Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Essential Idea Enzymes control the metabolism of the cell.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Essential Idea

• Enzymes control the metabolism of the cell.


Understandings

• Enzymes have an active site to which specific substrates bind.

• Enzyme catalysis involves molecular motion and the collision of substrates with the active site.


Overview: The Energy of Life

• The living cell is a miniature chemical factory where thousands of reactions occur

• The cell extracts energy and applies energy to perform work

• Some organisms even convert chemical energy to light, as in bioluminescence


An organism’s metabolism transforms matter and energy

• Metabolism is the totality of an organism’s chemical reactions

• Metabolism is a property of life that arises from interactions between molecules within the cell


IB Assessment Statement

• Define enzyme and active site.


What is an Enzyme?

• globular proteins

• catalysts which speed up biological reactions

• unchanged by the reaction

• specific to their substrate

• active site is the position on the enzyme occupied by the substrate

• affected by temperature and pH


Organization of the Chemistry of Life into Metabolic Pathways

• A metabolic pathway begins with a specific molecule and ends with a product

• Each step is catalyzed by a specific enzyme


What is an enzyme animation?

• http://www.northland.cc.mn.us/biology/biology1111/animations/enzyme.swf


Enzymes speed up metabolic reactions by lowering energy barriers

• A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction

• An enzyme is a catalytic protein

• Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction

LE 8-13

• In the below chemical reaction, the enzyme sucrase catalyzes the hydrolysis reaction where sucrose is broken in to glucose & fructose.

SucroseC12H22O11

GlucoseC6H12O6

FructoseC6H12O6


The Activation Energy Barrier

• Every chemical reaction between molecules involves bond breaking and bond forming

• The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA)

• Activation energy is often supplied in the form of heat from the surroundings


How Enzymes Lower the EA Barrier

• Enzymes catalyze reactions by lowering the EA barrier

• Enzymes hasten reactions that would occur eventually



• Explain enzyme–substrate specificity. 3 The lock-and-key model can be used as a basis for the explanation. Refer to the three-dimensional structure.


Substrate Specificity of Enzymes

• The reactant that an enzyme acts on is called the enzyme’s substrate

• The enzyme binds to its substrate, forming an enzyme-substrate complex

• The active site is the region on the enzyme where the substrate binds

Substrate

Active site

Enzyme Enzyme-substratecomplex

The active site is the region on the enzyme where the substrate bindsThe active site is the region on the enzyme where the substrate binds


Catalysis in the Enzyme’s Active Site

• In an enzymatic reaction, the substrate binds to the active site

• The active site can lower an EA barrier by

– Orienting substrates correctly

– Straining substrate bonds

– Providing a favorable microenvironment

– Covalently bonding to the substrate

Explain enzyme–substrate specificity.

• a) Large globular protein enzyme

• b) Active Site where the substrate combines to the enzyme

• c)Substrate which fits the active site

• d) Activated complex. The substrate is weakened to allow the reaction.

• e)Unchanged enzyme/ re-used at low concentrations

• f) Product of the reaction

An enzyme’s structure allows only certain reactants to bind to the enzyme. This is describe by the Lock and Key Model.

– substrates

– active site

substrates (reactants)

enzyme

Substrates bind to anenzyme at certain places called active sites.

• The lock-and-key model helps illustrate how enzymes function.– substrates brought together– bonds in substrates weakened

Substrates bind to anenzyme at certain places called active sites.

The enzyme bringssubstrates together and weakens their bonds.

The catalyzed reaction formsa product that is releasedfrom the enzyme.

LE 8-17

Enzyme-substratecomplex

Substrates

Enzyme

Products

Substrates enter active site; enzymechanges shape so its active siteembraces the substrates (induced fit).

Substrates held inactive site by weakinteractions, such ashydrogen bonds andionic bonds.

Active site (and R groups ofits amino acids) can lower EA

and speed up a reaction by• acting as a template for substrate orientation,• stressing the substrates and stabilizing the transition state,• providing a favorable microenvironment,• participating directly in the catalytic reaction.

Substrates areconverted intoproducts.

Products arereleased.

Activesite is

availablefor two new

substratemolecules.


Lock and Key Model


How enzymes work animation

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_enzymes_work.html


Applications/ Skills

• Skill: Design of experiments to test the effect of temperature, pH and substrate concentration on the activity of enzymes.

• Skill: Experimental investigation of a factor affecting enzyme activity. (Practical 3)


Understandings

• Temperature, pH and substrate concentration affect the rate of activity of enzymes.

• Enzymes can be denatured.



• Explain the effects of temperature, pH and substrate concentration on enzyme activity.


Effects of Local Conditions on Enzyme Activity

• An enzyme’s activity can be affected by:

– General environmental factors, such as temperature and pH

– Chemicals that specifically influence the enzyme

– Increasing the concentration of an enzyme’s substrate will also increase enzyme activity.


Effects of Temperature and pH

• Each enzyme has an optimal temperature in which it can function

• Each enzyme has an optimal pH in which it can function


The Effect of Temperature on Enzyme Activity

• (a) As the temperature increases enzyme stability decreases.

– The kinetic energy of the enzyme atoms increases causing vibrations in the enzyme molecule that lead to the hydrogen bonds to breaking, shape changes in the active site.



(b) As the temperature increases the kinetic energy of the substrate and enzyme molecules also increases.

Therefore more collisions of the substrate with the active site and the formation of activated complex's and product. The rate of reaction is increasing.



• (c) The optimal temperature (X) is the highest rate of reaction. Compromise between decreasing enzyme stability and kinetic energy of the reactants.

• (d) Higher temperature increases the kinetic energy of the enzyme atoms so much that they break bonds, change shape of the active site.



• Define denaturation..


Denaturation

• Denaturation is a structural change in a protein that results in the loss (usually permanent) of its biological properties.


Denaturation & Temperature

• Temperature rises cause the average kinetic energy of the enzyme atoms to increase.

• This vibration breaks the weakest bonds first, which in the enzyme are the hydrogen bonds.

• The breaking of bonds, changes the shape of the enzyme.

• Change the shape of the enzyme changes the shape of the active site.

• Change the shape of the active site prevents substrate from entering.

• The rate of reaction reduces or stops.


Denaturation & pH

• At pH lower than the optimal pH the concentration of H+ in the solution will be higher than normal.

• The hydrogens will tend to be attracted to electronegative regions of the enzyme protein.

• Bonds are formed or changed as a consequence of the additional H+ which changes the shape of the enzyme molecule.

• Changes in shape, change the active site shape.

• Changes in active site shape reduces the ability of the substrate to bind with the active site.

• This reduces the rate of reaction that changes substrate to product.

• The rate of reaction reduces.

• For pH values above the optimum breaks bonds in the same way and have the same reductions in the rate of reaction

The effect of pH on the rate of an enzyme catalysed reaction:

• pH also affects the rate of reaction of an enzyme catalysed reaction.

• At the optimal pH (a) or (b) the maximum rate of reaction is achieved.

• Above or below the optimal pH the rate decreases.



• The change in rate is because bonds are made and broken which change the shape of the active site and therefore decrease the rate of reaction.



• The two enzyme shown in the image illustrate the fact that different enzymes can have very different optimal pH.

• e.g. Blue curve = pepsin (a)= pH3, Red curve =salivary amylase (b)= pH 7.2

The effect of substrate concentration on Enzyme Activity

• (a) As the substrate concentration is increased the rate of reaction increases.

• There are more collisions between the substrate and the enzyme such that more activated complex's are formed and therefore product per unit time.



• (b) Further increases in substrate also increase the rate but proportionately less than previously.

• The number of occupied active site is increasing and there is competition for the active site.



• (c) The rate is constant.

• The enzyme active site is fully saturated with substrate such that adding more substrate does not increase the rate of reaction. The enzymes molecules are fully occupied converting substrate to product and any substrate must await a free active site before conversion to product


Understandings

• Immobilized enzymes are widely used in industry.


Immobilized enzymes

The movement of enzymes can be restricted. This has many benefits to industries which use enzymes e.g.

1. Increases stability of the enzyme2.Easier to separate products3.Easy to recycle the enzyme4.Expose substrates to high enzyme

concentrations

You need to know the example of lactose free milk.


Applications/ Skills

• Application: Methods of production of lactose-free milk and its advantages.



• Explain the use of lactase in the production of lactose-free milk.


Explain the use of lactase in the production of lactose-free milk.

• Lactose is a disaccharide (glucose + Galactose) milk sugar

• Around 90% of all humans show some kind of lactose intolerance.

• People who are lactose intolerant can drink milk if it is lactose free.

• Lactase is an enzyme extracted from yeast that can digest the milk sugar to glucose and galactose.


Explain the use of lactase in the production of lactose-free milk.

• Milk products can be treat with lactase before consumption.

• Lactase treated milk products allow lactose intolerant people to digest milk products without problems




Reasons for converting lactose into glucose and galactose

1. it allows people who are lactose intolerant/have difficulty digesting lactose to consume milk (products);

2. galactose and glucose taste sweeter than lactose reducing need for additional sweetener (in flavoured milk products);

3. galactose and glucose are more soluble than lactose / gives smoother texture / reduces crystalization in ice cream;

4. (bacteria) ferment glucose and galactose more rapidly (than lactose) shortening production time (of yogurt/cottage cheese);

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Essential Idea Enzymes control the metabolism of the cell.

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