Variability in the catalytic and allosteric properties of rabbit liver fructose 1,6-diphosphatase. The prescence of tryptophan in homogeneous enzyme

Regulation of rabbit liver fructose-1,6-bisphosphatase by metals, nucleotides, and fructose 2,6-bisphosphate as determined from fluorescence studies

The fluorescent nucleotide analogue formycin 5'-monophosphate (FMP) inhibits rabbit liver fructose-1,6-bisphosphatase (I50 = 17 microM, Hill coefficient = 1.2), as does the natural regulator AMP (I50 = 13 microM, Hill coefficient = 2.3), but exhibits little or no cooperativity of inhibition. Binding of FMP to fructose-1,6-bisphosphatase can be monitored by the increased fluorescence emission intensity (a 2.7-fold enhancement) or the increased fluorescence polarization of the probe. A single dissociation constant for FMP binding of 6.6 microM (4 sites per tetramer) was determined by monitoring fluorescence intensity. AMP displaces FMP from the enzyme as evidenced by a decrease in FMP fluorescence and polarization. The substrates, fructose 6-phosphate and fructose 1,6-bisphosphate, and inhibitors, methyl alpha-D-fructofuranoside 1,6-bisphosphate and fructose 2,6-bisphosphate, all increase the maximal fluorescence of enzyme-bound FMP but have little or no effect on FMP binding. Weak metal binding sites on rabbit liver fructose-1,6-bisphosphatase have been detected by the effect of Zn2+, Mn2+, and Mg2+ in displacing FMP from the enzyme. This is observed as a decrease in FMP fluorescence intensity and polarization in the presence of enzyme as a function of divalent cation concentration. The order of binding by divalent cations is Zn2+ = Mn2+ greater than Mg2+, and the Kd for Mn2+ displacement of FMP is 91 microM. Methyl alpha-D-fructofuranoside 1,6-bisphosphate, as well as fructose 6-phosphate and inorganic phosphate, enhances metal-mediated FMP displacement from rabbit liver fructose-1,6-bisphosphatase.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficient purification and molecular properties of spinach chloroplast fructose 1,6-bisphosphatase

A relatively straightforward procedure has been developed for the purification of chloroplast fructose bisphosphatase from spinach leaves to apparent homogeneity and with 80% yield. The molecular weight of the enzyme was about 160 000. Chloroplast fructosebisphosphatase consists of four possibly identical subunits and, at pH 8.8, EASILY DISSOCIATES INTO EQUAL HALVES WITH LOWered activity. Sigmoid saturation curves with Hill coefficients between 3.0 and 3.7 were obtained for fructose 1,6-bisphosphate and Mg2+. Incubation of the enzyme with 20 mM dithiothreitol slowly altered the response to pH from no activity measured at pH 7.5 and full activity at pH 8.8 to equal activity at each of these pH values; at the same time the number of freely available sulphydryl groups increased from four to twelve per molecule. These properties are considered in the context of the observed activation of this enzyme following illumination of chloroplasts.

The allosteric properties of beef-liver fructose bisphosphatase

1. The activity of beef liver fructose bisphosphatase has been shown to respond cooperatively to increasing concentrations of the activating cations Mg2+ and Mn2+. The allosteric inhibitor AMP caused an increase in this cooperativity and a decrease in the apparent affinity of the enzyme for the activating cation. 2. The cooperative response of the enzyme to AMP is similarly increased by increasing cation concentrations with a concomitant decrease in the apparent affinity. 3. Direct binding experiments indicated that in the absence of either Mg2+ or Mn2+ the enzyme bound AMP non-cooperatively up to a maximum of two molecules per molecule of enzyme, a result that is indicative of half-sites reactivity. The binding became increasingly cooperative as the concentration of the activating cation was increased. 4. The substrate fructose bisphosphate had no effect on any of these cooperative responses. 5. These results may be most simply interpreted in terms of concerted model in which the activating cation functions both as an allosteric activator and as an essential cofactor for the reaction.

Changes in activity and molecular properties of fructose 1, 6-bisphosphatase during fasting and refeeding

During prolonged starvation, fructose 1,6bisphosphatase (EC 3.1.3.11) activity in rabbit liver and kidney shows a transient decrease during the first 36 hr, before rising at 96 hr to levels severalfold higher than those found in the livers of fed animals. Proteolytic activity appears in the 105,000 x g supernatant fraction within several hours of starvation, and continues to increase during the entire 96-hr period. On refeeding, the activities return to nearly the control levels within 24 hr. The catalytic properties of fructose 1,6-bisphosphatase isolated from the livers of fasted rabbits are similar to those of the enzyme from fed animals, but its structure is modified, since it no longer contains the single tryptophan residue located near the NH(2)-terminus in the native enzyme. Thus this tryptophan residue is not required for the neutral pH optimum. The structural changes and the transient decrease in activity may be related to the observed increase in "free" proteolytic activity.