Bioenergetics III: High Energy Compounds
High energy compounds
- The phosphate compounds found in living organisms can be arbitrarily divided into two groups based on their standard free energy changes of hydrolysis.
- ‘High-energy’ compounds have a ΔG‘0 of hydrolysis more negative than –25 kj/mol; ‘low-energy’ compounds have a less negative ΔG‘0.
- Based on this criterion, ATP, with a ΔG’o of hydrolysis of -30 kj/mol is a high-energy compound; glucose 6-phosphate is a low-energy compound (ΔG‘0 = -13,8 kj/mol)
- The term ‘high-energy phosphate bond’ was used by biochemists to describe P~O bond broken in hydrolysis reactions for a long time. But it is incorrect and misleading as it wrongly suggests that the bond itself contains the energy.
- In fact, the breaking of all chemical bonds requires an input of energy. The free energy released by hydrolysis of phosphate compounds does not come from the specific bond that is broken; it results from the products of the reaction having a lower free energy content than the reactant
