Anomeric specificity of enzymes of D-glucose metabolism.
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We report that glycolysis in human quiescent fibroblasts stimulated by serum addition is increased, and that the changes of the metabolic route reflect the activity of the phosphofructokinase. A possible role of fructose-2,6-bisphosphate as a positive modulator of the key enzyme is proposed.
Rat liver fructose 1,6-bisphosphatase can be protected against partial inactivation by N-ethylmaleimide by low concentrations of fructose 2,6-bisphosphate or high concentrations of fructose 1,6-bisphosphate. The partially inactivated enzyme has a much reduced sensitivity to high substrate inhibition and has lost the sigmoid component of the inhibition by fructose 2,6-bisphosphate; this compound is a simple linear competitive inhibitor of the modified enzyme. The results suggest that fructose 2,6-bisphosphate can bind to the enzyme at two distinct sites, the catalytic site and an allosteric site. High levels of fructose 1,6-bisphosphate probably inhibit by binding to the allosteric site.
Fructose 2,6-bisphosphate has been claimed to be both a substrate analogue and an allosteric inhibitor of fructose-1,6-bisphosphatase. The results reported here show that fructose 2,6-bisphosphate can be both an inhibitor and an activator of the enzyme, depending on the substrate concentration. This biphasic behaviour at saturating concentrations of substrate can only be due to an allosteric effect. In addition to the mechanistic implication it is possible that this finding may have physiological meaning.
Rat liver fructose-1,6-bisphosphatase was partially phosphorylated in vitro and separated into unphosphorylated and fully phosphorylated enzyme. The effects of fructose 2,6-bisphosphate and AMP on these two enzyme forms were examined. Unphosphorylated fructose-1,6-bisphosphatase was more easily inhibited by both effectors. Fructose 2,6-bisphosphate affected both K0.5 and Vmax, while the main effect of AMP was to lower Vmax. Fructose 2,6-bisphosphate and AMP together acted synergistically to decrease the activity of fructose-1,6-bisphosphatase, and since unphosphorylated and phosphorylated enzyme forms are affected differently, this might be a way to amplify the effect of phosphorylation.
Fructose 2,6-bisphosphate is present in the rat mammary gland, rising from a value of 1.4 nmol/g in pregnancy to 4.3 nmol/g tissue at 14 days lactation; the equivalent values calculated/ml intracellular water are 5.2 and 11.6 nmol, respectively. The tissue content of fructose 6-phosphate, fructose 1,6-bisphosphate, ATP and phosphoenolpyruvate remain relatively constant in the transition from pregnancy to the height of lactation. The changes in AMP, cyclic AMP, and citrate content of the mammary gland during lactation are such as to promote an increase in fructose 2,6-bisphosphate formation and flux through phosphofructokinase.
We report that fructose 2,6-bisphosphate (Fru-2,6-P2) levels in quiescent human fibroblasts stimulated with insulin treatment increase in a time- and dose-dependent manner. The biochemical role of the sugar phosphate in the cellular response to the hormone which controls carbohydrate metabolism and acts as mitogenic factor is discussed.