3.5 Energy-rich functional groups in substrates of glycolysis
You will have heard that the most abundant and important energy-rich metabolite in the cell is ATP. Within this molecule, the energy is stored in the energy-rich phosphodiester bonds. Cleavage of these bonds is exergonic, and the energy released in the cleavage drives the various reactions and processes powered by ATP. Conversely, in creating ATP from ADP, we require energy to form a new phosphodiester bond. We just saw where this energy is derived from in phosphoenolpyruvate, and we may say that the enolphosphate group is another energy-rich group.
The first ATP in glycolysis is formed by cleavage of the carboxyphosphate mixed anhydride in 1,3-bisphosphoglycerate, and so we may infer that such mixed anhydrides are energy-rich groups as well. Indeed, the same group also occurs in acetylphosphate and in succinylphosphate, and both of these are capable as well to drive the synthesis of a phosphodiester bond in ATP or GTP.
If we look again at Figure 3.4.3-1, we see that the mixed anhydride formed from ionic phosphate and a thioester bond. Since ionic phosphate represents a low-energy form of phosphate, it follows that the energy required for the formation of the mixed anhydride must be derived from the thioester. The energy required for the formation of the latter is derived from the oxidation of carbon 1, which changes from the aldehyde form to the more highly oxidized carboxylate form. In summary, the following functional groups can provide sufficient energy to drive the synthesis of ATP:
- thioesters (when cleaved),
- aldehydes (when oxidized to carboxylic acids),
- carboxyphosphates (when cleaved), and
- enolphosphates (when cleaved).