ATP is a highly unstable molecule. Unless quickly used to perform work, ATP spontaneously dissociates into ADP and inorganic phosphate (Pi), and the free energy released during this process is lost as heat. The energy released by ATP hydrolysis is used to perform work inside the cell and depends on a strategy called energy coupling. Cells couple the exergonic reaction of ATP hydrolysis with endergonic reactions, allowing them to proceed.
One example of energy coupling using ATP involves a transmembrane ion pump that is extremely important for cellular function. This sodium-potassium pump (Na+/K+ pump) drives sodium out of the cell and potassium into the cell. A large percentage of a cell's ATP is spent powering this pump because cellular processes regularly import great amounts of sodium into the cell and export great amounts of potassium out of the cell. The pump constantly works to stabilize cellular concentrations of sodium and potassium. In order for the pump to turn one cycle (exporting three Na+ ions and importing two K+ ions), one molecule of ATP must be hydrolyzed. When ATP is hydrolyzed, its gamma phosphate is transferred onto the pump protein.
This process of a phosphate group binding to a molecule is termed phosphorylation. As with most cases of ATP hydrolysis, a phosphate from ATP is transferred onto another molecule. In a phosphorylated state, the Na+/K+ pump has more free energy and is triggered to undergo a conformational change. This change allows it to release Na+ to the outside of the cell. It then binds extracellular K+, which, through another conformational change, causes the phosphate to detach from the pump. This release of phosphate triggers the K+ to be released to the inside of the cell. Essentially, the energy released from the hydrolysis of ATP is coupled with the energy required to power the pump and transport Na+ and K+ ions. ATP performs cellular work using this basic form of energy coupling through phosphorylation.
Often during cellular metabolic reactions, such as nutrient synthesis and breakdown, certain molecules must alter slightly in their conformation to become substrates for the next step in the reaction series. One example is during glycolysis, the very first steps of cellular respiration. In this first step, ATP is required to phosphorylate glucose, creating a high-energy but unstable intermediate. This phosphorylation reaction powers a conformational change that allows the phosphorylated glucose molecule to convert to the phosphorylated sugar fructose. Fructose is a necessary intermediate for glycolysis to move forward. Here, ATP hydrolysis' exergonic reaction, couples with the endergonic reaction of glucose phosphorylation constitutes an intermediate step in the pathway. Once again, the energy released by breaking a phosphate bond within ATP was used for phosphorylating another molecule, creating an unstable intermediate and powering an important conformational change.
This text is adapted from Openstax, Biology 2e, Section 6.4:ATP: Adenosine Triphosphate
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