Myoglobin& Hemoglobin · 2017. 2. 14. · Oxygen Dissociation Curve •A plot of % of degree of...
Transcript of Myoglobin& Hemoglobin · 2017. 2. 14. · Oxygen Dissociation Curve •A plot of % of degree of...
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