Correct answer = B. A Coenzymes–cosubstrates are small organic molecules that associate transiently with an enzyme and leave the enzyme in a changed form. Coenzyme–prosthetic groups are small organic molecules that associate permanently with an enzyme and are returned to their original form on the enzyme. Cofactors are metal ions. Heterotropic effectors are not substrates.
For Questions 5.3 and 5.4, use the graph below that shows the changes in free energy when a reactant is converted to a product in the presence and absence of an enzyme. Select the letter that best represents: .3. the activation energy of the catalyzed forward reaction. .4. the free energy of the reaction.
Correct answers = B; D. Enzymes (protein catalysts) provide an alternate reaction pathway with a lower activation energy. However, they do not change the free energy of the reactant or product. A is the activation energy of the uncatalyzed reaction. C is the activation energy of the catalyzed reverse reaction.
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I. OVERVIEW
Bioenergetics describes the transfer and utilization of energy in biologic systems. It concerns the initial and final energy states of the reaction components, not the reaction mechanism or how much time it takes for the chemical change to occur. Bioenergetics makes use of a few basic ideas from the field of thermodynamics, particularly the concept of free energy. Because changes in free energy provide a measure of the energetic feasibility of a chemical reaction, they allow prediction of whether a reaction or process can take place. In short, bioenergetics predicts if a process is possible, whereas kinetics measures the reaction rate (see p. 54).
II. FREE ENERGY
The direction and extent to which a chemical reaction proceeds are determined by the degree to which two factors change during the reaction. These are enthalpy (∆H, a measure of the change [∆] in heat content of the reactants and products) and entropy (∆S, a measure of the change in randomness or disorder of the reactants and products), as shown in Figure 6.1. Neither of these thermodynamic quantities by itself is sufficient to determine whether a chemical reaction will proceed spontaneously in the direction it is written. However, when combined mathematically (see Fig. 6.1), enthalpy and entropy can be used to define a third quantity, free energy (G), which predicts the direction in which a reaction will spontaneously proceed.
III. FREE ENERGY CHANGE
The change in free energy is represented in two ways, ∆G and ∆G0. The first, ∆G (without the superscript “0”), represents the change in free energy and, thus, the direction of a reaction at any specified concentration of products and reactants. ∆G, then, is a variable. This contrasts with the standard free energy change, ∆G0 (with the superscript “0”), which is the energy change when reactants and products are at a concentration of 1 mol/l. [Note: The concentration of protons (H+) is assumed to be 10−7 mol/l (that is, pH = 7). This may be shown by a prime sign (ʹ ), for example, ∆G0ʹ.] Although ∆G0, a constant, represents energy changes at these nonphysiologic concentrations of reactants and products, it is nonetheless useful in comparing the energy changes of different reactions. Furthermore, ∆G0 can readily be determined from measurement of the equilibrium constant (see p. 71). [Note: This section outlines the uses of ∆G, and ∆G0 is described in D. below.]
A. ∆G and reaction direction
The sign of ∆G can be used to predict the direction of a reaction at constant temperature and pressure. Consider the reaction: 1. Negative ∆G: If ∆G is negative, then there is a net loss of energy, and the reaction goes spontaneously as written (that is, A is converted into B) as shown in Figure 6.2A. The reaction is said to be exergonic.
2.