Structure and function of Structure and function of hemoglobinhemoglobin

GIT | 1 Lecture | Dr. Usman GhaniGIT | 1 Lecture | Dr. Usman Ghani

Hemoglobin (Hb)Hemoglobin (Hb)

A hemeprotein found only in red blood cellsA hemeprotein found only in red blood cells Oxygen transport functionOxygen transport function Contains heme as prosthetic groupContains heme as prosthetic group Heme reversibly binds to oxygenHeme reversibly binds to oxygen

The heme groupThe heme group A complex of protoporphyrin IX and ferrous A complex of protoporphyrin IX and ferrous

iron (Feiron (Fe2+2+)) FeFe2+2+ is present in the center of the heme is present in the center of the heme FeFe2+ 2+ binds to four nitrogen atoms of the binds to four nitrogen atoms of the

porphyrin ringporphyrin ring Forms two additional bonds with:Forms two additional bonds with:

Histidine residue of globin chainHistidine residue of globin chain OxygenOxygen

The heme group: Fe2+– porphyrin complex with bound O2

Types of HbTypes of HbNormal: HbA (97%)

HbA2 (2%)

HbF (1%)HbA1c

Abnormal:Abnormal: Carboxy HbMet HbSulf Hb

Hemoglobin A (HbA)Hemoglobin A (HbA) Major Hb in adultsMajor Hb in adults Composed of four polypetide chains:Composed of four polypetide chains:

Two Two αα and two and two ββ chains chains Contains two dimers of Contains two dimers of subunits subunits Held together by non-covalent interactionsHeld together by non-covalent interactions Each chain is a subunit with a heme group in the center Each chain is a subunit with a heme group in the center

that carries oxygenthat carries oxygen A Hb molecule contains 4 heme groups and carries 4 A Hb molecule contains 4 heme groups and carries 4

moelcules of Omoelcules of O22

HbA structureHbA structure

T-form of HbT-form of Hb

The deoxy form of HbThe deoxy form of Hb Taut formTaut form The movement of dimers is The movement of dimers is

constrainedconstrained Low-oxygen-affinity formLow-oxygen-affinity form

R-form of HbR-form of Hb

The oxygenated form of HbThe oxygenated form of Hb Relaxed formRelaxed form The dimers have more The dimers have more

freedom of movementfreedom of movement High-oxygen-affinity formHigh-oxygen-affinity form

Hemoglobin functionHemoglobin function

Carries oxygen from the lungs to tissuesCarries oxygen from the lungs to tissues Carries carbon dioxide from tissues back to Carries carbon dioxide from tissues back to

the lungsthe lungs Normal level (g/dL):Normal level (g/dL):

• Males: 14-16Males: 14-16• Females: 13-15Females: 13-15

Factors affecting oxygen bindingFactors affecting oxygen binding

Three allosteric effectors:Three allosteric effectors: pOpO22 (partial oxygen pressure) (partial oxygen pressure) pH of the environmentpH of the environment pCOpCO2 2 (partial carbon dioxide pressure)(partial carbon dioxide pressure) Availability of 2,3-bisphosphoglycerateAvailability of 2,3-bisphosphoglycerate

Oxygen Dissociation CurveOxygen Dissociation Curve The curve is sigmoidalThe curve is sigmoidal Indicates cooperation of Indicates cooperation of

subunits in Osubunits in O22 binding binding Binding of OBinding of O22 to one heme to one heme

group increases Ogroup increases O22 affinity affinity of othersof others

Heme-heme interactionHeme-heme interaction

PP5050

Indicates affinity of Hb to OIndicates affinity of Hb to O22 PP5050(mm Hg): the pressure at which Hb is 50% (mm Hg): the pressure at which Hb is 50%

saturated with Osaturated with O22 High affinity High affinity slow unloading of O slow unloading of O22 Low affinity Low affinity fast unloading of O fast unloading of O22 Lung pOLung pO22 is 100 mm is 100 mm Hb saturation 100% Hb saturation 100% Tissue pOTissue pO22 is 40 mm is 40 mm Hb saturation reduces Hb saturation reduces Hence OHence O22 is delivered to tissues is delivered to tissues

The Bohr effectThe Bohr effect Effect of pH and pCOEffect of pH and pCO22 on: on:

Oxygenation of Hb in the Oxygenation of Hb in the lungslungs

Deoxygenation in tissuesDeoxygenation in tissues Tissues have lower pH (acidic) Tissues have lower pH (acidic)

than lungsthan lungs Due to proton generation:Due to proton generation:

COCO22 + H + H220 0 HCO HCO33-- + H + H++

Protons reduce OProtons reduce O22 affinity of affinity of HbHb

The Bohr Effect The Bohr Effect Causing easier OCausing easier O22 release into the tissues release into the tissues The free Hb binds to two protonsThe free Hb binds to two protons Protons are released and react with HCOProtons are released and react with HCO33 –– to form CO to form CO22 gas ( gas (HCOHCO33

-- + H + H++COCO22 + H + H220 0 ))

The proton-poor Hb now has greater affinity for OThe proton-poor Hb now has greater affinity for O2 2 (in lungs)(in lungs) The Bohr effect removes insoluble COThe Bohr effect removes insoluble CO22 from blood stream from blood stream Produces soluble bicarbonateProduces soluble bicarbonate

Availability of 2,3 bisphosphoglycerateAvailability of 2,3 bisphosphoglycerate Binds to deoxy-hb and stabilizes the T-formBinds to deoxy-hb and stabilizes the T-form When oxygen binds to Hb, BPG is releasedWhen oxygen binds to Hb, BPG is released

At high altitudes:-RBC number increases-Hb conc. increases-BPG increases

High altitude and OHigh altitude and O22 affinity affinity

In hypoxia and high altitudeIn hypoxia and high altitude 2,3 BPG levels rise2,3 BPG levels rise This decreases OThis decreases O2 2 affinity of Hbaffinity of Hb Thus increases OThus increases O22 delivery to tissues delivery to tissues

High OHigh O22 affinity affinity

High OHigh O22 affinity is due to: affinity is due to: AlkalosisAlkalosis High levels of Hb FHigh levels of Hb F Multiple transfusion of 2,3 DPG-depleted Multiple transfusion of 2,3 DPG-depleted

bloodblood

Low affinity to O2

High affinity to O2

Fetal Hemoglobin (HbF)Fetal Hemoglobin (HbF) Major hemoglobin found in the fetus and Major hemoglobin found in the fetus and

newbornnewborn Tetramer with two Tetramer with two and two and two chains chains Higher affinity for OHigher affinity for O22 than HBA than HBA Transfers OTransfers O22 from maternal to fetal circulation from maternal to fetal circulation

across placentaacross placenta

HbAHbA22

Appears ~12 weeks after birthAppears ~12 weeks after birth Constitutes ~2% of total HbConstitutes ~2% of total Hb Composed of two Composed of two and two and two globin chains globin chains

HbAHbA1c1c

HbA undergoes non-HbA undergoes non-enzymatic glycosylationenzymatic glycosylation

Glycosylation depends on Glycosylation depends on plasma glucose levelsplasma glucose levels

HbA1c levels are high in HbA1c levels are high in patients with diabetes patients with diabetes mellitusmellitus

Abnormal HbsAbnormal Hbs

Unable to transport OUnable to transport O22 due to abnormal due to abnormal structurestructure

Carboxy-Hb: CO replaces OCarboxy-Hb: CO replaces O22 and binds 200X and binds 200X tighter than Otighter than O2 2 (in smokers)(in smokers)

Met-Hb: Contains oxidized FeMet-Hb: Contains oxidized Fe3+3+ (~2%) that (~2%) that cannot carry Ocannot carry O22

Sulf-HB: Forms due to high sulfur levels in Sulf-HB: Forms due to high sulfur levels in blood (irreversible reaction)blood (irreversible reaction)