how enzymes lower the ea barrier -...
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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