Structure of the Escherichia coli fumarate reductase respiratory complex

The integral membrane protein fumarate reductase catalyzes the final step o f anaerobic respiration when fumarate is the terminal electron acceptor. Th e homologous enzyme succinate dehydrogenase also plays a prominent role in cellular energetics as a member of the Krebs cycle and as complex II of the aerobic respiratory chain. Fumarate reductase consists of four subunits th at contain a covalently linked flavin adenine dinucleotide, three different iron-sulfur clusters, and at Least two quinones, The crystal structure of intact fumarate reductase has been solved at 3.3 angstrom resolution and de monstrates that the cofactors are arranged in a nearly linear manner from t he membrane-bound quinone to the active site flavin. Although fumarate redu ctase is not associated with any proton-pumping function, the two quinones are positioned on opposite sides of the membrane in an arrangement similar to that of the Q-cycle organization observed for cytochrome bc(1).