FREE ENERGY AND METABOLISM

The concept of free energy can be applied to the chemistry of life’s processes

© 2011 Pearson Education, Inc.

EXERGONIC AND ENDERGONIC REACTIONS IN METABOLISM

An exergonic reaction proceeds with a net release of free energy and is spontaneous

An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous

© 2011 Pearson Education, Inc.

FIGURE 8.6 (a) Exergonic reaction: energy released, spontaneous

(b) Endergonic reaction: energy required, nonspontaneous

Reactants

EnergyProducts

Progress of the reaction

Amount of energy

released(G 0)

ReactantsEnergy

Products

Amount of energy

required(G 0)

Progress of the reaction

Fre

e e

ne

rgy

Fre

e e

ne

rgy

FIGURE 8.6A

(a) Exergonic reaction: energy released, spontaneous

Reactants

EnergyProducts

Progress of the reaction

Amount of energy

released(G 0)

Fre

e en

erg

y

FIGURE 8.6B

(b) Endergonic reaction: energy required, nonspontaneous

ReactantsEnergy

Products

Amount of energy

required(G 0)

Progress of the reaction

Fre

e en

erg

y

FIGURE 8.7C

(c) A multistep open hydroelectric system

G 0

G 0

G 0

CONCEPT 8.4: 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

© 2011 Pearson Education, Inc.

FIGURE 8.UN02

Sucrase

Sucrose(C12H22O11)

Glucose(C6H12O6)

Fructose(C6H12O6)

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 thermal energy that the reactant molecules absorb from their surroundings

© 2011 Pearson Education, Inc.

FIGURE 8.12

Transition state

Reactants

Products

Progress of the reaction

Fre

e e

ner

gy EA

G O

A B

C D

A B

C D

A B

C D

HOW ENZYMES LOWER THE EA BARRIER

Enzymes catalyze reactions by lowering the EA barrier

Enzymes do not affect the change in free energy (∆G); instead, they hasten reactions that would occur eventually

© 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc.

Animation: How Enzymes WorkRight-click slide / select “Play”

FIGURE 8.13

Course ofreactionwithoutenzyme

EA

withoutenzyme EA with

enzymeis lower

Course ofreactionwith enzyme

Reactants

Products

G is unaffectedby enzyme

Progress of the reaction

Fre

e en

erg

y

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

Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction

© 2011 Pearson Education, Inc.

FIGURE 8.14

Substrate

Active site

Enzyme Enzyme-substratecomplex

(a) (b)

CATALYSIS IN THE ENZYME’S ACTIVE SITE

In an enzymatic reaction, the substrate binds to the active site of the enzyme

The active site can lower an EA barrier by Orienting substrates correctly Straining substrate bonds Providing a favorable microenvironment Covalently bonding to the substrate

© 2011 Pearson Education, Inc.

FIGURE 8.15-1

Substrates

Substrates enter active site.

Enzyme-substratecomplex

Substrates are heldin active site by weakinteractions.

12

Enzyme

Activesite

FIGURE 8.15-2

Substrates

Substrates enter active site.

Enzyme-substratecomplex

Substrates are heldin active site by weakinteractions.

Active site canlower EA and speedup a reaction.

12

3

Substrates areconverted toproducts.

4

Enzyme

Activesite

FIGURE 8.15-3

Substrates

Substrates enter active site.

Enzyme-substratecomplex

Enzyme

Products

Substrates are heldin active site by weakinteractions.

Active site canlower EA and speedup a reaction.

Activesite is

availablefor two new

substratemolecules.

Products arereleased.

Substrates areconverted toproducts.

12

3

45

6

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

© 2011 Pearson Education, Inc.

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

Optimal conditions favor the most active shape for the enzyme molecule

© 2011 Pearson Education, Inc.

FIGURE 8.16

Optimal temperature fortypical human enzyme (37°C)

Optimal temperature forenzyme of thermophilic

(heat-tolerant)bacteria (77°C)

Temperature (°C)(a) Optimal temperature for two enzymes

Ra

te o

f re

ac

tio

nR

ate

of

rea

cti

on

120100806040200

0 1 2 3 4 5 6 7 8 9 10pH

(b) Optimal pH for two enzymes

Optimal pH for pepsin(stomachenzyme)

Optimal pH for trypsin(intestinal

enzyme)

FIGURE 8.16A

Optimal temperature fortypical human enzyme (37°C)

Optimal temperature forenzyme of thermophilic

(heat-tolerant)bacteria (77°C)

Temperature (°C)(a) Optimal temperature for two enzymes

Rat

e o

f re

acti

on

120100806040200

FIGURE 8.16B

Rat

e o

f re

acti

on

0 1 2 3 4 5 6 7 8 9 10pH

(b) Optimal pH for two enzymes

Optimal pH for pepsin(stomachenzyme)

Optimal pH for trypsin(intestinal

enzyme)

IB QUESTIONS?

3.6.1 Define enzyme and active site3.6.2 Explain enzyme-substrate specificity.3.6.3 Explain the effects of temperature, pH and substrate concentration on enzyme activity.3.6.4 Define denaturation.3.6.5 Explain the production of lactase in the production of lactose free milk.

COFACTORS

Cofactors are nonprotein enzyme helpers Cofactors may be inorganic (such as a metal

in ionic form) or organic An organic cofactor is called a coenzyme Coenzymes include vitamins

© 2011 Pearson Education, Inc.

ENZYME 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

© 2011 Pearson Education, Inc.

FIGURE 8.17

(a) Normal binding (b) Competitive inhibition (c) Noncompetitive inhibition

Substrate

Activesite

Enzyme

Competitiveinhibitor

Noncompetitiveinhibitor

THE EVOLUTION OF ENZYMES

Enzymes are proteins encoded by genes Changes (mutations) in genes lead to

changes in amino acid composition of an enzyme

Altered amino acids in enzymes may alter their substrate specificity

Under new environmental conditions a novel form of an enzyme might be favored

© 2011 Pearson Education, Inc.

FIGURE 8.18

Two changed amino acids werefound near the active site.

Active site

Two changed amino acidswere found in the active site.

Two changed amino acidswere found on the surface.

CONCEPT 8.5: REGULATION OF ENZYME ACTIVITY HELPS CONTROL METABOLISM

Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated

A cell does this by switching on or off the genes that encode specific enzymes or by regulating the activity of enzymes

© 2011 Pearson Education, Inc.

ALLOSTERIC REGULATION OF ENZYMES

Allosteric regulation may either inhibit or stimulate an enzyme’s activity

Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site

© 2011 Pearson Education, Inc.

ALLOSTERIC ACTIVATION AND INHIBITION

Most allosterically regulated enzymes are made from polypeptide subunits

Each enzyme has active and inactive forms

The binding of an activator stabilizes the active form of the enzyme

The binding of an inhibitor stabilizes the inactive form of the enzyme

© 2011 Pearson Education, Inc.

FIGURE 8.19

Regulatorysite (oneof four)

(a) Allosteric activators and inhibitors

Allosteric enzymewith four subunits

Active site(one of four)

Active form

Activator

Stabilized active form

Oscillation

Non-functionalactive site

Inactive formInhibitor

Stabilized inactiveform

Inactive form

Substrate

Stabilized activeform

(b) Cooperativity: another type of allosteric activation

FIGURE 8.19A

Regulatory site (one of four)

(a) Allosteric activators and inhibitors

Allosteric enzymewith four subunits

Active site(one of four)

Active form

Activator

Stabilized active form

Oscillation

Nonfunctionalactive site

Inactive formInhibitor

Stabilized inactive form

FIGURE 8.19B

Inactive form

Substrate

Stabilized activeform

(b) Cooperativity: another type of allosteric activation

Cooperativity is a form of allosteric regulation that can amplify enzyme activity

One substrate molecule primes an enzyme to act on additional substrate molecules more readily

Cooperativity is allosteric because binding by a substrate to one active site affects catalysis in a different active site

© 2011 Pearson Education, Inc.

IDENTIFICATION OF ALLOSTERIC REGULATORS

Allosteric regulators are attractive drug candidates for enzyme regulation because of their specificity

Inhibition of proteolytic enzymes called caspases may help management of inappropriate inflammatory responses

© 2011 Pearson Education, Inc.

FIGURE 8.20

Caspase 1 Activesite

Substrate

SH SH

SH

Known active form Active form canbind substrate

Allostericbinding site

Allostericinhibitor

Hypothesis: allostericinhibitor locks enzymein inactive form

Caspase 1

Active form Allostericallyinhibited form

Inhibitor

Inactive form

EXPERIMENT

RESULTS

Known inactive form

FIGURE 8.20A

Caspase 1 Activesite

Substrate

SH SH

SH

Known active form Active form canbind substrate

Allostericbinding site

Allostericinhibitor

Hypothesis: allostericinhibitor locks enzymein inactive form

EXPERIMENT

Known inactive form

FIGURE 8.20B

Caspase 1

Active form Allostericallyinhibited form

Inhibitor

Inactive form

RESULTS

FEEDBACK INHIBITION

In feedback inhibition, the end product of a metabolic pathway shuts down the pathway

Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is needed

© 2011 Pearson Education, Inc.

FIGURE 8.21

Active siteavailable

Isoleucineused up bycell

Feedbackinhibition

Active site ofenzyme 1 isno longer ableto catalyze theconversionof threonine tointermediate A;pathway isswitched off. Isoleucine

binds toallostericsite.

Initial substrate(threonine)

Threoninein active site

Enzyme 1(threoninedeaminase)

Intermediate A

Intermediate B

Intermediate C

Intermediate D

Enzyme 2

Enzyme 3

Enzyme 4

Enzyme 5

End product(isoleucine)

SPECIFIC LOCALIZATION OF ENZYMES WITHIN THE CELL

Structures within the cell help bring order to metabolic pathways

Some enzymes act as structural components of membranes

In eukaryotic cells, some enzymes reside in specific organelles; for example, enzymes for cellular respiration are located in mitochondria

© 2011 Pearson Education, Inc.

FIGURE 8.22

Mitochondria

The matrix containsenzymes in solution that

are involved in one stageof cellular respiration.

Enzymes for anotherstage of cellular

respiration areembedded in theinner membrane.

1 m