Chapter Summary

• Energy is the capacity to do work. In a biological system, the usable energy is called free energy (G). The unusable energy is entropy (S), a measure of the disorder in the system.

• Potential energy is the energy of state or position; it includes the energy stored in chemical bonds. Kinetic energy is the energy of motion; it is the type of energy that can do work.

• The laws of thermodynamics apply to living organisms. The first law states that energy cannot be created or destroyed. The second law states that energy transformations decrease the amount of energy available to do work (free energy) and increase disorder. Review Figure 8.2

• The change in free energy (ΔG) of a reaction determines its chemical equilibrium, the point at which the forward and reverse reactions proceed at the same rate.

• An exergonic reaction releases free energy and has a negative ΔG. An endergonic reaction consumes or requires free energy and has a positive ΔG. Endergonic reactions proceed only if free energy is provided. Review Figure 8.3

• Metabolism is the sum of all the biochemical (metabolic) reactions occurring in an organism at a given time. Catabolic reactions are associated with the breakdown of complex molecules and release energy (are exergonic). Anabolic reactions build complexity in the cell and are endergonic.

• Adenosine triphosphate (ATP) serves as an energy currency in cells. Hydrolysis of ATP releases a relatively large amount of free energy.

• The ATP cycle couples exergonic and endergonic reactions, harvesting free energy from exergonic reactions, and providing free energy for endergonic reactions. Review Figure 8.6, Activity 8.1

• The rate of a chemical reaction is independent of ΔG but is determined by the energy barrier. Review Focus: Key Figure 8.8

• Enzymes are protein catalysts that affect the rates of biological reactions by lowering the energy barrier, supplying the activation energy (E_a) needed to initiate reactions. Review Figure 8.10, Activity 8.2

• A substrate binds to the enzyme’s active site—the site of catalysis—forming an enzyme–substrate complex (ES). Enzymes are highly specific for their substrates. Review Figure 8.9

• Enzymes can be classified as to the type of chemical reaction catalyzed.

• At the active site, a substrate can be oriented correctly, chemically modified, or strained. Any of these factors can induce the substrate to readily reach its transition state, allowing the reaction to proceed. Review Figure 8.11

• Binding substrate causes many enzymes to change shape, exposing their active site(s) and allowing catalysis. The change in enzyme shape caused by substrate binding is known as induced fit. Review Figure 8.12

• Some enzymes require other substances, known as cofactors, to carry out catalysis. Prosthetic groups are permanently bound to enzymes; coenzymes are not. A coenzyme can be considered a substrate, as it is changed by the reaction and then released from the enzyme.

• Substrate concentration affects the rate of an enzyme-catalyzed reaction. Review Figure 8.13

• Metabolism is organized into pathways in which the product of one reaction is a reactant for the next reaction. Each reaction in the pathway is catalyzed by a different enzyme. Review Activity 8.3

• Enzyme activity is subject to regulation. Some inhibitors bind irreversibly to enzymes. Others bind reversibly. Review Figures 8.15, 8.16, Animation 8.1

• An allosteric effector binds to a site other than the active site and stabilizes the active or inactive form of an enzyme. Review Figure 8.17, Animation 8.2

• The end product of a metabolic pathway may inhibit an enzyme that catalyzes the commitment step of that pathway. Review Figure 8.18

• Reversible phosphorylation is an important mechanism for regulating enzyme activity.

• Enzymes are sensitive to their environments. Both pH and temperature affect enzyme activity. Review Figures 8.19, 8.20