Characterization of a Human Placental Fructose-6-Phosphate, 2-Kinase/Fructose-2,6-Bisphosphatase.

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A full-length cDNA, which encodes a human placental fructose-6-phosphate, 2-kinase/fructose-2, 6-bisphosphatase, was constructed and expressed in Escherichia coli. The expressed protein, purified to homogeneity, showed a molecular weight of 58, 000 by gel electrophoresis under denaturing conditions, compared to the deduced molecular weight of 59, 410. The N-terminal sequence of 15 amino acids coincided with that of the deduced sequence. The active enzyme was a dimer as judged by molecular sieve filtration. The expressed enzyme was bifunctional with Vmax values of 142 and 0.2milliunits/mg for the kinase and phosphatase activities, respectively. The phosphatase activity was extremely low, because one phosphatase active site residue was mutated, and consequently the kinase/phosphatase ratio was the highest among the known isozymes. Furthermore, the enzyme was phosphorylated by cAMP-dependent protein kinase, protein kinase C and also by [2-32P]fructose-2, 6-bisphosphate. Phosphorylation by cAMP-dependent protein kinase and protein kinase C increased the maximal Fru-6-P, 2-kinase activities by 1.8- and 1.1-fold, respectively. These results suggested that placental fructose-6-phosphate, 2-kinase/ fructose-2, 6-bisphosphatase is important in maintaining and regulating a relatively high rate of glycolysis in placenta.
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