MadSci Network: Biochemistry |
The form in which energy is released when ATP is broken down depends on the particular reaction involved. For instance, the hydrolysis of ATP in solution to give ADP and phosphate only results in the formation of heat. However, most reactions involving the breakdown of ATP in living organisms couple the energy released from the hydrolysis of ATP to another chemical reaction, in this way the energy is trapped as chemical potential energy. Typically, this involves the coversion of a "low energy" substrate to a higher enery substrate (by converting it to a phosphorylated compound) such that the overall reaction becomes energetically favorable (i.e. it releases energy (exothermic) instead of requiring enery (endothermic)). The hydrolysis of ATP is also coupled to the active transport of ions and molecules across membranes, and the performance of mechanical work (in muscle). The importance of the phosphorous atom is to provide the core of the phosphate group that is the basis of the phosphoanhydride bond that has a relatively high stndard free energy of hydrolysis. This provides a useful biochemical "currency" since the hydrolysis reaction is chemically very simple (only requiring water), but containing plenty of energy. In this respect it is very different to carbon-carbon bonds, which may contain plenty of energy, but are chemically very hard to make and break. Oxygen is a special atom in energy production as it is a powerful oxidant and as it accepts electrons to form water, the energy released bu its reduction can be coupled to the formation of many molecules of ATP. Although the phosphorous atom in ATP cannot be replaced by other elements, high energy bonds employing other elements are used in biochemistry. For instance, the hydrolysis of acetyl coenzyme A to acetate and free CoA releases a similar amount of energy as the hydrolysis of ATP. In this case the high energy bond os a carbon-sulfur bond.
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