ATP (adenosine 5′-triphosphate) is the main energy currency in living cells. It undergoes a type of reaction called hydrolysis where one or two of the terminal phosphate groups are released.
These reactions are accompanied by a considerable release of energy and that's why ATP is such an important molecule. It is synthesized by a special reaction that is not the reverse of the hydrolysis reaction. Instead it utilizes the energy of a proton gradient across a membrane to make ATP [How Cells Make ATP: ATP Synthase]. Since ATP is very stable inside the cells it can serve as an energy storage molecule until it is ready to be used.
One of the important features of enzymes is their ability to couple reactions that would otherwise not occur. One of the ways that enzymes do this is by bringing together two different substrates to form a reactive intermediate. There are dozens of molecules that can be used in a wide variety of different reactions and these are referred to as coenzymes or cofactors. ATP is one of them.1
Here's an example of how ATP can be used to make a reaction proceed when it would otherwise not take place because it requires too much energy. The formation of glutamine from glutamate requires the attachment of an ammonia group to glutamate. This reaction will not take place inside the cell because the direct energy requirement is too high.
Instead, the enzyme glutamine synthetase utilizes the energy of ATP to make the reaction go in two steps.
In the first step, ATP is hydrolyzed to ADP and the phosphate group is attached to glutamate to make a "high energy" intermediate called γ-glutamyl phosphate. The enzyme does not release this product; it holds on to it until a molecule of ammonia enters the active site to displace the phosphate group and create glutamine. In this way the overall reaction can proceed because each of the intermediate steps is favorable. (Hydrolysis of ATP in step one and hydrolysis of γ-glutamyl phosphate in step two.)
The enzyme has coupled the overall hydrolysis of ATP to ADP + Pi to the formation of glutamine from glutamate. ADP will be used to synthesize another molecule of ATP so that the store of energy currency remains constant inside the cell.
1. ATP was first discovered as an essential factor in fermentation and muscle contraction. Hans von Euler-Chelpin received the Nobel Prize in 1929 for recognizing the importance of adenosine phosphate "cozymases."