NDUFA9: Proteins and Enzymes

ATP is generated via oxidative phosphorylation (Oxphos) in the mitochondria of almost all cells. The protein components of Oxphos, 4 respiratory chain complexes (I-IV) and an ATP synthase, are encoded in both the mitochondrial and nuclear genomes. Of the 4 respiratory chain complexes, complex I is the largest at 900 kD and contains at least 41 polypeptide subunits, 7 of which are encoded in the mitochondria, the remaining subunits are encoded in the nucleus. The multisubunit NADH:ubiquinone oxidoreductase is the first enzyme complex. By use of chaotropic agents, complex I can be fragmented into 3 different fractions. The flavoprotein fraction contains the NDUFV1, NDUFV2, and NDUFV3 subunits. The iron-sulfur protein (IP) fraction contains at least 7 subunits, NDUFS1-NDUFS6 and NDUFA5. The remaining subunits are part of the hydrophobic protein (HP) fraction. NDUFA9 is part of the hydrophobic protein fraction of the enzyme complex. However, it is predominantly hydrophilic, and appears to lie mostly outside the lipid bilayer.