Myoglobin &Hemoglobin

Dr. Sameh Sarray Hlaoui

Lecture5

Myoglobin andHemoglobin

-Myoglobin and Hemoglobin are hemeproteins(metalloprotein containing a heme prosthetic group).

- Function as transporters of oxygen .

-Myoglobin (monomeric) binds 1 oxygen molecule.

-Hemoglobin is tetrameric (4 subunits) composed of 2a and 2b globinchains, held together by non covalent interactions, and binds 4 moleculesof oxygen.

Myoglobin

• foundinthe muscletissue

• Amonomer thatfoldsinto8separateright-handeda-helices(AthroughH)connectedbyshortnonhelicalregions.

• Itcontains1heme group

• Mostofthehydrophobicaminoacidsarelocated

inthecore(insidetheprotein),whilehydropholic

aminoacidsareonthesurfaceexceptfortwo

histidines sidechains,oneoneachsideofheme:

His93(F8)closetotheheme andbindstheiron andHis

64(E7)Theotherisfaraway

Myoglobin structure

The 'hole' between the iron heme and Histidine E7 is where the oxygen (O2) binds,to form oxymyoglobin

• Foundexclusively inRedbloodcells(RBCs)

• Itisatetramer composedof– Twoα-subunits andtwo β-subunits(globins)– Structure?– Eachglobin contains1hemegroupwithacentralFe2+ion(ferrous

ion)

Hemoglobin

Quaternarystructure

• Hb exist in 2 different forms: T-form and R-form

• T-form (T=“tense”) has a much lower affinity to O2 than the R-form.

• The subunits of Hb are held together by non-covalent bonds. The binding of the firstO2 molecule to subunit of the T-form leads to local conformational change thatweakens the association between the subunits R-form (“Relaxed).

• Cooperative process: The binding of the first O2 to Hb enhances the binding offurther O2molecules until all 4 are saturated Allosteric effect (Defined aschange in the shape and activity of an enzyme, resulting from binding of aregulatory substance).

Hemoglobin

OxygenDissociationCurve

•Aplotof%ofdegreeofsaturationmeasuredatdifferentpartialpressureofO2(PO2)is

called: oxygen dissociation curve

• Myoglobin:hyperbolicshape:myoglobin takeupO2atalowerpressurethanHb.IthasahigheraffinityforO2atallO2levelscomparedwithHb (lowO2butsaturationishigh)

•Hemoglobin: sigmoidalshape(S-shape)showsthatsubunitsco-operateinbindingofO2:when1subunithasboundO2theothersubunitshasgreateraffinity.

OxygenDissociationCurves100

80

60

40

20

60 804020 100

PO2 mmHg

Saturatio

n(%

)

Hb

Mb

lungs

tissues

• P50 is thepartialpressureofoxygeninthebloodatwhichthehemoglobinis50%saturated .

• TheP50forMbis1mmHgand26mmHgforHb

AgentsthatinfluenceO2 binding

•Theseregulatorysubstancesarecalledallostericeffectors:

- pCO2

- pHoftheenvironment,

- 2,3biphosphoglycerate (2,3-BPG)

IsMyoglobin isinfluencedbyallosteric effect? NO

TheBohrEffect

DiscoveredbyChristianBohr

ThechangeinoxygenaffinitywithpHisknownastheBohreffect

TheBohrEffect

-When pH is low: decrease in O2 affinity to Hb

stabilize “T” form

and the curve shift to the right

- Conversely, High pH will increase the affinity and stabilize the “R” form

Carbondioxidediminishesoxygenbinding

§ TissuesproduceCO2 thatreactswithwatertoformtheweakcarbonicacid

§ Carbonicaciddissociates spontanouslyandgenerateprotons(H+)(pH)andbicarbonate(majorbloodbuffer)CO2 +H2O⇄ H2CO3 ⇄ HCO3

- +H+

§ TheseprotonsaretakenupbyHbO2releasingoxygen

HbO2 +H+ ⇄ HbH +O2

§ Shiftofthedissociationcurvetorightby­ PCO2toenhanceoxygenationofbloodandthereleaseofO2inthetissues

¯ inH2CO3 concà H+conc ¯ )à ShiftofO2-Hbcurvetoleft&moreavidbindingofO2toHbà ­ inquantityofO2boundtoHbà ­ O2transporttotissues.

§ Thereversemechanismoccursinthelungs:

- whereO2 concentrationishigh and

- CO2 concentrationislow

• 2,3-bisphosphoglycerate (2,3-BPG), generated from glucosedegradation in RBC.

• 2,3-BPG only binds to deoxyhemoglobin deoxyHb is thusstabilized.

2,3-Bisphosphoglycerate

Thebindingof2,3-bisphosphoglyceratetotheβ-subunits ofdeoxyhemoglobin

Oxyhemoglobin DissociationCurves

IncreasesinCO2,H+ orBPG shiftthecurvetotheright,i.e.decreasetheaffinityforoxygen

(pH)

100

80

60

40

20

10080604020

HbO

2( %

)

PO2 (mmHg)

˜ 2,3 BPG˜ H+

˜ PCO2

—2,3 BPG—H+

—PCO2(Bohr effect)

100

80

60

40

20

10080604020

HbO

2( %

)

PO2 (mmHg)

˜ 2,3 BPG˜ H+

˜ PCO2

—2,3 BPG—H+

—PCO2(Bohr effect)

HumanHemoglobins

-HbA orHbA1:(2a and 2b), majorhemoglobininhuman-HbA2:(2a and 2g), minorcomponentofnormaladultHb (2%)-HbF:(2a and 2g chains), synthesizedonlyduringfetaldevelopment-HBA1c:(2a and 2b-glucose),aglucoseresidueisattachedtotheb-chain;increaseamountinRBCofdiabetesmellituspatient

Fetal Hemoglobin (Hb F)• Inthefetustheβ-subunitofhemoglobinisreplacedwithag-

subunit.Inchildrenandadults,HbF isreplacedmainlybyHbA (α2β2)

andasmallamountofHbA2 (α2d2).

• Adulthemoglobin(Hb A)canbindto2,3BPG,butHb Fcannot

thereforeHb FhashigheraffinityforO2thanthemother,sooxygen

istransferredtothefetus

Glycohemoglobin(HbA1c)

• Isformedspontaneously(bynonenzymaticreaction)withglucose.

• PeoplewithDMhavemoreHbA1c THANNORMAL(>7%).

• MeasurementofHbA1cinbloodgivesanindicationofthelevelofglucoseoverthepast120days(RBClifespan)

HEMOGLOBINOPATHIES(mutationsinhemoglobin)

sicklecelldisease• It’s ageneticdisordercausedbymutationinb-globin chain.

• Themutationoccursinthe6th positionofb-chainwhereGlutamic acid isreplacedbyValine,(Anegativechargewillbereplacedbyahydrophobicgroupthatwillappearonthesurface).

• Valine residuesaggregatetogetherbyhydrophobicinteractionsratherthanwiththecellularenvironmentofredbloodcells(similartowater)leadingtoprecipitationofHb withinRBCs.Andthen

• RBCsassumeasickle shapedleadingtofragilityoftheirwallsandhighrateofhemolysis

ThalassemiaAgroupofgeneticdiseasesinwhichadefectoccurintherateofsynthesisofoneormoreHbchains butthechainsarestructurallynormal.

Thisisduetoadefectorabsenceofoneormoreofgenesresponsibleofa andb chainsleadingtoprematuredeathofRBCs.

b-thalassemia:whensynthesisofb-chainsisdecreasedorabsent.Individualswithb-globingenedefectshaveeither:

- b-thalassemia minor(b-thalassemia trait):whenthesynthesisofonlyoneb-globingeneisdefectiveorabsent.Thosepersonmakesomeb chainsandusuallydon’tneedspecifictreatment.

- b-thalassemia major:whenbothgenesaredefective.

a-thalassemia:inwhichsynthesisofa globin chainisdefectiveorabsent.Thereare4copiesofgeneresponsibleforsynthesisofaglobin chainssopatientsmayhave:

- silentcarrierofa-thalassemia withnosymptoms:ifonegeneisdefective- a-thalassemia trait:if2genesaredefective.Minoranemiapresent- HbH disease:if3genesaredefectives.Moderateanemiapresent- Hydrops fetalis:lacks all4genes.Fetusmaysurvivetillbirththendies.

THE END!

N

N

N

N

CH3 HC

CH3

S CH2

CH3

CH S CH2

CH3

CH2

CH2

COO-

CH3

H3C

CH2CH2-OOC

protein

protein

Fe

Heme c

Heme isa cofactor consistingofanFe2+ (ferrous) ion containedinthecentreofa heterocyclic macrocycle organiccompound calleda porphyrin,madeupoffour pyrrolic groupsjoinedtogetherby methine bridges