The effects of anions on fumarate reductase isolated from the cytoplasmic membrane of Escherichia coli

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Abstract
A broad range of anions stimulated the Vmax of purified fumarate reductase isolated from the cytoplasmic membrane of E. coli, while leaving the Km for fumarate unaffected. Reducing agents potentiate the effects of anions on the activity but have no effect by themselves. Thermal stability, conformation as monitored by circular dichroism and susceptibility to the thiol reagent 5,5''-dithiobis-(2-nitrobenzoic acid) are also altered by anions. The apparent Km for succinate in the reverse reaction (succinate dehydrogenase activity) varies as a function of anion concentration, but the Vmax is not affected. The membrane-bound activity is not stimulated by anions and its properties closely resemble those of the purified enzyme in the presence of anions. Anions apparently alter the physical and chemical properties of fumarate reductase, so that it more closely resembles the membrane-bound form.