mitochondrial matrix to the intermembrane space [2, 3]
Summary
Complex I (NADH ubiquinone oxidoreductase) catalyzes the first stages of oxidative phosphorylation in mitochondria by oxidizing NADH in the matrix and transferring the electrons via flavin mononucleotide and a series of iron-sulfur centers to ubiquinone in the inner membrane [1, 2]. The component was found associated with both the intact complex and subcomplex I, which represents most of the membrane arm of the complex, and it could not be resolved chromatographically from subunit SGDH (the subunit of bovine complex I with the N-terminal sequence Ser-Gly-Asp-His).
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