NADH enters the respiratory chain at Complex I Authored: Jassal, B, 2005-05-10 13:13:07 Complex I (NADH:ubiquinone oxidoreductase or NADH dehydrogenase) utilizes NADH formed from glycolysis and the TCA cycle to pump protons out of the mitochondrial matrix. It is the largest enzyme complex in the electron transport chain, containing 45 subunits. Seven subunits (ND1-6, ND4L) are encoded by mitochondrial DNA (Loeffen et al [1998]), the remainder are encoded in the nucleus. The enzyme has a FMN prosthetic group and 8 Iron-Sulfur (Fe-S) clusters. The electrons from NADH oxidation pass through the flavin (FMN) and Fe-S clusters to ubiquinone (CoQ). This electron transfer is coupled with the translocation of protons from the mitochondrial matrix to the intermembrane space. For each electron transferred, 2 protons can be pumped out of the matrix. As there are 2 electrons transferred, 4 protons can be pumped out. Complex I is made up of 3 sub-complexes - Iron-Sulfur protein fraction (IP), Flavoprotein fraction (FP) and the Hydrophobic protein fraction (HP), probably arranged in an L-shaped structure with the IP and FP fractions protruding into the mitochondrial matrix and the HP arm lying within the inner mitochondrial membrane. The overall reaction can be summed as below: NADH + Ubiquinone + 5H+matrix -> NAD+ + Ubiquinol + 4H+memb. space The electrons from complex I are transferred to ubiquinone (Coenzyme Q, CoQ), a small mobile carrier of electrons located within the inner membrane. Ubiquinone is reduced to ubiquinol during this process. EC Number: 1.6.5.3 GENE ONTOLOGYGO:0006120 Pubmed11695836 Pubmed12231006 Pubmed12837546 Pubmed14741580 Pubmed15250827 Pubmed9878551 Reactome Database ID Release 43163217 Reactome, http://www.reactome.org ReactomeREACT_6310 has a Stoichiometric coefficient of 4 has a Stoichiometric coefficient of 5