Control of Energy Release from Stored Glycogen When the Body Needs Additional Energy: Effect of ATP and ADP Cell Concentrations in Controlling the Rate of Glycolysis
Continual release of energy from glucose when energy is not needed by the cells would be an extremely waste-ful process. Instead, glycolysis and the subsequent oxi-dation of hydrogen atoms are continually controlled in accordance with the cells’ need for ATP. This control is accomplished by multiple feedback control mechanisms within the chemical schemata. Among the more impor-tant of these are the effects of cell concentrations of both ADP and ATP in controlling the rates of chemical reactions in the energy metabolism sequence.
One important way in which ATP helps control energy metabolism is to inhibit the enzyme phospho-fructokinase. Because this enzyme promotes the forma-tion of fructose-1,6-diphosphate, one of the initial steps in the glycolytic series of reactions, the net effect of excess cellular ATP is to slow or even stop glycolysis, which in turn stops most carbohydrate metabolism. Conversely, ADP (and AMP as well) causes the oppo-site change in this enzyme, greatly increasing its activ-ity. Whenever ATP is used by the tissues for energizing a major fraction of almost all intracellular chemical reactions, this reduces the ATP inhibition of the enzyme phosphofructokinase and at the same time increases its activity as a result of the excess ADP formed. Thus, the glycolytic process is set in motion, and the total cellular store of ATP is replenished.
Another control linkage is the citrate ion formed in the citric acid cycle. An excess of this ion also stronglyinhibits phosphofructokinase, thus preventing theglycolytic process from getting ahead of the citric acid cycle’s ability to use the pyruvic acid formed during glycolysis.
A third way by which the ATP-ADP-AMP system controls carbohydrate metabolism, as well as control-ling energy release from fats and proteins, is the fol-lowing: Referring back to the various chemical reactions for energy release, we see that if all the ADP in the cell has already been converted into ATP, additional ATP simply cannot be formed. As a result, the entire sequence involved in the use of foodstuffs—glucose, fats, and proteins—to form ATP is stopped. Then, when ATP is used by the cell to energize the different phy-siologic functions in the cell, the newly formed ADP and AMP turn on the energy processes again, and ADP and AMP are almost instantly returned to the ATP state. In this way, essentially a full store of ATP is automatically maintained, except during extreme cellular activity, such as very strenuous exercise.
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