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:http://www.wiley.com/legacy/college/boyer/0470003790/animations/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 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?

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 removedCompetitive inhibitors bind to the active siteNoncompetitive inhibitors bind somewhere other than the active site and change the conformation of the active site

Irreversible: the inhibitor cannot be removedExamples: 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 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)

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