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the enzymatic transfer of a phosphoric acid residue (phosphategroup, —PO32–) from one compound to another in living cells. Transphosphorylation links the most important reactions of metabolism in the cell, carrying out an exchange of energy between various processes by forming and breaking energy-rich phosphate bonds. In the majority of transphosphorylation reactions, the phosphate is transferred to the hydroxyl group of an alcohol or carbohydrate, with the formation of an energy-poor bond. A molecule of adenosine triphosphate (ATP) usually serves as the donor of the phosphategroup. The transphosphorylation reaction is catalyzed by phosphotransferase enzymes, which usually require the presence of Mg2+ to become active.