Regulatory properties of Rana esculenta liver D-fructose-1,6-bisphosphate 1-phosphohydrolase and their comparison with properties of other vertebrate liver isoenzymes

Kinetic properties of Pelophylax esculentus muscle FBPase

D-Fructose-1,6-bisphosphate 1-phosphohydrolase FBPase; [EC 3.1.3.11] was isolated from Pelophylax esculentus muscle in an electrophoretically homogeneous form with ca 30% yield. Its subunit molecular mass is ca 37 kDa. In this study, we determined the basic kinetic properties of the frog muscle enzyme. FBPase exhibited a maximum activity at pH 7.5. Like other FBPases the frog enzyme requires magnesium ions for its activity (K(a)=263 microM) and is activated by potassium ions (K(a)=63.6 microM). I(0.5) for calcium ion (91 microM) is 100 times higher than the corresponding value of mammalian muscle FBPase. K(s) for the substrate was 1.68 microM. Substrate excess inhibited the enzyme (K(si)=55 microM). AMP and fructose-2,6-bisphosphate (Fru-2,6P(2)) are potent inhibitors of frog muscle FBPase with I(0.5) of 0.2 microM and K(i) of 114 nM, respectively. Both inhibitors act synergistically on the frog muscle FBPase. In the presence of 0.05-0.5 microM of AMP, K(i) for Fru-2,6P(2) is 92 and 28 nM. I(0.5) for AMP for P. esculentus muscle FBPase is 55 times lower than the corresponding value for P. esculentus liver isozyme.

The interaction of FBPase with aldolase: a kinetic and fluorescence investigation on chicken muscle enzymes

Fructose-1,6-bisphosphatase (FBPase; EC 3.1.3.11) is strongly inhibited by AMP in vitro and, therefore, at physiological concentrations of substrate and AMP, FBPase should be completely inhibited. Desensitization of rabbit muscle FBPase against AMP inhibition was previously observed in the presence of rabbit muscle aldolase. In this study, we analysed the kinetics of an FBPase catalyzed reaction and interaction between chicken muscle FBPase and chicken muscle aldolase. The initial rate of FBPase reaction vs. substrate concentration shows a maximum activity at a concentration of 20 microM Fru-1,6P2 and then decreases. Assuming rapid equilibrium kinetics, the enzyme-catalyzed reaction was described by the substrate inhibition model, with Ks approximately 5 microM and Ksi approximately 39 microM and factor beta approximately 0.2, describing change in the rate constant (k) of product formation from the ES and ESSi complexes. Based on ultracentrifugation studies, aldolase and FBPase form a hetero-complex with approximately 1:1 stoichiometry with a dissociation constant (Kd) of 3.8 microM. The FBPase-aldolase interaction was confirmed via fluorescence investigation. The aldolase-FBPase interaction results in aldolase fluorescence quenching and its maximum emission spectrum shifting from 344 to 356 nm. The Kd of the FBPase-aldolase complex, determined on the basis of fluorescence changes, is 0.4 microM at 25 degrees C with almost 1:1 stoichiometry. This interaction increases the I(0.5) for the AMP inhibition of FBPase threefold, and slightly affects FBPase affinity to magnesium ions, increasing the Ka and Hill coefficient (n). No effect of aldolase on the FBPase pH optimum was observed. Thus, the decrease in FBPase sensitivity to AMP inhibition enables FBPase to function in vivo thanks to aldolase.