ATP Currency
ATP (adenosine triphosphate) has three phosphates linked in series. The bonds between the second and third phosphates, and between the first and second, are phosphoanhydride bonds. They are “high-energy” because:
- Hydrolysis products (ADP + Pi) have lower energy than ATP (resonance stabilization of Pi).
- The negatively charged phosphates in ATP repel each other; breaking them reduces strain.
- Hydrolysis produces protons and is favored at cellular pH.
Standard ΔG°’ for ATP → ADP + Pi is about -7.3 kcal/mol. Under cellular conditions, the actual ΔG is closer to -11 or -12 kcal/mol because cells keep ATP high relative to ADP + Pi.
The Energy Continuum
- ATP → ADP + Pi: ΔG°’ ≈ -7.3 kcal/mol.
- ATP → AMP + PPi: ΔG°’ ≈ -10 kcal/mol (extra push from PPi hydrolysis downstream).
- Phosphoenolpyruvate (PEP) → pyruvate: ΔG°’ ≈ -14.8 kcal/mol. Higher energy than ATP - can transfer phosphate TO ADP.
- Creatine phosphate → creatine: ΔG°’ ≈ -10 kcal/mol. Stores energy for quick ATP regeneration in muscle.
- Glucose-6-phosphate → glucose + Pi: ΔG°’ ≈ -3.3 kcal/mol. Lower energy than ATP.
Why Cells Keep Making ATP
Cells never store ATP in huge amounts. ATP half-life in a working cell is a few seconds. Instead, cells maintain a high ATP/ADP ratio (about 10:1 or more) by constantly regenerating ATP at the rate it is consumed. Any drop in ATP immediately activates catabolic pathways to restore the ratio (respiratory control).
Why is the bond between the last two phosphates of ATP called "high-energy"?
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Several reasons. Hydrolysis products (ADP + Pi) are resonance-stabilized. The negatively charged phosphates in ATP electrostatically repel each other, so breaking the bond relieves strain. Hydrolysis also releases a proton, which is favored at physiologic pH. The net ΔG°' is about -7.3 kcal/mol - enough to drive most coupled unfavorable reactions.
Why is PEP able to phosphorylate ADP to make ATP?
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PEP has a higher phosphate transfer potential (more negative ΔG°' of hydrolysis, ~-14.8 kcal/mol) than ATP (~-7.3 kcal/mol). In pyruvate kinase (step 10 of glycolysis), PEP transfers its phosphate to ADP. The reaction is strongly exergonic because the drop from PEP's energy to ATP's energy is favorable. This is substrate-level phosphorylation.
How do muscles rapidly regenerate ATP during intense exercise?
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Creatine phosphate. Creatine kinase transfers the phosphate from creatine-P directly to ADP, regenerating ATP without going through glycolysis or oxidative phosphorylation. This provides a burst of ATP lasting 5-15 seconds, perfect for the start of a sprint. When creatine-P is depleted, glycolysis and eventually oxidative phosphorylation take over.