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Structure, function, and
metabolism of hemoglobin
Vladimíra Kvasnicová
Structure of hemoglobin
• hemoprotein
(complex protein: globin + prosthetic group)
• quaternary structure: 4 subunits
• prosthetic group of each of the subunit = heme
4 polypetide chains
4 molecules of heme
4 ferrous (Fe2+) ions
Mr = 64 500
The figure is found at http://dtc.pima.edu/~biology/202alpha/lesson1/hemoglobin.jpg (March 2007)
HEME
MYOGLOBIN
hemoglobin
• it has not a quarternary structure: only 1 polypeptide chain
• found in muscles: binds O2 „for storrage“
• higher affinity to oxygen than hemoglobin
The figures are found at http://www.virtuallaboratory.net/Biofundamentals/lectureNotes/AllGraphics/myoglobinSurface.jpg
and http://courses.washington.edu/conj/protein/hemo.gif (March 2007)
Types of hemoglobin and its subunits
• adult hemoglobin:
HbA1 = 22
HbA2 = 22
( 2% from total Hb of adults)
• fetal hemoglobin
HbF = 22
! higher affinity to O2 than HbA !
binds oxygen more firmly at lower pO2 (placenta!)
The figure is found at http://www.labcorp.com/datasets/labcorp/html/img/fethgb.jpg (March 2007)
Structure of heme
• 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 (Fe2+)
→ in the middle of the tetrapyrrole structure
by coordination-covalent bonds
Pyrrole
hemoglobin
The figures are found at http://www.medical-definitions.net/images/hemoglobin.jpg
and http://omlc.bme.ogi.edu/spectra/hemoglobin/hemestruct/heme-struct.gif (March 2007)
Pyrrole
The figures are found at http://www.medical-definitions.net/images/hemoglobin.jpg
and 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 by
noncovalent bonds to form tetramer of Hb
• hemoglobin is an intracellular protein: within ery
concentration of Hb in blood:
female
120 – 162 g/l
male
135 – 172 g/l
Synthesis of hemoglobin
Disorders:
• THALASSEMIA = group of genetically determined
disorders: absence or reduced synthesis of a globin chain
( or  thalassemia)
• ANEMIA
(= decreased oxygen-carrier capacity of blood)
 sideropenic anemia – insufficient concentration of Fe
 sickle cell anemia – point mutation
in the -globin gene forms abnormal
HbS (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:
 -aminolevulinic acid (= 5-aminolevulinic, ALA)
 porphobilinogen (PBG = pyrrol derivative)
 uroporphyrinogen III (= 1st porphyrinogen – precursor of heme)
 protophorphyrine IX (= direct precursor of heme)
The figure is from: Color Atlas of Biochemistry / J. Koolman, K.H.Röhm. Thieme 1996. ISBN 0-86577-584-2
Synthesis of heme - regulation
ALA-synthase
 the key regulatory enzyme in all tissues
 pyridoxal phosphate dependent
ALA-synthase 1 (liver)
 inhibited by heme (feed back inhibition)
 regulation of transcription and by allosteric mechanism
 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 heme
synthesis
PORPHYRIAS
• 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, photosensitivity
lead poisoning – accumulation of ALA (blood, urine)
(inhibition of porphobilinogen synthase)
Degradation of hemoglobin
• cells of reticulo-endothelial system (RES) of
spleen, bone marrow, liver, and skin
• Hb released from erythrocytes in blood vesels is
bound by haptoglobin → RES
• free heme is transported by hemopexin
HEMOGLOBIN → 4x globin + 4x heme
• globins chains → amino acids
• heme → Fe3+ + CO + biliverdin→bile pigments→feaces
The figure is from: Color Atlas of Biochemistry / J. Koolman, K.H.Röhm. Thieme 1996. ISBN 0-86577-584-2
Transport of blood gases
Air composition:
78% N2
21% O2
1% water, inert gases, CO2 (0,04%)
Air pressure:
1 atm = 101 325 Pa (~ 101 kPa) = 760 Torr (= mmHg)
1 mmHg = 0,1333 kPa
1 kPa = 7,5 mmHg
Transport of blood gases
pO2
pCO2
arterial blood
venose blood
13,33 kPa
5,33 kPa
100 mmHg
40 mmHg
5,33 kPa
6,13 kPa
40 mmHg
46 mmHg
(alveols)
The figure is found at http://people.eku.edu/ritchisong/RITCHISO//301notes6.htm (March 2007)
Transport of blood gases
- function of hemoglobin -
• it transports O2 and part of CO2 (and CO)
• it binds H+ (reacts as a buffer)
• O2 and CO: bound to Fe2+ in heme → 4 O2 / 1 Hb
„oxyhemoglobin“ HbO2 /„carbonylhemoglobin“ COHb
• CO2 is bound to globin!
(-NH2 of side chains of amino acids)
„carbaminohemoglobin“ HbCO2
• H+ is bound to residues of His
„deoxyhemoglobin“ HHb
Transport of blood gases
- transport of CO2 -
1. largely in a form of HCO3- (~ 70%)
CO2 + H2O  H2CO3  HCO3- + H+
enzyme: carbonic anhydrase
spontaneous dissociation
(in erytrocytes)
2. bound to hemoglobin (~ 23%)
3. freely disolved (~ 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 -
tissues:
CO2 + H2O → H2CO3 → HCO3- + H+
H+ + HbO2 → HHb + O2 → aerobic metabolism
lungs:
HHb + O2 → HbO2 + H+
H+ + HCO3- → H2CO3 → H2O + CO2 → excreted
O2
O2
The 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)
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://www.biocrawler.com/encyclopedia/Fetal_hemoglobin (March 2007)
Saturation of hemoglobin by oxygen
• quaternary structure of hemoglobin
allosteric effect
T-conformation: lower affinity to O2 (deoxy Hb)
R-conformation: higher affinity to O2 (oxyHb)
T  R
Hb + O2  HbO2
The figure is found at http://employees.csbsju.edu/hjakubowski/classes/ch331/bind/MbHbbindcurve.gif (March 2007)
The animation is found at http://en.wikipedia.org/wiki/Image:Hb-animation2.gif (March 2007)
Saturation of hemoglobin with oxygen
Factors affecting the saturation:
 alkaline pH and  pO2 stabilize R-conformation
(IN LUNGS)
 acidic pH,  pCO2,  temperature and 2,3-BPG
stabilize T-conformation, i.e. deoxyHb
(IN PERIPHERY)
shift of the saturation curve toward right
Bohr´s effect
= the saturation of Hb by O2 drops because lowering pH
The 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)
Patological forms of hemoglobin
1. methemoglobin (over 3%)
metHb
 Fe3+ instad of Fe2+
 unable to transport oxygen !!!
2. glycohemoglobin (over 6%)
HbA1c
 after long term increased glycemia
3. carbonylhemoglobin (over 2%)
 after CO poisoning
4. sulfhemoglobin, cyanhemoglobin
 poisoning by H2S, HCN or by cyanides
COHb
Carbon monoxide poisoning
•
CO has 200x higher affinity to Hb than O2
•
it forms COHb = carbonyl hemoglobin
(formerly called carboxyhemoglobin)
•
max. allowed concentration in the air: 0.003%
•
intoxication by CO depends on pCO and a time
of its exposition (0.04%  strong headache, 2-3 hours:
unconsciousness; 1%  death after a few minutes)
CO binds
to Fe2+ instead of
oxygen
The 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 poisoning
may result due to:
• exposure to automobile exhaust
• smoke inhalation
• an improperly ventilated gas heater
• or other appliance (incomplete burning)
Carbon monoxide poisoning
CONSEQUENCES
• decreased oxygen-carrying capacity of Hb
• decreased delivery of oxygen to cells
 CO prevents reversible displacement of O2 on Hb
 CO shifts the O2-hemoglobin dissociation curve
to the left
 CO inhibits the intracellular respiration
 CO may bind directly to cardiac and skeletal muscle
to cause direct toxicity and to components of the
nervous system to cause demyelination and neurologic
symptoms
„cherry red coloration to the skin“
The figure is found at http://www.acsu.buffalo.edu/~lcscott/carbonmonoxide.html (March 2007)
Saturation of
hemoglobin
with CO
physiological value:
 2%
The figure is found at
http://www.uhseast.com/134221.cfm
(March 2007)
COHb / total Hb
(ratio in %)
The figure is from http://www.coheadquarters.com/CORisk/figco32x.htm (March 2007)
Carbon monoxide poisoning
TREATEMENT
• 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 history
of loss of consciousness, carbonyl hemoglobin
saturation greater than 25%, metabolic acidosis
and cerebellar findings on neurologic exam