CHAPTER 20

1. CHAPTER 20 ENZYMES AND VITAMINS

2. A. Enzymes • Are biological catalysts • Catalyze nearly all of the chemical reactions that take place in the body • Enzymes increase the rate of a reaction, but are unchanged themselves at the end of the reaction • An uncatalyzed reaction might eventually take place, but not at a rate quickly enough to meet the body’s demands -- this is why we need enzymes!

3. How Does An Enzyme Work? • Lowers the activation energy for a reaction. • As a result, less energy is needed to convert reactants to products. This allows more molecules to form product. • The enzyme does not affect the equilibrium position of the reaction.

4. Enzymes Lower Activation Energy, But Don’t Change Equilibrium Position

5. Names and Classification of Enzymes • The enzyme name often describes the reaction taking place, and the enzyme name always ends with the suffix -ase. • Examples: oxidase catalyzes oxidation lipid is hydrolyzed by lipase • What type of reaction would you think is catalyzed by a hydrolase? An isomerase? An oxidoreductase? • While I don’t need you to memorize the classes and subclasses in table 20.1, I could ask you to tell me the type of reaction catalyzed by some of the more obvious classes on the list.

6. B. Enzyme Action • Each enzyme has a unique three-dimensional shape that binds and recognizes a group of reacting molecules called substrates. • The active site of the enzyme is a small pocket to which the substrate directly binds. • Some enzymes are specific only to one substrate; others can bind more than one substrate.

7. Enzyme-Substrate Binding

8. Models of Enzyme Action • Early theory: lock-and-key model. Active site (lock) had the same shape as the substrate (key). Only the right shape key could bind. • Current theory: induced fit model. Active site closely resembles but does not exactly bind the substrate. • Allows for more flexibility in type of substrate • Also explains how the reaction itself occurs. As the substrate flexes to fit the active site, bonds in the substrate are flexed and stressed -- this causes changes/conversion to product.

9. More Detail on Binding • An interactive animation on enzyme specificity and binding: http://www.wiley.com/legacy/college/boyer/0470003790/animations/enzyme_binding/enzyme_binding.swf

10. C. Factors Affecting Enzyme Activity • Enzyme activity is defined as how fast an enzyme catalyzes its reaction. • Many factors affect enzyme activity: • Temperature: most have an optimum temp around 37oC • pH: most cellular enzymes are optimal around physiological pH, but enzymes in the stomach have a lower optimum pH • Concentration of enzyme and substrate: have all of the enzyme molecules been used up, even though substrate is still available?

11. Reaction Rate vs. Enzyme and Substrate Conc.

12. D. Enzyme Inhibition • Inhibitors stop the catalytic activity of the enzyme. • There are different methods of inhibition: • Reversible: the inhibitor can be removed Competitive inhibitors bind to the active site Noncompetitive inhibitors bind somewhere other than the active site and change the conformation of the active site • Irreversible: the inhibitor cannot be removed Examples: toxins that form a permanent bond to the enzyme, antibiotics (prevent bacterial cell wall formation)

13. How a Noncompetitive Inhibitor Works

14. Thinking about Inhibition… • What kind of inhibitor competes with the substrate for the active site? • Competitive • In what kind of inhibition does the addition of more substrate reverse the inhibition? • Reversible, competitive • In what kind of inhibition is the structure of the inhibitor not similar to that of the substrate? • Noncompetitive

15. E. Control of Enzyme Activity • We don’t always need high levels of products of enzyme-catalyzed reactions around. What kind of control system is used to regulate amounts of enzyme and products? • Two main methods: zymogens, and feedback control.

16. Zymogens • Many enzymes are active as soon as they’re made. • However, some are made in an inactive form and stored. This inactive form is called a zymogen or proenzyme. • To become active, the body needs only to cleave off a small peptide fragment. • Many digestive enzymes are produced initially as zymogens… why?

17. Feedback Control • Some enzymes (allosteric enzymes) bind molecules called regulators (different from the substrate) that can affect the enzyme either positively or negatively • Positive regulator: speeds up the reaction by changing the shape of the active site -- substrate binds more effectively • Negative regulator: slows down reaction by preventing proper substrate binding, again, by changing enzyme shape • Feedback control: the end product acts as a negative regulator. If there is enough of the end product, it will slow down the first enzyme in a pathway. Why does it slow down the first, and not the third, or fourth?

18. Feedback Control

19. F. Enzyme Cofactors and Vitamins • Many enzymes require small molecules or metal ions called cofactors to catalyze reactions properly. • Some metal ions (such as Fe2+ and Cu2+) participate in redox reactions with oxidases • Other metal ions stabilize either the enzyme or substrate over the course of the reaction • Vitamins: molecules essential for normal health that must be obtained from the diet (body does not synthesize) • Classified as either water-soluble (contain polar groups) or fat-soluble (nonpolar compounds)

20. Vitamins • Water soluble vitamins: not stored in the body, excess are eliminated • Many are enzyme cofactors (B vitamins, vitamin C) • Fat soluble vitamins: stored in the body and not eliminated -- can be toxic if you take too much • Not coenzymes or cofactors but play various important roles in the body