Three-dimensional structure of bovine heart NADH: ubiquinone oxidoreductase (complex I) by electron microscopy of a single negatively stained two-dimensional crystal.

Bovine heart NADH:ubiquinone oxidoreductase (complex I), which is the largest (about 1 MDa) membrane protein complex in the mitochondrial respiratory chain, catalyzes the electron transfer from NADH to ubiquinone, coupled with proton pumping. We have crystallized bovine complex I in reconstituted lipid bilayers and obtained a three-dimensional density map by the electron crystallographic analysis of a single negatively stained two-dimensional crystal. The asymmetric unit with dimensions of a = 388 Å, b = 129 Å and γ = 90° contains two molecules and is of P1 symmetry. Structural differences between the two molecules indicate flexibility of the hydrophilic domain relative to the membrane-embedded domain.