Structure of the Escherichia coli fumarate reductase respiratory complex. Science 284

The integral membrane protein fumarate reductase catalyzes the Þnal step of anaerobic respiration when fumarate is the terminal electron acceptor. The 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 that contain a covalently linked ßavin 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 demon-strates that the cofactors are arranged in a nearly linear manner from the membrane-bound quinone to the active site ßavin. Although fumarate reduc-tase 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 bc1. Because oxygen has a high affinity for electrons, aerobic respiration represents