Structure, function, and metabolism of hemoglobin

Structure of hemoglobinStructure, function, andmetabolism of hemoglobinVladimíra Kvasnicová• hemoprotein (complex protein: globin + prosthetic group)• quaternary structure: 4 subunits• prosthetic group of each of the subunit = heme4 polypetide chains4 molecules of heme4 ferrous (Fe 2+ ) ionsHEMEMYOGLOBINhemoglobinM r = 64 500The figure is found at http://dtc.pima.edu/~biology/202alpha/lesson1/hemoglobin.jpg (March 2007)• it has not a quarternary structure: only 1 polypeptide chain• found in muscles: binds O 2 „for storrage“• higher affinity to oxygen than hemoglobinThe figures are found at http://www.virtuallaboratory.net/Biofundamentals/lectureNotes/AllGraphics/myoglobinSurface.jpgand http://courses.washington.edu/conj/protein/hemo.gif (March 2007)

Types of hemoglobin and its subunits• adult hemoglobin:HbA 1 = α 2 β 2HbA 2 = α 2 δ 2(∼ 2% from total Hb of adults)• fetal hemoglobinHbF = α 2 γ 2 ! higher affinity to O 2 than HbA !binds oxygen more firmly at lower pO 2 (placenta!)The figure is found at http://www.labcorp.com/datasets/labcorp/html/img/fethgb.jpg (March 2007)Structure of hemePyrrole• cyclic tetrapyrrole• the pyrrols has different substituents• belongs among porphyrins (heme = Fe-protoporphyrine IX)• it contains: conjugated double bonds → red color 4 nitrogen atoms (N) 1 ferrous ion (Fe 2+ )→ in the middle of the tetrapyrrole structureby coordination-covalent bondshemoglobinThe figures are found at http://www.medical-definitions.net/images/hemoglobin.jpgand http://omlc.bme.ogi.edu/spectra/hemoglobin/hemestruct/heme-struct.gif (March 2007)

PyrroleQiuz1. What is the concentration of Hbin blood?2. Describe the structure of Hb3. Where is oxygen bound into Hb?4. How many O 2 can be bound to Hb?5. Draw the saturation curve of HbThe figures are found at http://www.medical-definitions.net/images/hemoglobin.jpgand http://omlc.bme.ogi.edu/spectra/hemoglobin/hemestruct/heme-struct.gif (March 2007)Synthesis of hemoglobin• bone marrow• in erytroblasts, not in erythrocytes• 4 individual subunits are connected bynoncovalent bonds to form tetramer of Hb• hemoglobin is an intracellular protein: within eryconcentration of Hb in blood:femalemale120 – 162 g/l135 – 172 g/l

Disorders:Synthesis of hemoglobin• THALASSEMIA = group of genetically determineddisorders: absence or reduced synthesis of a globin chain(α or β thalassemia)• ANEMIA (= decreased oxygen-carrier capacity of blood) sideropenic anemia – insufficient concentration of Fe sickle cell anemia – point mutationin the β-globin gene forms abnormalHbS (Glu → Val)Synthesis of heme - REPETITION• mainly in the bone marrow (Hb) and in the liver(cytochroms)• mitochondria / cytoplasm / mitochondria• substrates: succinyl-CoA + glycine• important intermediates: δ-aminolevulinic acid (= 5-aminolevulinic, ALA) porphobilinogen (PBG = pyrrol derivative) uroporphyrinogen III (= 1 st porphyrinogen – precursor of heme) protophorphyrine IX (= direct precursor of heme)Synthesis of heme - regulationALA-synthase the key regulatory enzyme in all tissues pyridoxal phosphate dependent (vit. B6)ALA-synthase 1 (liver) inhibited by heme (feed back inhibition) regulation of transcription and by allosteric mechanismThe figure is from: Color Atlas of Biochemistry / J. Koolman, K.H.Röhm. Thieme 1996. ISBN 0-86577-584-2 some drugs ↑ amount of ALA-synthase (↓ conc. of heme)ALA-synthase 2 (erythroblasts) neither feed back inhibition nor induction by drugs regulated on the level of iron availability

Disorders of hemesynthesisPORPHYRIAS• inborn or acquired• classification by defect enzyme• accumulation of heme precursors in the body(skin) and their excretion with urine or feaces(dark color)• neurogical symptomps, photosensitivitylead poisoning – accumulation of ALA (blood, urine)(inhibition of porphobilinogen synthase)Degradation of hemoglobin• cells of reticulo-endothelial system (RES) ofspleen, bone marrow, liver, and skin• Hb released from erythrocytes in blood vesels isbound by haptoglobin→RES• free heme is transported by hemopexinHEMOGLOBIN → 4x globin + 4x heme• globins chains → amino acids• heme→Fe 3+ + CO + biliverdin→bile pigments→feacesQiuz1. Where is Hb synthesized?2. What failures of Hb synthesis do youknow?3. What substrates are needed for thesynthesis of heme?4. What is the source of iron for thesynthesis of heme?5. What is the cause of jaundice during anexcessive degradation of erytrocytes?The figure is from: Color Atlas of Biochemistry / J. Koolman, K.H.Röhm. Thieme 1996. ISBN 0-86577-584-2

Transport of blood gasesAir composition:78% N 2 21% O 2 1% water, inert gases, CO 2 (0,04%)Air pressure:1 atm = 101 325 Pa (~ 101 kPa) = 760 Torr (= mmHg)1 mmHg = 0,1333 kPa1 kPa = 7,5 mmHgTransport of blood gasesarterial blood venose bloodpO 2 13,33 kPa 5,33 kPa100 mmHg 40 mmHgpCO 2 5,33 kPa 6,13 kPa40 mmHg 46 mmHg(alveols)Transport of blood gases- function of hemoglobin -• it transports O 2 and part of CO 2 (and CO)• it binds H + (reacts as a buffer)• O 2 and CO: bound to Fe 2+ in heme → 4 O 2 / 1 Hb„oxyhemoglobin“ HbO 2 /„carbonylhemoglobin“ COHb• CO 2 is bound to globin! (-NH 2 of side chains of amino acids)„carbaminohemoglobin“ HbCO 2The figure is found at http://people.eku.edu/ritchisong/RITCHISO//301notes6.htm (March 2007)• H + is bound to residues of His„deoxyhemoglobin“ HHb

Transport of blood gases- transport of CO 2 -1. largely in a form of HCO 3- (~ 70%)CO 2 + H 2 O ↔ H 2 CO 3 ↔ HCO 3- + H +enzyme: carbonic anhydrase spontaneous dissociation(in erytrocytes)2. bound to hemoglobin (~ 23%)3. freely dissolved (~ 7%)The figure is found at http://fig.cox.miami.edu/~cmallery/150/physiol/sf41x11.jpg (March 2007)Transport of blood gases- reactions in erytrocytes -O 2tissues:CO 2 + H 2 O → H 2 CO 3 → HCO 3- + H +H + + HbO 2 → HHb + O 2 → aerobic metabolismlungs:HHb + O 2 → HbO 2 + H +O 2H + + HCO 3- → H 2 CO 3 → H 2 O + CO 2 → excretedThe figure is from http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecPhysio/42-29-BloodCO2Transport-AL.gif (March 07)

Hemoglobin saturation curve- saturation with oxygen -The figure is found at http://employees.csbsju.edu/hjakubowski/classes/ch331/bind/MbHbbindcurve.gif(March 2007) The figure is found at http://dr-amy.com/rich/oxygen/fig1.gif (March 2007)HbF is left-shifted(it has higher affinity to oxygen)The figure is found at http://dr-amy.com/rich/oxygen/fig1.gif (March 2007)The figure is found at http://www.biocrawler.com/encyclopedia/Fetal_hemoglobin (March 2007)

Saturation of hemoglobin by oxygen• quaternary structure of hemoglobinallosteric effectT-conformation: lower affinity to O 2 (deoxy Hb)R-conformation: higher affinity to O 2 (oxyHb)T ↔ RHb + O 2 ↔ HbO 2The figure is found at http://employees.csbsju.edu/hjakubowski/classes/ch331/bind/MbHbbindcurve.gif (March 2007)Saturation of hemoglobin with oxygenFactors affecting the saturation: alkaline pH and ↑ pO 2 stabilize R-conformation(IN LUNGS) acidic pH, ↑ pCO 2 , ↑ temperature and 2,3-BPGstabilize T-conformation, i.e. deoxyHb(IN PERIPHERY)The animation is found at http://en.wikipedia.org/wiki/Image:Hb-animation2.gif (March 2007)shift of the saturation curve toward right

Bohr´s effect= the saturation of Hb by O 2 drops because lowering pHThe figure is found at http://employees.csbsju.edu/hjakubowski/classes/ch331/bind/MbHbbindcurve.gif(March 2007)The figure is found at http://www.nd.edu/~aseriann/dpg.html (March 2007)Qiuz1. What is the % proportion of O 2and CO 2 in air?2. What is pO 2 in arterial blood?3. What is pCO 2 in arterial blood?4. How is CO 2 transported in thehuman body?5. Summarize factors decreasingthe affinity of Hb to oxygenPatological forms of hemoglobin1. methemoglobin (over 3%) metHb Fe 3+ instad of Fe 2+ unable to transport oxygen !!!2. glycohemoglobin (over 6%) HbA 1c after long term increased glycemia3. carbonylhemoglobin (over 2%) COHb after CO poisoning4. sulfhemoglobin, cyanhemoglobin poisoning by H 2 S, HCN or by cyanides

QiuzCarbon monoxide poisoning1. Compare the fetal and adult Hb2. What is methemoglobin?3. What is glycohemoglobin?4. What is carbonylhemoglobin?5. What is carboxyhemoglobin?• CO has 200x higher affinity to Hb than O 2• it forms COHb = carbonyl hemoglobin(formerly called carboxyhemoglobin)• max. allowed concentration in the air: 0.003%• intoxication by CO depends on pCO and a timeof its exposition (0.04% ∼ strong headache, 2-3 hours:unconsciousness; 1% ∼ death after a few minutes)CO bindsto Fe 2+ instead ofoxygenThe figure is found at http://www.orthosmoke.org/index.php/pt/Carbon%20Monoxide (March 2007)The figure is found at http://dr-amy.com/rich/oxygen/fig1.gif (March 2007)

Carbon monoxide poisoningmay result due to:• exposure to automobile exhaust• smoke inhalation• an improperly ventilated gas heater• or other appliance (incomplete burning)CONSEQUENCESCarbon monoxide poisoning• decreased oxygen-carrying capacity of Hb• decreased delivery of oxygen to cells CO prevents reversible displacement of O 2 on Hb CO shifts the O 2 -hemoglobin dissociation curveto the left CO inhibits the intracellular respiration CO may bind directly to cardiac and skeletal muscleto cause direct toxicity and to components of thenervous system to cause demyelination and neurologicsymptomsSaturation ofhemoglobinwith COCOHb / total Hb(ratio in %)physiological value:< 2%„cherry red coloration to the skin“The figure is found at http://www.acsu.buffalo.edu/~lcscott/carbonmonoxide.html (March 2007)The figure is found athttp://www.uhseast.com/134221.cfm(March 2007)

QiuzDescribe the first aid in case of anintoxication of a person bycarbon monoxideThe figure is from http://www.coheadquarters.com/CORisk/figco32x.htm (March 2007)Carbon monoxide poisoningTREATEMENT• fresh air• exposure to high concentrations of oxygen(the 100% oxygen is administered by a face mask) it is recommended in patients who have a historyof loss of consciousness, carbonyl hemoglobinsaturation greater than 25%, metabolic acidosisand cerebellar findings on neurologic exam