Binding and kinetic data for rabbit liver fructose-1,6-bisphosphatase with Zn2+ as cofactor

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Atomic absorption determinations of zinc content were employed to demonstrate the technique to obtain zinc-free rabbit liver fructose-1,6-bisphosphatase (D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11). Reactivation of the apoenzyme by Zn2+ is rapid (within 1 min) and restores up to 96% of the initial specific activity. Gel filtration measurements showed that the enzyme contains four binding sites for zinc per molecule, one per subunit. The dissociation constants for the initial two binding sites are less than 0.1 μM. In the presence of a substrate analog, (α + β) methyl D-fructofuranoside 1,6-bisphosphate, at a level where two analog molecules are bound per phosphatase molecule, a total of eight Zn2+ ions bind at 8 μM Zn2+, revealing the presence of additional binding sites, including the catalytic one. The activity in the presence of Zn2+ is maximal at ca. 8 μM Zn2+, which corresponds to saturation of the four subunit sites plus the catalytic sites in the presence of substrate. At metal ion concentrations less than 10 μM, the order of activation is Zn2+ > Mn2+ > Mg2+. In kinetic assays with two metal cofactors the effect of Zn2+ at concentrations less than 10 μM on either the Mg2+ or the Mn2+ assays is inhibitory owing to the apparent formation of mixed (two different elements) metal ion-enzyme complexes possessing a catalytic activity that is measureable but lower than anticipated if the catalysis by the various metal ions is simply additive. Hence the activation by EDTA of the Mg2+ and Mn2+ assays is explicable in terms of Zn2+ removal, thus eliminating mixed metal species. Collectively these observations suggest that fructose-1,6-bisphosphatase may function in vivo as a Zn2+ metalloprotein.

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