Chapter 1 Visualizing functional flexibility by three-dimensional electron microscopy reconstructing complex I of the mitochondrial respiratory chain

Complex I is the major entry point in the bacterial and mitochondrial respiratory chain. Structural knowledge of the enzyme is still limited because of its large size and complicated architecture. Only the structure of the hydrophilic domain of a bacterial Complex I has been solved to high resolution by X-ray crystallography. To date, no X-ray structure of the complete enzyme has been reported, and most structural information of the holoenzyme has been obtained by 3-D electron microscopy. In this chapter the methods are described used for determining the 3-D reconstruction of Complex I that revealed for the first time a detailed and reproducible domain structure. Complex I is a highly flexible molecule, and methods for calculating the 3-D reconstruction from electron micrographs must take into account this heterogeneity. The techniques described in this chapter can be modified and adapted for the study of more heterogeneous preparations, such as functionalized Complex I. In addition, these techniques are not restricted to the structure determination of Complex I but are appropriate for the 3-D reconstruction of macromolecular assemblies from electron micrographs when inhomogeneities may be present.