In 1941–1942 Fritz Lipmann, Hermann M. Kalckar and Feodor Lynen described a new principle of regulation of the energy metabolism by the adenylic acid system, which Fritz Lipmann called the “phosphate pump”. The energy consuming “endergonic” reactions are connected with a cleavage of ATP to yield ADP and orthophosphate. The ATP-producing “exergonic” reactions depend on stoichiometric amounts of orthophosphate and ADP. Thus ATP is produced in such a proportion as it was consumed before. It is a “tight” control of metabolism by stoichiometric participation of the controlling metabolites in the controlled enzyme reaction. A second possibility for regulation of metabolism by the adenylic acid system consists in the allosteric effect of partners of the adenylic acid system on enzyme activity. The allosteric inhibition of phosphofructokinase by ATP is a well studied example of such a “loose” control mechanism. Phosphofructokinase is a key enzyme in the ATP-producing carbohydrate degradation. Therefore, this mechanism as well as the “phosphate pump” is useful for regulation of the energy metabolism. A third possibility for the regulation of enzymes by the adenylic acid system is the metabolite-controlled, enzyme catalyzed, ATP-dependent chemical modification of enzymes. Hitherto 4 enzymes regulated in this way have been found and elucidated: glycogen phosphorylase, glycogen synthetase and pyruvate dehydrogenase from mammalian tissues, and glutamine synthetase from E. coli. In all 4 cases ATP participates stoichiometrically in the chemical modification of the enzymes. The modification consists in an ATP-dependent phosphorylation or adenylylation and in the back reactions, i.e. dephosphorylation or deadenylylation, respectively. Moreover different partners of the adenylic acid system control the activity of the modifying enzymes in an allosteric manner and thus contribute to the regulation. Evidence for an enzyme-catalyzed chemical modification of phosphofructokinase from yeast by partners of the adenylic acid system will be presented and discussed.