The first step in glycolysis is phosphorylation of Glc by a family of enzymes called HKs to form G6P. In the liver, an isozyme of hexokinase called GCK is used, which differs primarily in regulatory properties. This reaction consumes 1 ATP, but the energy is well-spent - it keeps [Glc]i low as to allow continuous entry of Glc through its plasma membrane transporters; prevents Glc leakage out - the cell lacks such transporters for G6P; activates Glc preparing it for the next metabolic changes.
G6P is then rearranged into F6P by GPI. Fru can also enter the glycolytic pathway via phosphorylation at this point.
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