structure, function, and metabolism of hemoglobin vladimíra kvasnicová

Structure, function, and metabolism of

hemoglobinVladimíra 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

The figure is found at http://dtc.pima.edu/~biology/202alpha/lesson1/hemoglobin.jpg (March 2007)

Mr = 64 500

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)

hemoglobin

HEME

MYOGLOBIN

• it has not a quarternary structure: only 1 polypeptide chain

• found in muscles: binds O2 „for storrage“

• higher affinity to oxygen than hemoglobin

Types of hemoglobin and its subunits

• adult hemoglobin:

HbA1 = 22

HbA2 = 22 ( 2% from total Hb of adults)

• fetal hemoglobin

HbF = 22 ! 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

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

hemoglobin

Pyrrole

The 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 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 mutationin the -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:

-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

arterial blood venose blood

pO2 13,33 kPa 5,33 kPa

100 mmHg 40 mmHg

pCO2 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

The figure is from http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecPhysio/42-29-BloodCO2Transport-AL.gif (March 07)

O2

O2

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)

The figure is found at http://www.biocrawler.com/encyclopedia/Fetal_hemoglobin (March 2007)

HbF is left-shifted(it has higher affinity to oxygen)

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

The figure is found at http://employees.csbsju.edu/hjakubowski/classes/ch331/bind/MbHbbindcurve.gif(March 2007)

Bohr´s effect= the saturation of Hb by O2 drops because lowering pH

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%) COHb after CO poisoning

4. sulfhemoglobin, cyanhemoglobin poisoning by H2S, HCN or by cyanides

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)

The figure is found at http://www.orthosmoke.org/index.php/pt/Carbon%20Monoxide (March 2007)

CO binds

to Fe2+ instead of oxygen

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)

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 curveto 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

Carbon monoxide poisoning

The figure is found at http://www.acsu.buffalo.edu/~lcscott/carbonmonoxide.html (March 2007)

„cherry red coloration to the skin“

Saturation of

hemoglobin with CO

The figure is found at http://www.uhseast.com/134221.cfm

(March 2007)

COHb / total Hb (ratio in %)

physiological value:

2%

The figure is from http://www.coheadquarters.com/CORisk/figco32x.htm (March 2007)

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

Carbon monoxide poisoning