Enzyme 1 Enzyme 2 Enzyme 3

DCBAReaction 1 Reaction 3Reaction 2

Startingmolecule

Product

1

Climbing up converts the kineticenergy of muscle movementto potential energy.

A diver has less potentialenergy in the waterthan on the platform.

Diving convertspotential energy tokinetic energy.

A diver has more potentialenergy on the platformthan in the water.

2

(a) First law of thermodynamics (b) Second law of thermodynamics

Chemicalenergy

Heat CO2

H2O

+

3

(a) Gravitational motion (b) Diffusion (c) Chemical reaction

• More free energy (higher G)• Less stable• Greater work capacity

In a spontaneous change• The free energy of the system decreases (∆G < 0)• The system becomes more stable• The released free energy can be harnessed to do work

• Less free energy (lower G)• More stable• Less work capacity

4

Reactants

Energy

Fre

e e

ne

rgy

Products

Amount ofenergy

released(∆G < 0)

Progress of the reaction

(a) Exergonic reaction: energy released

Products

ReactantsEnergy

Fre

e e

ne

rgy

Amount ofenergy

required(∆G > 0)

(b) Endergonic reaction: energy required

Progress of the reaction

5

(a) An isolated hydroelectric system

∆G < 0 ∆G = 0

(b) An open hydroelectric system ∆G < 0

∆G < 0

∆G < 0

∆G < 0

(c) A multistep open hydroelectric system

6

Phosphate groupsRibose

Adenine

7

Inorganic phosphate

Energy

Adenosine triphosphate (ATP)

Adenosine diphosphate (ADP)

P P

P P P

P ++

H2O

i

8

(b) Coupled with ATP hydrolysis, an exergonic reaction

Ammonia displacesthe phosphate group,forming glutamine.

(a) Endergonic reaction

(c) Overall free-energy change

PP

GluNH3

NH2

Glu i

GluADP+

PATP+

+

Glu

ATP phosphorylatesglutamic acid,making the aminoacid less stable.

GluNH3

NH2

Glu+

Glutamicacid

GlutamineAmmonia

∆G = +3.4 kcal/mol

+2

1

9

(b) Mechanical work: ATP binds noncovalently to motor proteins, then is hydrolyzed

Membrane protein

P i

ADP+

P

Solute Solute transported

Pi

Vesicle Cytoskeletal track

Motor protein Protein moved

(a) Transport work: ATP phosphorylates transport proteins

ATP

ATP

10

P iADP +

Energy fromcatabolism (exergonic,energy-releasingprocesses)

Energy for cellularwork (endergonic,energy-consumingprocesses)

ATP + H2O

11

Sucrose (C12H22O11)

Glucose (C6H12O6) Fructose (C6H12O6)

Sucrase

12

Progress of the reaction

Products

Reactants

∆G < O

Transition state

Fre

e en

erg

y EA

DC

BA

D

D

C

C

B

B

A

A

13

Progress of the reaction

Products

Reactants

∆G is unaffectedby enzyme

Course ofreactionwithoutenzyme

Fre

e en

erg

y

EA

withoutenzyme EA with

enzymeis lower

Course ofreactionwith enzyme

14

Substrate

Active site

Enzyme Enzyme-substratecomplex

(b)(a)

15

Substrates

Enzyme

Products arereleased.

Products

Substrates areconverted toproducts.

Active site can lower EA

and speed up a reaction.

Substrates held in active site by weakinteractions, such as hydrogen bonds andionic bonds.

Substrates enter active site; enzyme changes shape such that its active siteenfolds the substrates (induced fit).

Activesite is

availablefor two new

substratemolecules.

Enzyme-substratecomplex

5

3

21

6

4

16

Ra

te o

f re

ac

tio

n

Optimal temperature forenzyme of thermophilic

(heat-tolerant) bacteria

Optimal temperature fortypical human enzyme

(a) Optimal temperature for two enzymes

(b) Optimal pH for two enzymes

Ra

te o

f re

ac

tio

n

Optimal pH for pepsin(stomach enzyme)

Optimal pHfor trypsin(intestinalenzyme)

Temperature (ºC)

pH543210 6 7 8 9 10

0 20 40 80 60 100

17

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

Noncompetitive inhibitor

Active siteCompetitive inhibitor

Substrate

Enzyme

18

Allosteric enyzmewith four subunits

Active site(one of four)

Regulatorysite (oneof four)

Active form

Activator

Stabilized active form

Oscillation

Non-functionalactivesite

InhibitorInactive form Stabilized inactive

form

(a) Allosteric activators and inhibitors

Substrate

Inactive form Stabilized activeform

(b) Cooperativity: another type of allosteric activation

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Intermediate C

Feedbackinhibition

Isoleucineused up bycell

Enzyme 1(threoninedeaminase)

End product(isoleucine)

Enzyme 5

Intermediate D

Intermediate B

Intermediate A

Enzyme 4

Enzyme 2

Enzyme 3

Initial substrate(threonine)

Threoninein active site

Active siteavailable

Active site ofenzyme 1 nolonger bindsthreonine;pathway isswitched off.

Isoleucinebinds toallostericsite

20