The Allosteric Properties of Beef‐Liver Fructose Bisphosphatase

Abstract

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 a concerted model in which the activating cation functions both as an allosteric activator and as an essential cofactor for the reaction.