HOW ENZYMES FUNCTION

5.14 Activation Energy

• Activation energy (EA): Energy available to break bonds and form new ones

• Although there is a lot of potential energy in biological molecules, such as carbohydrates, it is not released spontaneously

Reactionwithoutenzyme

EA with enzyme

Ener

gy Reactants

Reaction withenzyme

EA withoutenzyme

Netchangein energy(the same)

ProductsProgress of the reaction

• Purpose: to lower the Energy of Activation Ea

• Enzymes are proteins and specific in shape

• If shape changes the enzyme will not function b/c substrate is specific for active site

5.14 Enzymes- Purpose

5.15 Enzymes- Specific Structure

• Enzymes have unique three-dimensional shapes

– As a result of its shape, the enzyme has an active site where the enzyme interacts with the enzyme’s substrate

– Consequently, the substrate’s chemistry is altered to form the product of the enzyme reaction

Substrate

Active site

Enzyme Enzyme-substratecomplex

Enzyme availablewith empty activesite

Active site

1

Enzyme(sucrase)

Enzyme availablewith empty activesite

Active site

1

Enzyme(sucrase)

Substrate bindsto enzyme withinduced fit

2

Substrate(sucrose)

Enzyme availablewith empty activesite

Active site

1

Enzyme(sucrase)

Substrate bindsto enzyme withinduced fit

2

Substrate(sucrose)

Substrate isconverted toproducts

3

Enzyme availablewith empty activesite

Active site

1

Enzyme(sucrase)

Substrate bindsto enzyme withinduced fit

2

Substrate(sucrose)

Substrate isconverted toproducts

3Products arereleased

4

Fructose

Glucose

5.15 Enzymes- Environmental Conditions

• Enzymes require certain environmental conditions

– Temperature is very important– human enzymes function best at 37ºC, or body temperature– High temperature will denature (change the shape of)

human enzymes

– Enzymes also require a pH – around neutral for best results

5.15 Enzymes- Environmental Conditions

5.15 Enzymes- Cofactors

• Some enzymes require non-protein helpers

– Cofactors are inorganic, such as zinc, iron, or copper

– Coenzymes are organic molecules and are often vitamins

5.16 Enzymes- Inhibitors

• Competitive inhibitors bind to the active site of an enzyme, competing with the substrate

• Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective

• Examples of inhibitors include toxins, poisons, pesticides, and antibiotics

• BOTH type inhibit enzyme activity!

Substrate

Enzyme

Active site

Normal binding of substrate

Competitiveinhibitor

Enzyme inhibition

Noncompetitiveinhibitor

5.16 Enzyme Inhibitors

• Is inhibition reversible?

Covalent bonds form between Enzyme and Inhibitor!

5.16 Enzyme Inhibitors

• Enzyme inhibitors are important in regulating cell metabolism

• Feedback inhibition: when the product of a reaction can serve as an inhibitor of one enzyme in the same reaction

– The more product formed, the greater the inhibition = regulation of the reaction

5.16 Enzyme Inhibitors

• What would happen if they did not produce feedback inhibitors??

5.16 Enzyme Inhibitors

• There would be chaos in the cell!

• Too many chemical reactions!