Thermodynamics tells us that all biochemical pathways must be characterized by a decrease in free energy, This is clearly the case for degradative pathways, in which thermodynamically unstable food molecules are converted to more stable compounds, such as carbon dioxide and water, with the evolution of heat. All degradative pathways have two primary purposes: (1) to produce the small organic fragments necessary as building blocks for larger organic molecules and (2) to conserve a significant fraction of the free energy of the original food molecule in a form that can do work. This latter purpose is accomplished by coupling some of the steps in degradative pathways with the simultaneous formation of high-energy molecules such as ATP, which can store free energy.
Not all the free energy of a food molecule is converted into the free energy of high-energy molecules. If this were the case, a degradative pathway would not be characterized by a decrease in free energy, and there would be no driving force to favor the breakdown of food molecules. Instead, we find that all degradative pathways are characterized by a conversion of at least one-half the free energy of the food molecule into heat or entropy, for example, it is estimated that in cells, approximately 40% of the free energy of glucose is used to make new high-energy compounds, the remainder being dissipated into heat energy and entropy.
A high-energy molecule contains one or more bonds whose breakdown by water, called hydrolysis, is accompanied by a large decrease in free energy (5 kcal/mol). The specific bonds whose hydrolysis yields these large negative AG values are called high-energy bonds, a somewhat misleading term, since it is not the bond energy but the free energy of hydrolysis that is high. Nonetheless, the term high-energy bond is generally employed, and tor convenience, we shall continue this usage by marking high-energy bonds with the symbol
The energy of hydrolysis of the average high-energy bond (7 kcal/mol) is very much smaller than the amount of energy that would be released if a glucose molecule were to be completely degraded in one step (688 kcal/mol). A one-step breakdow n of glucose would be inefficient in making high-energy bonds. This is undoubtedly the reason why biological glucose degradation requires so many steps, in this way, the amount of energy released per degradative step is of the same order of magnitude as thfi free energy of hydrolysis of a high-energy bond.
The most important high-energy compound is ATP. It is formed from inorganic phosphate Q and ADP, using energy obtained either from degradative reactions or from the sun, a process known as photosynthesis. There are, however, many other important high-energy compounds. Some are directly formed during degradative reactions* others are formed using some of the free energy of ATP. Table 4-2 lists the most important types of high-energy bonds. All involve either phosphate or sulfur atoms. The high-energy pyrophosphate bonds of ATP arise from the union of phosphate groups. The pyrophosphate linkage (©-©) is not, however, the only kind of high-energy phosphate bond: The attachment of a phosphate group to the oxygen atom of a carboxyl group creates a high-energy acyl bond. It is now clear that high-energy bonds involving sulfur atoms play almost as important a role in energy
Free Energy in Biomolecuhs
TABLE 4-2 important Classes of High-Energy Bonds
AG of Reaction, kcal/mol
Nucleoside diphosphates adenosine —© — © <ADP)
Was this article helpful?