Structure of hemoglobin
• hemoprotein
Structure, function, and
metabolism of hemoglobin
(complex protein: globin + prosthetic group)
• quaternary structure: 4 subunits
• prosthetic group of each of the subunit = heme
Vladimíra Kvasnicová
4 polypetide chains
4 molecules of heme
4 ferrous (Fe2+) ions
HEME
hemoglobin
MYOGLOBIN
• it has not a quarternary structure: only 1 polypeptide chain
• found in muscles: binds O2 „for storrage“
Mr = 64 500
The figure is found at http://dtc.pima.edu/~biology/202alpha/lesson1/hemoglobin.jpg (March 2007)
• 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 = α2β2
HbA2 = α2δ2
(∼ 2% from total Hb of adults)
• fetal hemoglobin
HbF = α2γ2
! 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
Pyrrole
• 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
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
Qiuz
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
1.
What is the concentration of Hb
in blood?
2.
Describe the structure of Hb
3.
Where is oxygen bound into Hb?
4.
How many O2 can be bound to Hb?
5.
Draw the saturation curve of Hb
Synthesis of hemoglobin
Synthesis of heme - REPETITION
Disorders:
• THALASSEMIA = group of genetically determined
disorders: absence or reduced synthesis of a globin chain
(α or β thalassemia)
• mainly in the bone marrow (Hb) and in the liver
(cytochroms)
• mitochondria / cytoplasm / mitochondria
• substrates: succinyl-CoA + glycine
• ANEMIA
(= decreased oxygen-carrier capacity of blood)
• important intermediates:
sideropenic anemia – insufficient concentration of Fe
δ-aminolevulinic acid (= 5-aminolevulinic, ALA)
sickle cell anemia – point mutation
in the β-globin gene forms abnormal
HbS (Glu → Val)
porphobilinogen (PBG = pyrrol derivative)
uroporphyrinogen III (= 1st porphyrinogen – precursor of heme)
protophorphyrine IX (= direct precursor of heme)
Synthesis of heme - regulation
ALA-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 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
The figure is from: Color Atlas of Biochemistry / J. Koolman, K.H.Röhm. Thieme 1996. ISBN 0-86577-584-2
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
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
lead poisoning – accumulation of ALA (blood, urine)
(inhibition of porphobilinogen synthase)
• heme → Fe3+ + CO + biliverdin→bile pigments→feaces
Qiuz
The figure is from: Color Atlas of Biochemistry / J. Koolman, K.H.Röhm. Thieme 1996. ISBN 0-86577-584-2
1.
Where is Hb synthesized?
2.
What failures of Hb synthesis do you
know?
3.
What substrates are needed for the
synthesis of heme?
4.
What is the source of iron for the
synthesis of heme?
5.
What is the cause of jaundice during an
excessive degradation of erytrocytes?
Transport of blood gases
Transport of blood gases
Air composition:
78% N2
21% O2
1% water, inert gases, CO2 (0,04%)
pO2
Air pressure:
arterial blood
venose blood
13,33 kPa
5,33 kPa
100 mmHg
40 mmHg
5,33 kPa
6,13 kPa
40 mmHg
46 mmHg
1 atm = 101 325 Pa (~ 101 kPa) = 760 Torr (= mmHg)
pCO2
1 mmHg = 0,1333 kPa
1 kPa = 7,5 mmHg
(alveols)
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
The figure is found at http://people.eku.edu/ritchisong/RITCHISO//301notes6.htm (March 2007)
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 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 -
O2
tissues:
CO2 + H2O → H2CO3 → HCO3- + H+
H+ + HbO2 → HHb + O2 → aerobic metabolism
lungs:
HHb + O2 → HbO2 +
O2
H+
H+ + HCO3- → H2CO3 → H2O + CO2 → excreted
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)
HbF is left-shifted
(it has higher affinity to oxygen)
The figure is found at http://www.biocrawler.com/encyclopedia/Fetal_hemoglobin (March 2007)
The figure is found at http://dr-amy.com/rich/oxygen/fig1.gif (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)
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
The animation is found at http://en.wikipedia.org/wiki/Image:Hb-animation2.gif (March 2007)
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
Qiuz
1. methemoglobin (over 3%)
1.
What is the % proportion of O2
and CO2 in air?
2.
What is pO2 in arterial blood?
3.
What is pCO2 in arterial blood?
4.
How is CO2 transported in the
human body?
5.
Summarize factors decreasing
the affinity of Hb to oxygen
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
Qiuz
1.
Compare the fetal and adult Hb
2.
What is methemoglobin?
3.
What is glycohemoglobin?
4.
What is carbonylhemoglobin?
5.
What is carboxyhemoglobin?
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
Carbon monoxide poisoning
CONSEQUENCES
may result due to:
• exposure to automobile exhaust
• smoke inhalation
• an improperly ventilated gas heater
• or other appliance (incomplete burning)
• 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
Saturation of
hemoglobin
with CO
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 at
http://www.uhseast.com/134221.cfm
(March 2007)
COHb / total Hb
(ratio in %)
Qiuz
Describe the first aid in case of an
intoxication of a person by
carbon monoxide
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