Three-dimensional Structure of Bovine NADH:ubiquinone Oxidoreductase (complex I) at 22 A in Ice

NADH:ubiquinone oxidoreductase (complex I) is the first and largest complex in the electron transport chain of mitochondria. The bovine complex purified from cardiac muscle consists of at least 42 different subunits with a combined molecular mass of about 890 kDa. The three-dimensional structure of the complex was determined at 22 A from single particles embedded in vitrified ice using electron cryo-microscopy. The structure was calculated using a new program to align particles, to correct for the contrast transfer function of the microscope, and to carry out the three-dimensional reconstruction of the complex. The bovine complex has the overall L-shaped appearance found in earlier studies of the closely related complex I from Neurospora crassa, but it differs by having a thin stalk region linking the membrane-bound globular arm with the intrinsic membrane domain. Thus, the stalk which measures about 30 A in diameter is likely to contain part of the electron transfer pathway linking the NADH binding site in the globular arm with the ubiquinone binding site in the membrane domain. The globular domain of bovine complex I is significantly bigger than that of the N. crassa enzyme, suggesting that the apparent additional subunit complexity of the bovine enzyme is associated with the globular part.