Crystal structure of mitochondrial respiratory membrane protein Complex II

Zihe Rao’s group was first to successfully determine the three-dimensional structure of the mitochondrial respiratory membrane protein Complex II (1), thus expanding the research fields of mitochondrial structural biology and cell biology. It is an important achievement which should have significant impacts in life science and medical research, since mutations in Complex II are implicated in a number of mitochondrial diseases including familial or non-familial head and neck paraganglioma, familial or non-familial pheochromocytoma, midgut carcinoid, Merkel cell carcinoma and Leigh syndrome.

The role of Complex II in the mitochondrial respiratory chain.

Mitochondria, as cellular organelles, are the "energy factory" of the cell and are mainly responsible for cell aerobic respiration. They realize energy transformation through the oxidation-phosphorylation process and provide most of the energy for cell activity. The oxidation process in mitochondria is carried out by four respiratory membrane protein complexes inside the mitochondrial inner membrane (Complex I, II, III and IV). Since the 1990s, determining the structures of these four membrane protein complexes has been a major challenge and leading scientists from the United States, Japan, England and Germany have made great efforts in this field. Until recently, only scientists from the United States and Japan had been successful in determining the crystal structures of mitochondrial Complex III and Complex IV. No breakthroughs had been made on either mitochondrial Complex I or Complex II until the work by our group.

(left) Overall structure of mitochondrial respiratory Complex II.A transparent surface is superimposed onto a ribbon representation of Complex II. FAD binding protein (Fp) is shown in blue; the iron-sulfur protein (Ip) is shown in cream; the transmembrane proteins CybL and CybS are shown in pink and gold, respectively. (right) Prosthetic groups constituting the electron transfer pathway. FAD, [2Fe-2S],[4Fe-4S], [3Fe-4S] and heme b are shown with ubiquinone (UQ), together with their edge-to-edge distances and midpoint redox potentials.

We began our structural studies of the mitochondrial respiratory membrane protein Complex II in 2001 using new methods. They chose to extract and purify the membrane protein complex from porcine heart and finally determined the structure of this complex in 2005. Complex II, also known as succinate:ubiquinone oxidoreductase, is comprised of four different protein subunits: the flavoprotein (622 amino acids), iron-sulfur protein (252 amino acids), and two membrane-anchor proteins (CybL, 140 amino acids and CybS, 103 amino acids) with a total of six trans-membrane helices. The Complex II structure also includes prosthetic groups required for electron transfer from succinate to ubiquinone. The structure correlates the protein environments around prosthetic groups with their unique midpoint redox potentials. Two ubiquinone binding sites are discussed and elucidated by a complex structure with inhibitors 3-nitropropionate and 2-thenoyltrifluoroacetone (TTFA). The availability of the Complex II structure provides a bona fide model for the study of human mitochondrial diseases related to mutations in this complex and helps to complete our understanding of the mitochondrial respiratory electron transfer chain.

The structure of the mitochondrial respiratory Complex II was published in Cell. Incidentally, this was the first paper by Chinese scientists working entirely in mainland China to be published in Cell for 25 years.


  1. Sun F, X. Huo, Y. Zhai, A. Wang, J. Xu, D. Su, M. Bartlam, and Z. Rao*. 2005. Crystal structure of mitochondrial respiratory membrane protein complex II. Cell 121(7):1043-57.