[25] The adenylate energy charge in the study of enzymes in vitro

Publisher Summary Enzymes are increasingly being regarded not merely as interesting catalysts, but as the functioning elements responsible for the remarkable chemical properties of living cells. It is not enough for enzymes to catalyze reactions; many enzymes must be modulated in response to metabolic needs of the cell. The regulatory and correlative properties of enzymes are as important as their catalytic properties. Many regulatory needs can be met by responses to the concentrations of individual metabolites. The adenine nucleotide system contains much chemical energy because the concentration ratios [ATP]/[ADP] and [ATP]/[AMP] remain very far from their equilibrium values. These nonequilibrium ratios are involved in most metabolic energy transductions. The fluxes through all metabolic sequences are kinetically controlled by the energy status of the adenine nucleotide pool, which can be expressed quantitatively by the energy charge. To measure the responses of enzymes to variation in the value of the energy charge, it is necessary to make up solutions containing a constant total adenylate pool (ATP + ADP + AMP) at the desired values of energy charge.