This project is intended to investigate the integrated events that occur during electron transport in membrane-bound respiratory Complex II. Two model systems are used, succinate- ubiquinone oxidoreductase (SQR, succinate dehydrogenase) and menaquinol-fumarate oxidoreductase (QFR, fumarate reductase) from Escherichia coli. Both are excellent model systems for investigating the function of Complex II that plays an important role in the metabolic processes that occur in mitochondria and is thus important for energy generation by the cell. These enzymes are structurally like their eukaryotic counterparts but much easier to manipulate genetically and for ease of production of large quantities of the proteins that can be studied by biochemical and biophysical methods. The enzyme complexes appear to have evolved from a common evolutionary precursor and are structurally and functionally very similar. Fumarate reductase usually functions in an anaerobic environment whereas, SQR a component of the Krebs cycle functions during aerobic metabolism. Functionally the enzymes can carry out the same reactions of oxidizing succinate to fumarate and reducing fumarate to succinate, however, QFR is much more efficient in both reactions than is SQR in vivo. These studies are intended to determine the structural and functional reasons for the differences in catalytic efficiency of the two enzymes. A recent high resolution structure of QFR allows design of specific site- directed mutations that will address the differences between the two enzymes around the catalytic site and covalent flavin cofactor. High resolution structures with inhibitors bound at the quinone binding sites will be obtained during these studies, as will the identity of amino acids involved in protonation/deprotonation with quinones. Additional studies will investigate how electron transport between the quinone and heme cofactors in QFR and SQR occurs. This research will help to understand the structure and function of quinone binding sites in respiratory complexes and the electron transfer reactions that take place at protein stabilized semiquinones in Complex II.