Summary: Two distinct forms of fructose 1,6-bisphosphatase, designated form A and form B, have been partially purified from autotrophically grown cells of the hydrogen bacterium 1b. Form A was present when the organism was grown under autotrophic conditions but was not synthesized during heterotrophic growth. Form B was detected in both autotrophically and heterotrophically grown organisms, suggesting that it is a constitutive enzyme. The molecular weights were estimated to be 140000 for form A and 190000 for form B. Both purified enzymes showed fructose bisphosphatase as well as sedoheptulose bisphosphatase activity, with pH optima of 8.2 and 8.8 to 9.0 for forms A and B, respectively. The ratio of fructose bisphosphatase to sedoheptulose bisphosphatase activity was 0.6 for form A and about 4.5 for form B. The enzymes required Mg for activity; Mm could only partially replace Mg. Form A was inhibited by ATP and ribulose bisphosphate. AMP exerted a strong allosteric inhibition on form B which was also inhibited by NAD and NADH. Phospho/pyruvate activated form B. On the basis of these results, different physiological roles for the two forms of fructose bisphosphatase are suggested.


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