CARBONICANHYDRASE:WhyZinc?

By:AmirSafi

WhatReactionisCatalyzed?

H2O+CO2 H+ +HCO3

CarbonicAnhydrase CatalyzestheReactionofwater+carbondioxidetoformbicarbonate.

Rate of Catalysis

kuncat = 10-11 s-1kcat = 104 s-1

CO2 + H2O = HCO3- + H+

Role of Zn(II) in Proteins1. Catalytic: involved directly in catalytic steps

(e.g., CA)

2. Structural: required for the stability of a protein structure, or arrangement of an active site (e.g., CuZnSOD)

3. Regulatory: Zn-free enzyme has activity, addition of Zn2+enhance or inhibit activity

(e.g., zinc finger family of proteins)

Zinc Fingers

Jamieson, Andrew C.; Miller, Jeffrey C.; Pabo, Carl O. Nature Rev. Drug Discovery (2003), 2, 361-368.

Cys2His2 zinc fingers are found in 2% of all human genes

Hydrolytic reactions

Hydrolytic reactions

BackgroundonCarbonicAnhydrase

CarbonicAnhydrase isanenzymethatislocatedinredbloodcells.

Itcontainsabout.31to.34%zinc.

Thisisimportantbecauseitsthefirstknowndirectphysiologicalfunctionofzinc.

http://www.pdb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb49_1.html

CarbonAnhydrase Mechanism

Step1:deprotonation

1.Berg,JeremyM.(2007).Biochemistry,6thEd.

Step2:Carboxylation Theanionicoxygenthenperformsanucleophilicattackonthecarbon.

Step3:FormationofRingStructure

AnionicoxygenformsabondwithZn2+tomakearinglikeresonance.

Step4:FormationofBicarbonate

Additionofwaterdisplacesthebicarbonateion.

CarbonicAnhydrase Mechanism

BIGQUESTION:WHYDOESCARBONANHYDRASEUSE

ZINC?

HistoryofZinc Zinccompoundshavebeenusedforatleast2,500years

Zincderiveditsname,supposedly,fromtheGermanword Zinke,whichmeans'toothlike'or'pointed'.

MetalliczincwasfirstproducedinIndiasometimeinthe1400sbyheatingthemineralcalamine(ZnCO3)withwool.

ZincwasrediscoveredbyAndreasSigismundMarggraf in1746byheatingcalaminewithcharcoal.

http://www.buzzle.com/articles/historyofzinc.html

Fe,Zn,Cu:themostcommontransitionmetalionsinbiology

ELEMENT Sea Water(M)x10^8 Human Body(M)x10^8

Fe .0052 2230

Zn 8 1720

Cu 1 1650

PropertiesofZinc

2+istheonlyaccessibleoxidationstate(d10) Bindsbothhardandsoftdonors Tetrahedralgeometrypreferred 56CNareaccessible GoodLewisacidbecausehighcharge/radiusratio

Zincisd10 ZincIoniscolorless

NotRedox Active

Hard/SoftAcidBaseTheory

GoodLewisacidbecausehighcharge/radiusratio

Borderline

Metals:

Fe2+,Ni2+,Zn2+,Co2+,Cu2+

Ligands:

SR,his,X,RCO2

ZincGeometryanddorbitalSplittingdiagrams.

RECAPWhyZinc:

1. 2+istheonlyaccessibleoxidationstate(d10) Nocomplicationsfromredox activity.

2.Bindsbothhardandsoftdonors

3.Tetrahedralgeometrypreferred/56CNareaccessible

MedicalApplication:TreatGlaucoma

Carbonicanhydraseinhibitorsareapplicableduringemergenciesbecausetheyareabletorapidlyreducethepressurethathasbuiltupinsidetheeyeby40%to60%.Thelessfluidthatisallowedtobuildupyieldslesspressurebehindtheeye.

http://www.pdb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb49_2.html

Acetazolamide

Diuretic:Diureticsaredrugsthatincreasetherateofurineflow.SoldasthedrugDiamox

http://jpet.aspetjournals.org/content/115/3/251.abstract

CarbonicAnhydrase Inhibition

BadNewsorGoodNews

Glaucoma: Themechanismofactionto

lowerpressureisnotknown.Therouteofadministrationcanbeoral,intravenous,topical,orinhaledbysmoking,anditdictatesmanyofthebehavioralandphysiologicconsequences.Oftheseroutes,smokinghasthemorerapidonsetofpsychoactiveandotherpharmacologiceffect

IsZincBoring?http://www.youtube.com/watch?v=99wPiMbk0o

CARBONIC ANHYDRASE:Why Zinc?What Reaction is Catalyzed?Rate of CatalysisRole of Zn(II) in ProteinsSlide Number 5Hydrolytic reactionsSlide Number 7Background on Carbonic AnhydraseCarbon Anhydrase MechanismStep 1: deprotonationStep 2: CarboxylationStep 3: Formation of Ring StructureStep 4: Formation of Bicarbonate Carbonic Anhydrase MechanismBIG QUESTION:History of ZincFe, Zn, Cu: the most common transition metal ions in biologyProperties of ZincZinc is d10Hard/ Soft Acid Base TheoryBorderlineZinc Geometry and d orbital Splitting diagrams.RECAPMedical Application: Treat GlaucomaAcetazolamideCarbonic Anhydrase InhibitionBad News or Good NewsIs Zinc Boring?