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High-Energy Phosphates

Terms such as the "high energy phosphate bond," and "high energy phosphates" have, for decades, led to confusion for students and teachers trying to reconcile the principles of bond breaking and making in chemistry with the flow of energy in the metabolism of the cell. These terms are not likely to vanish from general biology and biochemistry texts and will be continued to be taught in the classroom. However, an improved harmonization of biology and chemistry can be achieved, with clear descriptions, in terms of physical chemistry, of the role these molecules play.

The "high energy phosphates" as well as other molecules that are capable of providing or storing energy, share common chemical characteristics. These fuels and energy messengers contain relatively weak bonds. These molecules that therefore undergo reactions in which the energy input involved in breaking these weak bonds is more than compensated for decreases in energy accompanying the formation of stronger bonds in the products.

The molecules are thermodynamically unstable, or potentially reactive, but kinetically stable under physiological conditions. While ATP, for example, does spontaneously hydrolyze in aqueous solution at 37șC, in the absence of a specific enzyme (ATPase) that reaction is very slow. A dandy reference on the kinetic versus the thermodynamic stability of phosphates by Prof. F.H. Westheimer (Why Nature Chose Phosphates) appears in the References.

While entropy changes play a role in the determining the spontaneity of reactions in general, the negative free energy changes associated with combustion reactions, and to a significant extent, hydrolysis reactions, are dominated by their exothermic nature arising from the replacement of weaker bonds in the reactants with stronger bonds in the products.

The decomposition of decomposition of nitroglycerin represents an extreme example of an exothermic reaction. The = 1425 kJ/mol.

This is more than twice the decrease in enthalpy associated with the oxidation of a mole of glucose. In both cases an equal number of bonds are broken and made. Where does the large net decrease in energy come from? Adding the energies (enthalpies) of the bonds broken in the reactant with those formed in the products results in:

= 5652 7077 = 1425 kJ/mol

i.e. nitroglycercin could be classified as a "high energy molecule" which liberates a large amount of energy when its bonds are broken. The weak or "squiggle bonds" in this case would be either the CO, or ONO2, bonds. However, it is clear that it is the replacement of weak bonds in the nitroglycerin with much stronger bonds in the gaseous products that is the source of the highly exothermic nature of this reaction.



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