SUMMARY

• 6.1 Enzymes Are Powerful and Highly Specific Catalysts

  The catalysts in biological systems are enzymes, and nearly all enzymes are proteins. Enzymes are highly specific and have great catalytic power. They can enhance reaction rates by factors of 10⁶ or more.

• 6.2 Many Enzymes Require Cofactors for Activity

  Cofactors required by enzymes for activity can be small, vitamin-derived organic molecules called coenzymes, or they can be metals.

• 6.3 Gibbs Free Energy Is a Useful Thermodynamic Function for Understanding Enzymes

  Free energy (G) is the most valuable thermodynamic function for determining whether a reaction can take place and for understanding the energetics of catalysis. A reaction can take place spontaneously only if the change in free energy (ΔG) is negative. The free-energy change of a reaction that takes place when reactants and products are at unit activity is called the standard free-energy change (ΔG°). Biochemists usually use ΔG°′, the standard free-energy change at pH 7. Enzymes do not alter reaction equilibria; rather, they increase reaction rates.

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• 6.4 Enzymes Facilitate the Formation of the Transition State

  Enzymes serve as catalysts by decreasing the free energy of activation of chemical reactions. Enzymes accelerate reactions by providing a reaction pathway in which the transition state (the highest-energy species) has a lower free energy and, hence, is more rapidly formed than in the uncatalyzed reaction.

  The first step in catalysis is the formation of an enzyme–substrate complex. Substrates are bound to enzymes at active-site clefts from which water is largely excluded when the substrate has been bound. The recognition of substrates by enzymes is often accompanied by conformational changes at active sites, and such changes facilitate the formation of the transition state.