HowEnzymesLowertheEHowEnzymesLowertheEAABarrierBarrier

• Enzymescatalyzereac.onsbyloweringtheEAbarrier

• donotaffectthechangeinfreeenergy(∆G)

• Insteadhastenreac.onsthatwouldoccureventually

Fig.8‐15Fig.8‐15

Progress of the reaction

Products

Reactants

∆G is unaffectedby enzyme

Course ofreactionwithoutenzyme

Free

ene

rgy

EAwithoutenzyme EA with

enzymeis lower

Course ofreactionwith enzyme

SubstrateSpecificityofEnzymesSubstrateSpecificityofEnzymes• Substrate

• Thereactantthatanenzymeactson

• Theenzymebindstoitssubstrate,forminganenzyme‐substrate

complex

• Ac>vesite

• regionontheenzymewherethesubstratebinds

• Inducedfit

• Forma.onofenzyme‐substratecomplextriggersconforma.onal

changeinenzyme

• bringschemicalgroupsoftheac.vesiteintoposi.onsthatenhance

theirabilitytocatalyzethereac.on

• Bycontor.ngandstressingbondsinsubstrate

Fig.8‐16Fig.8‐16

Substrate

Active site

Enzyme Enzyme-substratecomplex

(b)(a)

CatalysisintheEnzymeCatalysisintheEnzyme’’sAc>veSitesAc>veSite

• Inanenzyma.creac.on,thesubstratebindstotheac.vesiteofthe

enzyme

• Theac.vesitecanloweranEAbarrierby

• Orien.ngsubstratescorrectly

• Strainingsubstratebonds

• Providingafavorablemicroenvironment

• Covalentlybondingtothesubstrate

Fig.8‐17Fig.8‐17

Substrates

Enzyme

Products arereleased.

Products

Substrates areconverted toproducts.

Active site can lower EAand 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

EffectsofTemperatureandpHEffectsofTemperatureandpH

• Eachenzymehas

• Anop.maltemperatureforfunc.on

• Taqpolymeraseat95°C

• Anop.malpHforfunc.on

• Pepsinogen/pepsinatpH2

• Notnecessarilythesameforallenzymes

Fig.8‐18Fig.8‐18

Rat

e of

reac

tion

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

Rat

e of

reac

tion

Optimal pH for pepsin(stomach enzyme)

Optimal pHfor trypsin(intestinalenzyme)

Temperature (ºC)

pH543210 6 7 8 9 10

0 20 40 80 60 100

CofactorsCofactors

• Cofactors

• nonproteinenzymehelpers

• maybeinorganic(suchasametalinionicform)ororganic

• Coenzyme

• organiccofactor

• includevitamins

• VitaminC‐ascorbicacid

EnzymeInhibitorsEnzymeInhibitors

• Compe>>veinhibitors

• bindtotheac.vesiteofanenzyme,compe.ngwiththesubstrate

• Blockac.vesite

• Noncompe>>veinhibitors

• bindtoanotherpartofanenzyme

• causeenzymetochangeshapealteringac.vesite

• Examplesofinhibitorsincludetoxins,poisons,pes.cides,andan.bio.cs

Fig.8‐19Fig.8‐19

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

Noncompetitive inhibitor

Active siteCompetitive inhibitor

Substrate

Enzyme

AllostericRegula>onofEnzymesAllostericRegula>onofEnzymes

• Allostericregula>on

• mayeitherinhibitors.mulateanenzyme’sac.vity

• occurswhenaregulatorymoleculebindstoaproteinatonesite

andaffectstheprotein’sfunc.onatanothersite

• Canbeaformofnon‐compe..veinhibi.on

AllostericAc>va>onandInhibi>onAllostericAc>va>onandInhibi>on

• Mostallostericallyregulatedenzymesaremadefromindividual

polypep.desubunits

• Eachwithitsownac.vesite

• Eachenzymehasac.veandinac.veforms

• bindingofanac.vatorstabilizestheac.veformoftheenzyme

• bindingofaninhibitorstabilizestheinac.veformoftheenzyme

(a) Allosteric activators and inhibitors

InhibitorNon-functionalactivesite

Stabilized inactiveform

Inactive form

Oscillation

ActivatorActive form Stabilized active form

Regulatorysite (oneof four)

Allosteric enzymewith four subunits

Active site(one of four)

• Coopera>vity

• aformofallostericregula.onthatcanamplifyenzymeac.vity

• bindingbyasubstratetooneac.vesitestabilizesfavorable

conforma.onalchangesatallothersubunits

AllostericRegula>onAllostericRegula>on

(b) Cooperativity: another type of allosteric activation

Stabilized activeform

Substrate

Inactive form

FeedbackInhibi>onFeedbackInhibi>on

• Feedbackinhibi>on

• theendproductofametabolicpathwayshutsdownthepathway

• Feedbackinhibi.onpreventsacellfromwas.ngchemicalresources

bysynthesizingmoreproductthanisneeded

Fig.8‐22Fig.8‐22

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

Youshouldnowbeableto:Youshouldnowbeableto:

1. Dis.nguishbetweenthefollowingpairsofterms:catabolicandanabolicpathways;kine.candpoten.alenergy;openandclosedsystems;exergonicandendergonicreac.ons

2. Inyourownwords,explainthesecondlawofthermodynamicsandexplainwhyitisnotviolatedbylivingorganisms

3. Explainingeneraltermshowcellsobtaintheenergytodocellularwork

4. ExplainhowATPperformscellularwork

5. Explainwhyaninvestmentofac.va.onenergyisnecessarytoini.ateaspontaneousreac.on

6. Describethemechanismsbywhichenzymeslowerac.va.onenergy

7. Describehowallostericregulatorsmayinhibitors.mulatetheac.vityofanenzyme