BIOLOGY 12

Enzymes

Enzymes

Video

Enzyme BasicsEnzyme Basics

Like all catalysts, enzymes work by lowering the activation energy for a reaction, thus dramatically increasing the rate of the reaction. As a result, products are formed faster and reactions reach their equilibrium state more rapidly. Most enzyme reaction rates are millions of times faster than those of comparable un-catalyzed reactions. As with all catalysts, enzymes are not consumed by the reactions they catalyze, nor do they alter the equilibrium of these reactions.

Enzymes

proteins catalysts for biological reactions substrate specific they initiate chemical reactions without being

used in the reaction (reusable) usually named after their substrate, most end in

“ase” (ex. Maltase) “Lock and Key” Fit Induced Fit Model

Enzymes

they lower the activation energy, therefore increase the rate of reaction

Enzymes

activity is lost if they are denatured the unfolding and inactivation – by heating or

chemical denaturants, which disrupt the three-dimensional structure of the protein

Enzymes

How Enzymes Work

Enzyme Action

Lock and Key Model substrate binds to active site of enzyme enzyme-substrate complex forms substrate reacts and product is formed product is released (enzyme is unchanged by

reaction)

Enzyme Action

Enzyme Action

Induced Fit Model scientists have modified the “lock and key”

model enzyme structure is flexible – not rigid the shape of the enzyme’s active site and the

shape of the substrate do not need to match exactly

Enzyme Action

when the enzyme and substrate join, the enzyme changes its shape slightly (adjusts to fit)

this increases the range of substrate specificity

Enzyme Action

Catalytic Cycle of an Enzyme

Activation Energy (EA)

the minimum amount of energy required to start a chemical reaction

examples: Oxygen gas + Hydrogen gas + HeatHeat (A.E) =

Water Wood + Oxygen + Burning MatchBurning Match (A.E) =

Burning wood

Activation Energy (EA)

enzymes use a variety of mechanisms that lower activation energy and speed up a reaction

*see graphs on page 69 Activation Energy Graph

example of why we have enzymes: enzymes allow reactions to take place at lower

temperature (i.e. body temperature) the reactions of living organisms cannot depend

on heat as a source of activation energy high temperatures would destroy cells heat would be far too random of a source of

activation energy (not dependable) cells must carry out specific reactions at definite

times

Recap on Enzymes

each is specific – guides only one type of cell reaction

they are not permanently changed or used up during a reaction

can be used over and over again the shape of the enzyme is crucial to the

reaction

Factors Affecting Enzyme Activity

environmental factors

cofactors and coenzymes

enzyme inhibitors

Factors Affecting Enzyme Activity

temperature up to a point, the speed of the reaction

increases with the increase in temperature most enzymes function optimally around body

temperature (between 35o – 40oC) extreme temperatures can denature an enzyme

Factors Affecting Enzyme Activity

pH the optimal pH is between 6 and 8 (near

neutral)

Factors Affecting Enzyme Activity

cofactors inorganic non-protein helpers are sometimes

needed for optimal enzyme activity examples:

vitamins zinc iron must be present in the quaternary structure

of hemoglobin in order for it to pick up oxygen

Factors Affecting Enzyme Activity

enzyme inhibitors competitive:

has a structure similar to the substrate competes with the substrate for the active site occupies the active site

Factors Affecting Enzyme Activity

enzyme inhibitors non-competitive:

does not have a structure like the substrate binds to the enzyme but not active site changes the shape of enzyme and active site substrate cannot fit into altered active site no reaction occurs

Factors Affecting Enzyme Activity

Factors Affecting Enzyme Activity

How Enzymes Work

Lets Label It

Questions

1. You turn on a battery powered tape deck and the reels in the cassette begin to rotate.

How does this example explain energy transformation?

A. Potential energy (stored energy) in the batteries was converted to kinetic energy (mechanical energy).

Questions

2. Suppose you started a fire by using a magnifying glass to focus the sun’s image on some paper.What is the source of activation energy for this reaction?

A. The concentrated sunlight was absorbed and changed to heat. This heat provided the activation energy.

Questions

3. Explain how the enzyme maltase can control a reaction involving maltose, but not one involving sucrose.

A. Maltose consists of two bonded glucose units, whereas sucrose consists of a glucose bonded to a fructose.

If the active site of the enzyme maltase fits with maltose, it will not fit with sucrose.

Compare and Contrast

Kinetic Energy / Potential Energy Endergonic / Exergonic Enzyme/ Substrate Lock and Key Model / Induced Fit Model Enzyme / Coenzyme Competitive Inhibitors / Non-Competitive

Inhibitors

Questions

Page 77 #1, 4, 5, 6, 7, and 8.