ATP Hydrolysis and Resonance Stabilization

Study Notes

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

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

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

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

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

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