Dr. Michael Radermacher, University of Vermont

3D Electron Microscopy of Mitochondrial and Bacterial Complex I

Many complex organisms obtain energy by oxidation of fuel molecules. This process does not take place instantaneously, but is mediated by electron carrier complexes I-IV. Oxidation and concomitant production of ATP by the ATP synthase (Complex V) takes place in a series of
enzymes that form the oxidative phosphorylation system. In bacteria, the respiratory chain is located in the cytoplasmic membrane, and in eukaryotic cells in the inner mitochondrial membrane. In one pathway that oxidizes NADH, the electron transport chain, is composed of Complex I - the NADH-dehydrogenase - followed by complexes III (bc1-
complex) and IV (cytochrome c oxidase). Complex I converts between 30% and 50% of the energy generated in the electron transport chain. The structural knowledge of Complex I is limited and many of its functional principles are not yet understood. Complex I is the largest of the enzymes within the respiratory chain. The size of Complex I is species dependent, ranging from 550 kDa in bacteria and containing 14 individual subunits to approximately 1MDa in eukaryotes with 40 individual subunits. 3D reconstruction of complex I by electron microscopy is challenging, since the flexibility of the enzyme poses high demands on the image processing methods. We have determined the 3D structures of eukaryotic and prokaryotic complex I. The possible locations of the catalytic subunits have been determined by 3D difference imaging in conjunction with the fitting of the X-ray data of a hydrophilic fragment of bacterial complex I, and a comparison of the 3D structures of complex I from different species. The functional implications of these results will be discussed.