ATP Hydrolysis and Resonance Stabilization

Increase of Entropy

• The equation for ATP hydrolysis is ATP4− + H2O → ADP3− + Pi2− + H+
• Two moles of reactants give three moles of produced particles, increasing entropy
• ΔG = ΔH – TΔS, resulting in a decrease in free energy and favoring product formation

Resonance Stabilization

• The phosphoanhydride bonds in ATP have less resonance stabilization than the hydrolysis products: ADP and especially Pi
• Resonance stabilization is the delocalized sharing of electrons
• Four possible resonance structures for orthophosphate are shown

Energy Release upon Hydrolysis of ATP

• Hydrolysis of ATP is favorable under standard conditions
• An energy barrier must be overcome for hydrolysis of ATP, which occurs in the active site of enzymes
• ΔG0’ = -30.5 kJ/mol

Acyl Phosphates

• High energy release when bond is hydrolysed, ΔG0’ = -49.6 kJ/mol
• Higher energy level than ATP
• Used in substrate-level phosphorylation
• Enzymes, such as kinase, pass phosphate directly to ADP to make ATP
• Helps maintain required amount of ATP in cells
• Example: 1,3-Bisphosphoglycerate regenerates ATP through phosphoglycerate kinase

Enol Phosphates

• High energy release when bond is hydrolysed, ΔG0’ = -180 kJ/mol
• Can be used to make two molecules of ATP

FAD and FADH2

• FADH2 is the reduced form of FAD
• Electron carriers in the cell

ATP and Energy Carriers

• ATP is the energy carrier in the cell
• Acyl phosphates and enol phosphates are used to make ATP by substrate-level phosphorylation
• NADH, NADPH, and FADH2 are electron carriers

Bioenergetics and Thermodynamics

• Cells need energy, precursor metabolites, and reducing power (electrons)
• Catabolism and anabolism are linked
• Gibbs Free Energy (ΔG) tells us how much useful work can be obtained
• Enzymes lower the activation energy of a reaction by providing an energetically favorable pathway for a reaction to happen
• Many metabolic reactions in cells can flow both ways; they are freely reversible
• Key ‘checkpoint’ steps in a pathway are not reversible and will tend to have large ΔG values