Another issue to address is that of the energy changes associate with making and breaking phosphate bonds. Coupling of ATP hydrolysis to other processes provides the energy to drive them.
Consequently, phosphates such as ATP, below, are described in many texts as "energy rich". Alternatively, we read of the "high-energy" phosphate bond.
- These terms are very misleading, causing many people to believe that energy is released when a phosphate bond is broken. This is nonsense.
- What is being described is that the hydrolysis of ATP (and compounds like it) to ADP or AMP is exothermic:
- The standard free energy of hydrolysis of ATP to ADP and Pi is -7.2 kcal/mol, whereas hydrolysis to AMP and PPi is -7.6 kcal/mol.
- The terminology quoted above implies that DG is negative because ATP "contains" excess energy. It would be just as correct, or incorrect, to argue that AMP and ADP are "low energy".
- Arguments such as those in our text about ATP being "high energy" because of "competitive resonance" are simply hand waving.
- A very substantial part of the energy change associated with hydrolysis of ATP arises from increased solvation of the products as compared to the reactants; it has nothing to do with the supposed "energy content" of the molecule or its bonds.
Bonds and bond energies are a convenient fiction, an arbitrary way of partitioning the energy of a molecule to its component parts.
- As this is usually done, bond making is always exothermic, and bond breaking is always endothermic. No exceptions! Not among biological molecules or anywhere else.
- As useful as the scheme may be, we must always remember it is just a scheme; bond and bond energies have no physical reality. Check out my little essay on model building for more about this.
This page last modified 9:15 AM on Friday December 19th, 2008.
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