There is great variation in the amount of free energy possessed by specific molecules. This is because covalent bonds do not all have the same bond energy. As an example, the covalent bond between oxygen and hydrogen is considerably stronger than the bond between hydrogen and hydrogen, or oxygen and oxygen. The formation of an O—M bond at the expense of O—G or H-—H will thus release energy. Energy considerations, therefore, tell us thai a sufficiently concentrated mixture of oxygen and hydrogen will be transformed into water.
A molecule thus possesses a larger amount of free energy if linked logether bv weak covalent bonds than if it is linked together by strong bonds. This idea seems almost paradoxical at first glance since it means that the stronger the bond, the less energy it can give off. Rut the notion automatically makes sense when we realize that an atom that has formed a very strong bond has already lost a large amount of free energy in this process. Therefore, the best food molecules (molecules that donate energy) are those molecules that contain weak covalent bonds and are therefore thermodynamically unstable.
For example, glucose is an excellent food molecule since there is a great decrease in free energy when it is oxidized by oxygen to yield carbon dioxide and water. On thn other hand, carbon dioxide, composed of strong covalent double bonds between carbon and oxygen, known as carbonyl bonds, is not a food molecule in animals. In the absence of the energy donor ATP, carbon dioxide cannot be transformed spontaneously into more complex organic molecules, even with the help of specific enzymes. Carbon dioxide can be used as a primary source of carbon in planls only because the energy supplied by light quanta during photosynthesis results in the formation of ATP.
The chemical reactions, by which molecules are transformed into other molecules containing less free energy, do not occur at significant rates at physiological temperatures in the absence of a catalyst. This is because even a weak covalent bond is, in reality, very strong and is only rarely broken by thermal motion within a cell. For a covalent bond to be broken in the absence of a catalyst, energy must be supplied to push apart the bonded atoms. When the atoms are partially apart, they can recombine with new partners to form stronger bonds. In the process of recombination! the energy released is the sum of the free energy supplied to break the old bond plus the difference in free energy between the old and the new bond (Figure 4-1).
The energy that must be supplied to break the old covalent bond in a molecular transformation is called the activation energy. The activation energy is usually less than the energy of the original bond because molecular rearrangements generally clo not involve the production of completely free atoms. Instead, a collision between the two reacting molecules is required, followed by the temporary formation of a molecular complex called the activated state. In the activated state, the close proximity of the two molecules makes each other's bonds more labile, so that less energy is needed to break a bond than when the bond is present in a free molecule.
Most reactions of covalent bonds in cells are therefore described by
(A—B) + EC—D)-> (A—D) + (C—R) |Equalion 4-1]
figure 4-1 The energy of activation of a chemical reaction:
(A-- B) + (C—O)-* (A—D) * (C— B) This reaction is accompanied by a decrease in free energy.
activated state activated state
AG of the reaction progress of reaction activation energy
AG of the reaction progress of reaction
Enzymes Lower Activation Energies in Biochemical Reactions 57
The mass action expression for such a reaction is concA"D x concc s concA B X concc" D
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