Key Enzymes in Metabolism of D-Fructose

Fructokinase is found in liver,56,312-315 kidney,32 intestinal mucosa,33,316 adipose tissue,71,72 and lenses,317 but not in heart muscle, skeletal muscle, brain, and seminal vesicles.57 

Requirement of K+ by fructokinase has been demonstrated.312,319 The K+ ion acts as an allosteric activator, and it cooperatively increases the reaction velocity.320 The Mg2+ ion is also important, as the activity of fructokinase depends on the ratios of ATP4- MgATP2- and of KATP3- MgATP2-. The optimal ratios vary, depending upon the enzyme saturation and the K+ concentration.320 

This fructokinase catalyzes phosphorylation to afford D-fructose 6-phosphate this enzyme activity was detected in the cytoplasm of Streptococcus faecalis,321 Echinococcus granulosus,322 and in pea.323 

Accumulated reaction-products do not inhibit the enzyme.323 For the enzyme from E. granulosus,322 the Km value for D-fructose is 620 fiM, but is less than 1 mM for the enzyme from pea.323 The enzyme purified from Leuconostoc mesenteroides324,325 can phosphorylate either D-fructose or D-mannose at the same rate (Km 0.4 mM). However, the pH-activity curves are different for the two hexoses, with the ratio of activity of D-fructokinase to that of D-mannokinase being about 1 l at pH 6.9, 0.5 1 at pH 8.5, and 0.3 1 at pH 8.9. 

D-Fructose 1-phosphate kinase catalyzes the phosphorylation of D-fructose 1-phosphate to D-fructose 1,6-bisphosphate. The enzyme has been purified from Aerobacter aerogenes326 and Bacteroides symbiosis.327 

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