Question

How does ATP breakage provide energy to drive nonspontaneous reactions?

How would I use an annotated diagrams to explain why ATP hydrolysis is thermodynamically favourable, and then what is one example of a biological reaction or process where ATP breakage is used to “power” it?

Answer 1

Adenosine triphosphate (ATP) is composed of the nitrogenous base adenine, the sugar ribose, and three phosphate groups. Energy is stored in the phosphoanhydride bonds between the three phosphate groups in ATP. This energy is released by ATP hydrolysis, the chemical reaction in which ATP reacts with water to yield adenosine diphosphate (ADP) and an inorganic phosphate ion, denoted HOPO3 2- or Pi. For every mole of ATP hydrolyzed, 7.3 kilocalories of energy are released:

ATP + H2O → ADP + Pi

ΔG = -7.3 kcal/mol

The standard Gibbs free energy (ΔG) is the portion of free energy that can be used by a system to perform work when temperature and pressure are uniform. A water molecule hydrolyzes ATP into ADP and inorganic phosphate (Pi or HOPO3 2-) by breaking the bond between the two terminal phosphate groups in ATP. The reaction also releases energy and a proton. Energy is released because the bond energy in the phosphoanhydride bond is greater than the energy in the bonds of the products.

Because of the accompanying decrease in free energy, the hydrolysis of ATP can be used to drive other energy-requiring reactions within the cell. For example, the first reaction in glycolysis (discussed in the next section) is the conversion of glucose to glucose-6-phosphate. The reaction can be written as follows: Because this reaction is energetically unfavorable as written (ΔG°′= +3.3 kcal/mol), it must be driven in the forward direction by being coupled to ATP hydrolysis (ΔG°′= -7.3 kcal/mol):

Glucose Phosphate (HPO) Glucose-6-phosphate+ H20

ATP + H2O → ADP + HP042-
The combined reaction can be written as follows: The free-energy change for this reaction is the sum of the free-energy changes for the individual reactions, so for the coupled reaction ΔG°′= -4.0 kcal/mol, favoring glucose-6-phosphate formation.