Purification, characterization and crystallization of menaquinol:fumarate oxidoreductase from the green filamentous photosynthetic bacterium Chloroflexus aurantiacus

The integral membrane protein complex, menaquinol:fumarate oxidoreductase (mQFR) has been purified, identified and characterized from the thermophilic green filamentous anoxygenic photosynthetic bacterium Chloroflexus aurantiacus. The complex is composed of three subunits: a 74 kDa flavoprotein that contains a covalently bound flavin adenine dinucleotide, a 28 kDa iron-sulfur cluster-containing polypeptide, and a 27 kDa transmembrane polypeptide, which is also the binding site of two b-type hemes and two menaquinones. The purified complex has an apparent molecular mass of 260 kDa by blue-native PAGE, which is indicative of a native homodimeric form. The isolated complex is active in vitro in both fumarate reduction and succinate oxidation. It has been analyzed by visible absorption, redox titration, chemical analysis and EPR spectroscopy. In addition, phylogenetic analysis shows that the QFR of both C. aurantiacus and Chlorobium tepidum are most closely related to those found in the delta-proteobacteria. The purified enzyme was crystallized and X-ray diffraction data obtained up to 3.2 Å resolution.