Chapter 2 of 5 - Protein Physics & Bioenergetics Course
ATP & Phosphoryl Group Transfer Potential
Ranking phosphate compounds by hydrolysis free energy explains why some steps pay in ATP and why PEP can push flux even when downstream steps are costly.
Why ATP?
ATP is not the highest-energy phosphorylated metabolite. Its standard free energy of hydrolysis to ADP and inorganic phosphate, commonly cited as about ΔG°' ≈ -30.5 kJ/mol (magnitude depends slightly on ionic conditions and how activities are defined), places it in the middle of the biological phosphoryl transfer ladder. That position is exactly what makes it a versatile energy currency: ATP can accept phosphoryl groups from higher-potential donors (for example during glycolytic substrate-level phosphorylation) and donate them to lower-potential acceptors to drive biosynthesis, transport, and signaling.
Adenosine Triphosphate (ATP)
adenosine 5'-triphosphate
ATP is the universal energy currency of cells. Its hydrolysis to ADP and inorganic phosphate releases -30.5 kJ/mol under standard biochemical conditions.
Formula
C10H16N5O13P3
Mol. Weight
507.18 g/mol
Phosphoryl Transfer Potential Scale
The table lists representative ΔG°' values for hydrolysis (to defined products such as alcohols or water, depending on the compound class). More negative values correspond to higher phosphoryl transfer potential - a stronger tendency to transfer the group to a suitable acceptor.
| Compound | ΔG°' of hydrolysis (kJ/mol) |
|---|---|
| Phosphoenolpyruvate (PEP) | -61.9 |
| 1,3-Bisphosphoglycerate | -49.4 |
| Phosphocreatine | -43.1 |
| ATP (to ADP) | -30.5 |
| Glucose-6-phosphate | -13.8 |
| Glycerol-3-phosphate | -9.2 |
Phosphoenolpyruvate (PEP)
2-phosphonooxyprop-2-enoic acid
PEP has the highest phosphoryl group transfer potential of any common biological molecule (-61.9 kJ/mol). The large negative delta G of hydrolysis arises from tautomerization of the enol product to the more stable keto form (pyruvate).
Formula
C3H5O6P
Mol. Weight
167.98 g/mol
Chemical structure of ATP showing the adenine base, ribose sugar, and three phosphate groups connected by phosphoanhydride bonds.
Cacycle, Wikimedia Commons, CC BY-SA 3.0
Why Is ATP Hydrolysis So Favorable?
Even though ATP is mid-scale, its hydrolysis is still strongly exergonic. Four classical contributors are:
- Electrostatic relief - close packing of negative charge on the γ- and β-phosphate units is partially relieved when the anhydride link is cleaved.
- Resonance stabilization of products - inorganic phosphate (Pi) participates in extensive resonance that stabilizes it relative to the phosphoanhydride linkage in ATP.
- Better solvation - ADP and Pi are efficiently hydrated, lowering their chemical potentials compared with the ATP ground state in water.
- Entropy increase - one ATP molecule splitting into ADP and Pi increases translational and rotational freedom (a favorable ΔS contribution to ΔG).
Together, these effects make the net ΔG°' for ATP hydrolysis large and negative, while still leaving room above ATP on the ladder for PEP and other high-potential intermediates.
Quick Check
Which compound has the highest phosphoryl group transfer potential?
Fill in the Blank
ATP is an effective energy currency because its free energy of hydrolysis places it in the ________ of the phosphoryl transfer potential scale, allowing it to both accept and donate phosphoryl groups.
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