Bifidobacterium longum Requires a Fructokinase (Frk; ATP: d -Fructose 6-Phosphotransferase, EC 2.7.1.4) for Fructose Catabolism

ABSTRACT Although the ability of Bifidobacterium spp. to grow on fructose as a unique carbon source has been demonstrated, the enzyme(s) needed to incorporate fructose into a catabolic pathway has hitherto not been defined. This work demonstrates that intracellular fructose is metabolized via the fructose-6-P phosphoketolase pathway and suggests that a fructokinase (Frk; EC 2.7.1.4) is the enzyme that is necessary and sufficient for the assimilation of fructose into this catabolic route in Bifidobacterium longum . The B. longum A10C fructokinase-encoding gene ( frk ) was expressed in Escherichia coli from a pET28 vector with an attached N-terminal histidine tag. The expressed enzyme was purified by affinity chromatography on a Co 2+ -based column, and the pH and temperature optima were determined. A biochemical analysis revealed that Frk displays the same affinity for fructose and ATP ( K m fructose = 0.739 ± 0.18 mM and K m ATP = 0.756 ± 0.08 mM), is highly specific for d -fructose, and is inhibited by an excess of ATP (>12 mM). It was also found that frk is inducible by fructose and is subject to glucose-mediated repression. Consequently, this work presents the first characterization at the molecular and biochemical level of a fructokinase from a gram-positive bacterium that is highly specific for d -fructose.