6. Enzymes Regulation (1)

7/17/2019 6. Enzymes Regulation (1)

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Reginald H. Garrett

Charles M. Grisham

Chapter 15Enzyme Regulation



Outline

• What factors influence enzymatic actiity !• What are the general features of allosteric regulation !• Can allosteric regulation "e e#plained "y

conformational changes in proteins !•

What $inds of coalent modification regulate theactiity of enzymes !• %s the actiity of some enzymes controlled "y "oth

allosteric regulation and coalent modification !

• &pecial focus' is there an e#ample in nature thate#emplifies the relationship "et(een )uaternarystructure and the emergence of allosteric properties !*hemoglo"in and myoglo"in + paradigms of protein

structure and function,.



15.1 + What -actors %nfluence Enzymaticctiity!

• /he aaila"ility of su"strates and cofactors usuallydetermines ho( fast the reaction goes. s productaccumulates0 the apparent rate of the enzymaticreaction (ill decrease due to lac$ of su"strate.

•

Enzyme actiity can "e regulated through coalentmodification.• Enzyme actiity can "e regulated allosterically.• ymogens0 isozymes0 and modulator proteins may

play a role.• Genetic regulation of enzyme synthesis and decay

determines the amount of enzyme present at anymoment.



2eels of Enzyme Regulation

1. Enzyme leel' /he enzyme may "e actiated orinhi"ited "y either noncoalent or coalentinteractions. /his is the most rapid controlsystem.

3. Hormonal leel' hormone is secreted andcarries a message to the cell and in turn anenzyme is actiated or inhi"ited. /he speed ofthis control system is intermediate.

4. Gene leel' message is sent to the nucleuseither e#press or repress a gene. /hisdetermines the amount of enzyme producedand is the slo(est control measure.



Modes of regulation

• ymogens or proenzymes.• Enzyme cascades.•

%sozymes.• llosterism• Coalent modification.• &econd messengers.• Com"ined effects



15.1 + What -actors %nfluence Enzymatic ctiity!

-igure 15.3roinsulin is an 678residueprecursor to insulin

ymogens or proenzymes areinactie precursors of enzymes./ypically0 proteolytic cleaage

produces the actie enzyme.



-igure 15.4 /he proteolytic actiation ofchymotrypsinogen



roteolytic Enzymes of the 9igestie /ract



/he Cascade of ctiation &teps 2eading to:lood Clotting

-igure 15.;/he intrinsic ande#trinsic path(aysconerge at factor <0and the final common

path(ay inoles theactiation of throm"inand its conersion offi"rinogen into fi"rin0

(hich aggregates intoordered filamentousarrays that "ecomecrosslin$ed to formthe clot.



%sozymes re Enzymes With &lightly9ifferent &u"units

-igure 15.5 /heisozymes of lactatedehydrogenase *29H,.



15.3 + What re the General -eatures ofllosteric Regulation!

Action at "another site"

• llosteric enzymes hae )uaternary structure andregulation is at a site other than the actie site.

• Enzymes situated at $ey steps in meta"olicpath(ays are modulated "y allosteric effectors.

• /he effectors are usually produced else(here inthe path(ay *heterotropic,. & or = homotropic.

• Effectors may "e feed8for(ard actiators orfeed"ac$ inhi"itors.

• >inetics are sigmoid *?&8shaped?,0 see MM plot.



>inetics of

allosteric enzymes• Generally these don@t o"ey Michaelis8

Menten $inetics• Homotropic positie effectors produce

sigmoidal *&8shaped, $inetics curesrather than hyper"olae

•

/his reflects the fact that the "inding ofthe first su"strate accelerates "inding ofsecond and later ones

1 3 Wh h G l - f



15.3 + What re the General -eatures of llosteric Regulation!

-igure 15.7 &igmoid ersus A&B plot./he dotted line represents the hyper"olic plot

characteristic of normal Michaelis8Menten $inetics.

15 3 Wh t th G l - t f



15.3 + What re the General -eatures ofllosteric Regulation!

Effects

• positie effector actiates the enzyme *an

actiator,.• negatie effector inhi"its the enzyme *aninhi"itor,.

• ositie cooperatiity increases su"strate "inding

in an adacent su"unit.• Degatie cooperatiity decreases su"strate

"inding in an adacent su"unit.



15.4 llosteric Regulation andConformational Changes in &u"units.

• Monod0 Wyman0 Changeu# *MWC, Model'allosteric proteins can e#ist in t(o states' R*rela#ed, and / *taut or tight,.

• %n this t(o8state model0 all the su"units of anoligomer must "e in the same state *they allchange together, and is therefore termed theconcerted model.

•

/ state predominates in the a"sence ofsu"strate &.• & "inds much tighter to R than to /.



/he Concerted Model for llostericRegulation *MWC,

-igure 15. llosteric effects'

and % "inding to R and /0 respectiely.

C f



-igure 15. llosteric effects' and % "indingto R and /0respectiely.

/he Concerted Model for llostericRegulation

/h C t d M d l f ll t i



-igure 15. llosteric effects' and % "inding to R and /0 respectiely.

/he Concerted Model for llostericRegulation



More a"out the MWC model

• ositie cooperatiity is achieed "ecause &"inding increases the population of R0 (hichincreases the sites aaila"le to &.

• %n the MWC ligands such as & are positiehomotropic effectors.

• Molecules that influence the "inding ofsomething other than themseles are

heterotropic effectors.• /he MWC model does not e#plain negatie

cooperatiity.

/h & ti l M d l f ll t i



•

n alternatie model + proposed "y >oshland0Demethy0 and -ilmer *the KNF model, relies onthe idea that ligand "inding triggers aconformation change in a protein.

•

%n this one8state model0 ligand8inducedconformation changes in one su"unit may lead toconformation changes in adacent su"units.

• /he >D- model e#plains ho( ligand8induced

conformation changes could cause su"units toadopt conformations (ith little affinity for theligand + i.e.0 negative cooperativity.

• /he >D- model is termed the sequential model.

/he &e)uential Model for llostericRegulation *>D-,




-igure 15.6

/he >oshland8Demethy8-ilmerse)uential model for allosteric"ehaior. *a, & "inding can0 "yinduced fit0 cause a conformation

change in the su"unit to (hich it"inds. *", %f su"unit interactionsare tightly coupled0 "inding of & toone su"unit may cause the othersu"unit to assume a conformationhaing a greater or lesser affinityfor &. /hat is0 the ligand8inducedconformational change in onesu"unit can affect the adoining

su"unit.

/he &e)uential Model for llostericRegulation




-igure 15.6/he >oshland8Demethy8-ilmermodel.

/heoretical curesfor the "inding of aligand to a proteinhaing four identicalsu"units0 each (ithone "inding site forthe ligand.

/he &e)uential Model for llostericRegulation



llosteric Models

S S S S S

S

S

S

S

S

S

S

S

S

S S

S

S

SS

MWC: Two state concerted

KNF: ne state sequential

15 ; Co alent Modification Reg late the



15.; Coalent Modification Regulate the ctiity of Enzymes• Enzyme actiity can "e regulated through reversi!le

phosphorylation.• /his is the most prominent form of coalent

modification in cellular regulation.• hosphorylation is accomplished "y protein $inases.• Each protein $inase targets specific proteins for

phosphorylation.• hosphoprotremoving phosphoryl groups "rom

proteins.ein phosphatases catalyze the reersereaction +• >inases and phosphatases themseles are targets of

regulation.

15 ; Coalent Modification Regulate the



15.; Coalent Modification Regulate the ctiity of Enzymes

Enzymes that catalyze phosphate transfer

• >inase'• n enzyme that transfers phosphate to 9 or M

or from /.

• hosphatase or phosphoprotein phosphatase'• n enzyme that hydrolyzes a phosphate off of a

su"strate.• hosphorylase'

• n enzyme that adds a phosphate to su"strate "utdoes not use / to do so.




15.; + Coalent Modification Regulate the ctiity of Enzymes

-igure 15.1 Enzyme regulation "y reversi!le covalent

modi"ication. 9epending on the enzyme0phosphorylation may actiate or inactiate its catalyticfunction.





• rotein $inases phosphorylate &er0 /hr0 and /yrresidues in target proteins.

• >inases typically recognize specific amino acid

se)uences in their targets.• %n spite of this specificity0 all $inases share a

common catalytic mechanism "ased on a conseredcore $inase domain of a"out 37F residues0 -ig. 15..







M> >inase

Human ER>5 *M>, protein domains. DE&1 and DE&30 "ipartitenuclear e#portation signal :18:90 :1 *ho# and :em domain 1,"inding domain >inase 9omain0 catalytic $inase domain T#$%

sequence moti" containing #&K' regulatory phosphorylation

residues( R81 and R830 proline rich domains /ranscriptional trans8actiation0 transcriptional actiity domain. /he ER>5 D8terminus domainresem"les the typical M> catalytic domain and includes the M>8consered /<I actiation se)uence */316EI33F, in the actiation loop. The

activation o" #&K' occurs via interaction with and dualphosphorylation in its T#$ moti" !y MKK' *Mody et al.0 3FF4,. M>>5mediated ER>5 actiation leads to ER>5 autophosphorylation in itsuni)ue C8terminal domain *Morimoto et al.0 3FF,.





-igure 15.rotein $inase is sho(ncomple#ed (ith apseudosu"strate peptide

*orange,.

/his comple# also includes / *red, and t(o Mn3J ions*yello(, "ound at the actie

site.



Cyclic M8dependent protein $inase

-igure 15.1F cyclic M8dependent protein $inase0 also$no(n as protein $inase *>,0 is a 15F8 to 1F8$9 R3C3

tetramer in mammalian cells.

/he t(o R *regulatory, su"units "ind cM cM "indingreleases the R su"units from the t(o C *catalytic, su"units.

C su"units are enzymatically actie as monomers.



Other Coalent Modification of rotein

• &eeral hundred different chemical modificationsof proteins hae "een discoered.

• Only a fe( of these are used to achiee meta"olicregulation through reversible conversion of anenzyme between active and inactive forms.

• fe( are summarized in /a"le 15.4.• /hree of the modifications in /a"le 15.4 re)uire

nucleoside triphosphates */0 K/, that arerelated to cellular energy status.



Other Coalent Modification of rotein

15 5 Enzymes Controlled "y :oth llosteric



15.5 Enzymes Controlled "y :oth llostericand Coalent Regulation!

• Glycogen phosphorylase *G, is an e#ample of the

many enzymes that are regulated "oth "y allostericcontrols and "y coalent modification.

• G uses i to attac$ glucose at an L*18;, lin$ageon the nonreducing ends of glycogen.

• /his conerts glycogen into readily usa"le fuel inthe form of glucose818phosphate.

• /his is a phosphorolysis reaction.

• Muscle G is a dimer of identical su"units0 each(ith 2 coalently lin$ed.

• /here is an allosteric effector site at the su"unitinterface.



Glycogen hosphorylase

-igure 15.11 /he glycogen phosphorylase reaction conertsglycogen into readily usa"le fuel in the form of glucose818./he enzyme cannot cleae an L*187, "ranchpoint.



hosphoglucomutase

-igure 15.13 /he phosphoglucomutase reaction conertsglucose818 into the glycolytic su"strate0 glucose878.



/he structure of glycogen phosphorylase

• Glycogen phosphorylase is a dimer of identical

su"units each haing 6;3 residues.• Each su"unit contains an actie site *at the center of

the su"unit, and an allosteric effector site near thesu"unit interface.

• regulatory phosphorylation site is located at &er1;on each su"unit.

• glycogen8"inding site e#erts regulatory control.•

Each su"unit contri"utes a to(er heli#N *residues373 to 36, to the su"unit8su"unit interface.• %n the dimer0 the to(er helices e#tend from their

respectie su"units and pac$ against each other.



/he structure of glycogen phosphorylase

-igure 15.14/he structure ofglycogenphosphorylase.

One monomer ofthe homodimer.

Glycogen hosphorylase ctiity is



Glycogen hosphorylase ctiity isRegulated llosterically

•

Muscle glycogen phosphorylase sho(scooperatiity in su"strate "inding.• / and glucose878 are allosteric inhi"itors of

glycogen phosphorylase.•

M is an allosteric actiator of glycogenphosphorylase.

• When / and glucose878 are a"undant0glycogen "rea$do(n is inhi"ited.

• When cellular energy reseres are lo( *i.e.0 highAMB and lo( A/B and AG878B, glycogencata"olism is stimulated.

Glycogen hosphorylase ctiity is



Glycogen hosphorylase ctiity isRegulated llosterically

-igure 15.1; ersus & cures for glycogen phosphorylase.*a, /he response to the concentration of the su"strate

phosphate *i,.*", / is a feed"ac$ inhi"itor.*c, M is a positie effector. %t "inds at the same site as /.

Glycogen phosphorylase conforms to the



Glycogen phosphorylase conforms to theMWC model• /he actie form of the enzyme is designated the

R state.• /he inactie form of the enzyme is denoted the /

state.• M promotes the conersion to the actie state.• /0 glucose8780 and caffeine faor conersion to

the inactie / state.• significant conformation change occurs at the

su"unit interface "et(een the / and R state.• /his conformational change at the interface is

lin$ed to a structural change at the actie site thataffects catalysis.

Glycogen hosphorylase is Controlled



Glycogen hosphorylase is Controlled:oth llosterically and Coalently

-igure 15.15/he mechanism of coalentmodification and allosteric

regulation of glycogenphosphorylase.

Favored

Favored

Conformation Change Regulates ctiity



Conformation Change Regulates ctiityof Glycogen hosphorylase

-igure 15.17

/he maor conformationalchange that occurs in theD8terminal residues uponphosphorylation of &er 1;.

&er 1;

is sho(n in red. D8terminal conformation ofphosphorylated enzyme*phosphorylase a,' yello(.

D8terminal conformation ofunphosphorylated enzyme*phosphorylase ",' cyan.

R l ti f G " C l t M difi ti



Regulation of G "y Coalent Modification

• %n 1570 Ed(in >re"s and Edmond -ischersho(ed that a conerting enzyme@ couldconert phosphorylase " to phosphorylase a.

• /hree years later0 >re"s and -ischer sho( thatthis conersion inoles coalentphosphorylation.

• /he enzyme is phosphorylase $inase and this

phosphorylation is mediated "y an enzymecascade *-igure 15.1,.

Glycogen phosphoryase is actiated "y a



Glycogen phosphoryase is actiated "y acascade of reactions

-igure 15.1 /he hormone8actiated enzymatic cascadethat leads to actiation of glycogen phosphorylase.

/h d l l C l R ti



/he denylyl Cyclase Reaction

-igure 15.16 /he adenylyl cyclase reaction. /he reaction isdrien for(ard "y su"se)uent hydrolysis of pyrophosphate "ythe enzyme inorganic pyrophosphatase.

/h d l l C l R ti



/he denylyl Cyclase Reaction

-igure 15.16 /he adenylyl cyclase reaction. /he reaction isdrien for(ard "y su"se)uent hydrolysis of pyrophosphate "ythe enzyme inorganic pyrophosphatase.

*&ho(n here arethe products of thereaction. /0 thereactant0 is sho(n

on the preiousslide.,

M i & d M



cM is a &econd Messenger

• Cyclic M is the intracellular agent ofe#tracellular hormones 8 thus a secondmessenger@.

•

Hormone "inding stimulates a G/8"indingprotein *G protein,0 releasing Gα*G/,.

• :inding of Gα*G/, stimulates adenylyl cyclase to

ma$e cM.

M i & d M



cM is a &econd Messenger

-igure 15.1 Hormone "inding to its receptor leads ia

G8 protein actiation to cM synthesis. denylylcyclase and the hormone receptor are integral mem"raneproteins GL and GβP are mem"rane8anchored proteins.

M d f l ti



Modes of regulation

• ymogens or proenzymes.• Enzyme cascades.• %sozymes.• llosterism• Coalent modification.• &econd messengers.• Com"ined effects

Hemoglo"in



Hemoglo"in

A classic example of allostery

• Hemoglo"in is an o#ygen transport proteinand myoglo"in is an O3 storage protein.

• 2oo$ at the o#ygen "inding cures forhemoglo"in and myoglo"in.

• Myoglo"in is monomeric hemoglo"in istetrameric0 L

3

Q3

.

• M"' 154 aa0 103FF MW.• H"' t(o L chains of 1;1 residues0 3 Q chains

of 1;7 residues.

-igure 15.3F O38"inding cures for



-igure 15.3F O3 "inding cures forhemoglo"in and myoglo"in

Myoglo"in 5F = 3.6 torr and Hemoglo"in 5F = 37 torr.

/he structure of myoglo"in is similar to that



/he structure of myoglo"in is similar to thatof the H" monomer

-igure 15.31/he myoglo"in andhemoglo"in structures.Each is a protein (ith a

heme *aromatic,.

Myoglo"in is monomeric.

Hemoglo"in is tetrameric.

M" and H" use porphryins to "ind -e3J



M" and H" use porphryins to "ind -e3J

-igure 15.33

Heme is formed (hen protoporphyrin %< "inds -e3J

-e3J is coordinated "y His -6




•

%ron interacts (ith si# ligands in H" and M".• -our of these are the D atoms of the porphyrin.• fifth ligand is donated "y the imidazole side

chain of amino acid residue His -6.

• */his residue is on the si#th or -N heli#0 and it isthe 6th residue in the heli#0 thus the name.,.

• When M" or H" "ind o#ygen0 the O3 moleculeadds to the heme iron as the si#th ligand.

• /he O3 molecule is tilted relatie to aperpendicular to the heme plane.





-igure 15.34/he si# ligandingpositions of an

iron atom in H"and M".

Myoglo"in &tructure



Myoglo"in &tructure

• M" is a monomeric heme protein.• M" polypeptide ?cradles? the heme group.• -e in M" is -e3J 8 ferrous iron 8 the form that

"inds o#ygen.• O#idation of -e yields 4J charge 8 ferric iron.• M" (ith -e4J is called metmyoglo!in and does

not "ind o#ygen.

O :inding lters M" Conformation



O3 :inding lters M" Conformation

• %n deo#ymyoglo"in0 the ferrous ion actually liesF.F55 nm a"oe the plane of the heme.

• When o#ygen "inds to -e in heme of M"0 theheme -e is dra(n to(ard the plane of the

porphyrin ring.• With o#ygen "ound0 the -e3J atom is only F.F37

nM a"oe the plane.• -or M"0 this small change has little conse)uence.• :ut a similar change in H" initiates a series of

conformational changes that are transmitted toadacent su"units.

H" Has an L Q /etrameric &tructure



H" Has an L3Q3 /etrameric &tructure

-igure 15.3; n LQ dimer of H"0(ith pac$ingcontacts indicated in

"lue.

/he sliding contactsmade (ith the other

dimer are sho(n inyello(. /he changesin these slidingcontacts are sho(nin -igure 15.35.

Cooperatie :inding of O#ygen %nfluences



p g ygHemoglo"in -unction

•

M"0 an o#ygen storage protein0 has a greateraffinity for o#ygen at all o#ygen pressures.• H" is different + it must "ind o#ygen in lungs and

release it in capillaries.

• H" "ecomes saturated (ith O3 in the lungs0 (herethe partial pressure of O3 is a"out 1FF torr.

• %n capillaries0 pO3 is a"out 4F torr0 and o#ygen is

released from H".• /he "inding of O3 to H" is cooperatie + "inding of

o#ygen to the first su"unit ma$es "inding to theother su"units more faora"le.

O "inding cures of M" and H"



O3 "inding cures of M" and H"

/he o#ygen "inding cure of

M" resem"les anenzyme'su"strate saturationcure.

n lternatie O :inding Cure for H"



n lternatie O3 :inding Cure for H"

O#ygen saturationcure for H" in theform of I ersus pO3

assuming n = ; and

5F

= 37 torr.

I is the fractionalsaturation of H".

4

2

4

2

[ ][ ]

pOY pO K

=

+

n lternatie O :inding Cure for H"



n lternatie O3 :inding Cure for H"

comparison of thee#perimentallyo"sered O3 cure for

H" yielding a alue for

n of 3.60the hypothetical cureif n = ;0and the cure if n = 1*non8interacting O38

"inding sites,.

/he Conformation Change



/he Conformation Change

The secret of Mb and Hb

• O#ygen "inding changes the M" conformation.• Without o#ygen "ound0 -e3J is out of heme plane.

• O#ygen "inding pulls the -e3J into the heme plane.• -e3J pulls its His -6 ligand along (ith it.• /he - heli# moes (hen o#ygen "inds.• /otal moement of -e3J is F.F3 nm + i.e.0 F.3 .• /his change means little to M"0 "ut lots to H"S

O#ygen :inding "y H" %nduces a



yg g yTuaternary &tructure Change

• When deo#y8H" crystals are e#posed to o#ygen0they shatter. /his is eidence of a large8scalestructural change.

• One alpha8"eta pair moes relatie to the other"y 15 degrees upon o#ygen "inding.

• /his massie change is induced "y moement of-e "y F.F4 nm (hen o#ygen "inds.

O#ygen "inding to H" results in a 15U



yg grotation of one LQ pair relatie to the other

-igure 15.35 &u"unit motion in hemoglo"in (hen themolecule goes from the *a, deo#y form to the *", o#y form.

-e3J Moement "y 2ess /han F.F; nm



y%nduces the Conformation Change in H"

• %n deo#y8H"0 the iron atom lies out of the heme

plane "y a"out F.F7 nm.• Kpon O3 "inding0 the -e3J atom moes a"out F.F4

nm closer to the plane of the heme.•

s if the O3 is dra(ing the heme iron into the plane.• /his may seem li$e a triial change0 "ut its

"iological conse)uences are far8reaching.• s -e3J moes0 it drags His -6 and the - heli# (ith

it.• /his change is transmitted to the su"unit interfaces0

(here conformation changes lead to the rupture ofsalt "ridges.

-e3J Moement "y 2ess /han F.F; nm



y%nduces the Conformation Change in H"

-igure 15.37Changes in theposition of the hemeiron atom upon

o#ygenation lead toconformationalchanges in thehemoglo"in

molecule.

&alt "ridges that sta"ilize deo#y8H" are



g y"ro$en in o#y8H"

-igure 15.3&alt "ridges "et(een differentsu"units in human deo#y8H"./hese noncoalent0 electrostatic

interactions are disrupted upono#ygenation.*a, /he salt "ridges and H8"onds

inoling interactions "et(een D8terminal and C8terminal residues

in the L8chains.*", /he salt "ridges and H "onds

inoling C8terminal residues of Q8chains

/he hysiological &ignificance of theH" O3 % t ti



H"'O3 %nteraction

• H" must "e a"le to "ind o#ygen in the lungs.• H" must "e a"le to release o#ygen in capillaries.• %f H" "ehaed li$e M"0 ery little o#ygen (ould

"e released in capillaries 8 see -igure 15.3FS• /he sigmoid0 cooperatie o#ygen "inding cure

of H" ma$es its physiological actions possi"leS• H" e#hi"its properties of "oth the MWC and

>D- models.• %n the / state only the L su"units can "ond O3

and in the R state all su"units can "ind O3.

HJ romotes 9issociation of O#ygen fromH l "i



Hemoglo"in

• :inding of O3 to H" is affected "y seeral agents0

including HJ0 CO30 3048"isphosphoglycerate andchloride ions.

• /he effect of HJ is particularly important. /his isthe :ohr effect.

• 9eo#y8H" has a higher affinity for HJ than o#y8H".• /hus0 as pH decreases0 dissociation of O3 from

hemoglo"in is enhanced.

• %gnoring the stoichiometry of O3 and HJ0 (e can(rite'

HJ romotes 9issociation of O#ygen fromH l "i



Hemoglo"in

-igure 15.36/he o#ygen saturation cures for myoglo"in and for

hemoglo"in at fie different pH alues' .70 .;0.30 .F0 7.6.

/he ntagonism of O3 :inding "y HJ is



/ermed the :ohr Effect

• /he effect of HJ on O3 "inding (as discoered "yChristian :ohr *the father of Deils :ohr0 the atomicphysicist,.

•

:inding of protons diminishes o#ygen "inding.• :inding of o#ygen diminishes proton "inding.• %mportant physiological significance.

• HH"O3 V== H"O3 J HJ p>a = 7.7

• HH" V== H" J HJ p>a = 6.3

-igure 15.3F O38"inding cures forh l "i d l "i



hemoglo"in and myoglo"in

Myoglo"in 5F = 3.6 torr and Hemoglo"in 5F = 37 torr.

CO3 lso romotes the 9issociation of O3 f H l "i



from Hemoglo"in

Carbon dioxide diminishes oxygen binding

1. Hydration of CO3 in tissues and e#tremities leadsto proton production'

• /hese protons are ta$en up "y H" as o#ygen

dissociates.• /he reerse occurs in the lungs.3. CO3 also "inds coalently to the D8terminus. nd

foors the / state.

&ummary of the hysiological Effects of HJ and CO on O :inding "y Hemoglo"in



and CO3 on O3 :inding "y Hemoglo"in

• t the tissue8capillary interface0 CO3 hydration

and glycolysis produce e#tra HJ0 promotingadditional dissociation of O3 (here it is neededmost.

• t the lung8artery interface0 "icar"onatedehydration *re)uired for CO3 e#halation,consumes e#tra HJ0 promoting O3 "inding.

3048:isphosphoglycerate



0 p p g y

An Allosteric Effector of Hemoglobin

• %n the a"sence of 3048:G0 o#ygen "inding toH" follo(s a rectangular hyper"olaS

• /he sigmoid "inding cure is only o"sered inthe presence of 3048:G.

• &ince 3048:G "inds at a site distant from the

-e (here o#ygen "inds0 it is called an allostericeffector.

:G :inding to H" Has %mportanth siological &ignificance



hysiological &ignificance

-igure 15.4F /he structure0 in ionic form of :G or 3048"isphosphoglycerate0 an important allosteric effector of H".

:G :inding to H" Has %mportanthysiological &ignificance




The "inside" story......

• Where does 3048:G "ind !• ?%nside.•

in the central caity of the tetramer.• What is special a"out 3048:G !

• Degatie charges interact (ith 3 2ys0 ; His03 D8termini.

• -etal H" 8 lo(er affinity for 3048:G0 higheraffinity for o#ygen0 so it can get o#ygen frommother.

:G :inding to H" Has %mportanthysiological &ignificance




-igure 15.41/he ionic "inding of:G to the t(o Q8su"units of H". :G

lies at the center of thecaity "et(een the t(oQ8su"units./he resulting

conformational changeprecludes O3 "inding.

CO3 lso romotes the 9issociation of O3 f H l "i



from Hemoglo"in

-igure 15.3O#ygen "inding cures of "loodand of hemoglo"in in thea"sence and presence of CO3

and :G.

-etal Hemoglo"in Has a Higher ffinity forO :ecause it has a 2o(er ffinity for :G



O3 :ecause it has a 2o(er ffinity for :G

• /he fetus depends on its mother for O30 "ut its

circulatory system is entirely independent.• Gas e#change ta$es place across the placenta.• -etal H" differs from adult H" + (ith P8chains in

place of Q8chains + and thus a L3P3 structure.• s a result0 fetal H" has a higher affinity for O3.

• Why does fetal H" "ind O3 more tightly !

•

-etal P8chains hae &er instead of His at position1;4 and thus lac$ t(o of the positie charges inthe :G "inding caity

• :G "inds less tightly and H" - thus loo$s more

li$e M" in its O3 "inding "ehaior.

-etal Hemoglo"in Has a Higher ffinity forO :ecause it has a 2o(er ffinity for :G



O3 :ecause it has a 2o(er ffinity for :G

-igure 15.43 Comparison of the o#ygen saturation cures

of H" and H" - under similar conditions of pH and A:GB.

&ic$le8Cell nemia is a Molecular 9isease



• &ic$le8cell anemia patients hae a"normally8

shaped red "lood cells.• /he erythrocytes are crescent8shaped instead of

disc8shaped.•

/he sic$le cells pass less freely through thecapillaries0 impairing circulation and causing tissuedamage.

• single amino acid su"stitution in the Q8chains of

H" causes sic$le8cell anemia.• Glu at position 7 of the Q8chains is replaced "y Xal.• s a result0 H" & molecules aggregate into long0

chainli$e polymeric structures.

Hemoglo"in and Ditric O#ide



g

•

Ditric o#ide *DOY, is a simple gaseous moleculethat acts as a neurotransmitter and as a secondmessenger in signal transduction *see Chapter43,.

• DOY is a high8affinity ligand for H"0 "inding to theheme iron 1F0FFF times more tightly than O3.

• &o (hy is DOY not "ound instantaneously to H"0

preenting its physiological effects !• DOY reacts (ith the +&H of Cys4Q0 forming an &8

nitroso deriatie'

Hemoglo"in and Ditric O#ide



g

• /he &8nitroso group is in e)uili"rium (ith other &8

nitroso compounds formed "y reaction of nitrico#ide (ith small8molecule thiols such as free Cysor glutathione'

• /hese small8molecule thiols transfer DOY fromerythrocytes to endothelial receptors0 (here ite#erts its physiological effects.



End Chapter 15Enzyme Regulation

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6. Enzymes Regulation (1)