Characterization of Two Fructose Bisphosphatase Isoenzymes from the Hydrogen Bacterium Nocardia opaca 1b

Abstract

Two distinct forms of fructose 1,6-bisphosphatase, form A and form B, were partially purified from autotrophically grown cells of N. opaca 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 autotrophically and heterotrophically grown organisms, suggesting that it is constitutive. The MW were estimated to be 140,000 for form A and 190,000 for form B. Both purified enzymes showed fructose bisphosphatase and sedoheptulose bisphosphatase activity, with pH optima of 8.2 and 8.8-9.0 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 Mg2+; Mn2+ could only partially replace Mg2+. 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. Phosphoenolpyruvate activated form B. Different physiological roles for the 2 forms of fructose bisphosphatase are suggested.