The Function fo the Subunits of the Fumarate Reductase Complex of Vibrio succinogenes

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Abstract
The membrane-bound fumarate reductase complex of V. succinogenes catalyzes the reduction of fumarate by 2,3-dimethyl-1,4-naphthohydroquinone (dimethylnaphthohydroquinone) and consists of 3 different peptides (MW 79,000, 31,000 and 25,000), the smallest of which is cytochrome b. The complex was cleaved with guanidinium chloride, the resulting subunits characterized and their functions within the complex investigated by reconstitutional experiments. The 79,000 MW subunit catalyzed the reduction of fumarate by benzylviologen radicals and the oxidation of succinate by methylene blue, but not fumarate reduction by dimethylnaphthohydroquinone. The spectral and the redox properties of the isolated cytochrome b (MW 25,000) were equivalent to those of the high-potential cytochrome b of the bacteria. The isolated cytochrome b had a midpoint potential of -15 mV and was reducible by dimethylnaphthohydroquinone in the absence of the other subunits. The 31,000 MW subunit did not catalyze any of the reactions mentioned above. For the reduction of cytochrome b by succinate in the presence of the 79,000 MW subunit, an amount of the 31,000 MW subunit was required which was equimolar to cytochrome b. The activity of fumarate reduction by dimethylnaphthohydroquinone could be restored by coprecipitation of the 3 subunits. The fumarate reductase complex has 2 different reactive sites, which are essential for its function in the phosphorylative electron transport of the bacterium. The site reacting with the substrates fumarate and succinate is situated on the 79,000 MW subunit, and that reacting with dimethylnaphthohydroquinone is cytochrome b. The 31,000 MW subunit mediates the electron transport between cytochrome b and the 79,000 MW subunit.

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