Enzymes are absolutely necessary for life. The function of enzymes is to speed up the rate of the chemical reactions requisite to cellular existence by lowering I he activation energies of molecular rearrangements lo values that can be supplied by the heat of motion (Figure 4-2). When a specific enzyme is present, there is no longer an effective barrier preventing the rapid formation of the reactants possessing the lowest amounts of free energy. Enzymes never affect the nature of an equilibrium: They merely speed up the rate at which it is reached. Thus, if the thermodynamic equilibrium is unfavorable for the formation of a molecule, the presence of an enzyme can in no way bring about the molecule's accumulation.
Because enzymes must catalyze essentially every cellular molecular rearrangement, knowing the free energy of various molecules cannot by itself tell us whether an energetically feasible rearrangement will, in fact, occur. The rate of the reactions must always be considered. Only if a cell possesses a suitable enzyme will the reaction be important.
activation "1 energy of
J catalyzed reaction prngress of reaction activation "1 energy of
J catalyzed reaction activa bon energy of uncatalyzed reaction
FIGURE 4-2 Enzymes (color curve) lower activation energies and thus speed up the rate of the reaction. Mole that AG remains the same because the equilibrium position remains unaltered prngress of reaction
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