CHAPTER 20 ENZYMES AND VITAMINS

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!

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.

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

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.

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.

Enzyme-Substrate Binding

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.

More Detail on Binding  An interactive animation on enzyme specificity and binding: ns/enzyme_binding/enzyme_binding.swf

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 37 o C  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?

Reaction Rate vs. Enzyme and Substrate Conc.

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)

How a Noncompetitive Inhibitor Works

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

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.

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?

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?

Feedback Control

F. Enzyme Cofactors and Vitamins  Many enzymes require small molecules or metal ions called cofactors to catalyze reactions properly.  Some metal ions (such as Fe 2+ and Cu 2+ ) 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)

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