The production of energy-rich ATP is always coupled with its breakdown to ADP (A). This is necessary since the body contains only a few grams of ATP, ADP, and AMP. In order to cover the daily caloric requirement of ~2000 kcal, 80 kg of free adenine nucleotides are needed. To achieve this, each ADP molecule has to be phosphorylated and dephosporylated several thousand times per day—and all of that in a strictly controlled fashion. These processes are regulated via a large number of interlocking mechanisms. The energy charge of a cell can be considered to function as a control value:
[ ATP] + 1 [ADP] Energy charge = [A TP] + [ADP ] + [AMP]
Its value will range from zero (only AMP) to one (only ATP). A high-energy charge inhibits ATP producing metabolic pathways and activates those using ATP.
Similar to the pH of cells, their energy charge is buffered and usually lies between 0.80 and 0.95. The activity of enzymes involved in energy metabolism may be controlled by a variety of mechanisms. The amount of an enzyme may be regulated via gene expression in the nucleus. For some enzymes, the speed at which they are broken down is regulated. Reversible modification is a particularly important mechanism of enzyme control. The following is an example of reversible modification: When glucose is needed, glycogen phosphorylase is activated by phosphorylation of a particular serine R-group in the enzyme. The activated glycogen phosphorylase then catalyzes the breakdown of glyco-gen, making glucose available.
Although theoretically coupled, ATP-using and ATP-producing processes can be uncoupled at times (B). While the cell uses ATP no ADP is available to the mitochondria. This inhibits ATP synthesis, leading to build-up of NADH. The resulting NADH/NAD+ ratio inhibits the citrate cycle, bringing energy production to a standstill. In reality, however, oxidation and phosphorylation can be uncoupled: electron transport in the respiratory chain continues unimpeded, albeit now producing heat instead of ATP.
Phosphofructokinase (PFK) is subject to allosteric inhibition by ATP (C). The catalytic reaction uses up ATP. Fructose-1,6-biphosphatase (FBPase) catalyzes the reverse reaction without using ATP. When both reactions occur simultaneously, a net "waste" of ATP results. Such "useless" cycles are called futile cycles. Individual differences in the activity of such cycles (fat hydrolysis and reesteri-fication) may account for up to 500 kcal per day. They provide a biochemical basis for differences in the efficiency of energy nutrient use and thereby for differences in body weight of people with identical nutritional habits and environments.
- A. Endergonic and Exergonic Processes
Precursor Final product
P E2 H2O FBPase 2
P E2 H2O FBPase 2
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