Succinate:quinone oxidoreductases structure

THE STRUCTURE OF Wolinella succinogenes QUINOL FUMARATE REDUCTASE AND ITS RELEVANCE TO THE SUPERFAMILY OF SUCCINATE QUINONE OXIDOREDUCTASES... 

The membrane-bound cytochrome 655s (Bacillus subtilis) contains, according to secondary structure predictions, five transmembrane helices. It functions to anchor two other subunits of the succinate quinone oxidoreductase complex (complex II, E.C. 1.3.5.1) in the cytoplasmic membrane (68). The 1.3-2.0 hemes per covalently bound flavin have been found with the isolated enzyme. The amino acid residues that bind the heme between the a-helices are likely bis(histidine). The EPR and NIR MCD spectra are consistent with this because the EPR spectra show a g value of 3.4 with a HALS lineshape, and the MCD spectra show a low-spin CT band at 1600 nm with Ae of 380 M cm at 4.2 K and 5 T (69). This appears to be another example of a bis(histi-dine)-coordinated heme with near perpendicular alignment of the ligands. 

Complex II is the succinate dehydrogenase of the tricarboxylic acid, or Krebs, cycle and catalyzes the oxidation of succinate to fumarate, coupled to the reduction of UQ to ubiquinol. It is part of a large family of related succinate quinone oxidoreductases and quinol fumarate oxidoreductases found in bacteria and mitochondria that have been classified based on subunit structure, number of cytochrome b haem centers and class of quinone substrate [77, 78]. The mitochondrial form belongs to the class that has a single h type haem bound to one of... 

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