1. Structure, function and metabolism of hemoglobin

2. Hemoglobin (Hb)
is a hemoprotein only found in the cytoplasm of erythrocytes (ery)
transports O2 and CO2 between lungs and various tissues
normal concentration of Hb in the blood:
adult males 135 – 175 g/L
adult females 120 – 168 g/L

3. Structure of hemoglobin
Hb is a spherical molecule consisting of 4 peptide subunits (globins) = quartenary structure
Hb of adults (Hb A) is a tetramer consisting of 2 - and 2 β- globins → each globin contains 1 heme group with a central Fe2+ ion (ferrous ion)

4. Heme structure Heme is a metaloporphyrine (cyclic tetrapyrrole)
Heme contains:
conjugated system of double bonds → red colour
4 nitrogen (N) atoms
1 iron cation (Fe2+) → bound in the middle of tetrapyrrole skelet by coordination covalent bonds
methine bridge pyrrole ring

5. Properties of iron in heme
Coordination number of iron in heme = 6
6 bonds:
4x pyrrole ring (A,B,C,D)
1x link to a protein
1x link to an oxygen

6. In which compounds can we find a heme group?
Hemoproteins
Hemoglobin (Hb)
Myoglobin (Mb)
Cytochromes
Peroxidases

7. Myoglobin (Mb)
is a single-chain globular protein of 153 AA, containing 1 heme group
transports O2 in skeletal and heart muscle
is found in cytosol within cells
is a marker of myocard damage

8. Heme biosynthesis in bone marrow (85% of Hb) and liver (cytochromes)
cell location: mitochondria / cytoplasm /
substrates: CoA + glycine
important intermediates:
-aminolevulinic acid ( 5-aminolevulinic acid, ALA)
porphobilinogen (PBG = pyrrole derivate)
uroporphyrinogen III ( porphyrinogen – heme precursor)
protoporphyrin IX ( direct heme precursor)
● key regulatory enzyme: ALA synthase

9. δ-aminolevulinic acid (ALA)
synthesis of heme starts in mitochondria
succinyl-CoA and Gly undergo a condensation → ALA
reaction is catalyzed by enzyme ALA synthase

10. Final formation of heme
Fe2+ is incorporated into protoporphyrin IX
reaction is catalyzed by enzyme ferrochelatase

11. Hemoglobin degradation
In the human body approx. 100 – 200 million ery are broken down every hour. Degradation of Hb begins in ER of reticuloendothelial cells (RES) of the liver, spleen, bone marrow and skin.
Hb is degraded to:
● globins → AAs → metabolism
● heme → bilirubin
● Fe2+ → transported with transferrin and used in the next heme biosynthesis
Not only Hb but other hemoproteins also contain heme groups which are degraded by the same pathway.

12. Convertion of heme to bilirubin
green
red-orange

13. Fate of bilirubin
Bilirubin (Bil) is released from RES into the blood. BUT! Bil is only poorly soluble in plasma, and therefore during transport it is bound to albumin („nonconjugated Bil“). ↓
LIVER
In the hepatocytes, Bil is conjugated by 2 molecules of glucuronic acid → bilirubin diglucuronide (soluble in water, „conjugated Bil“)
BILE
INTESTINE
Bil is reduced to urobilinogen

14. In intestine: Urobilinogen can be:
reabsorbed and returned to the liver
(= enterohepatic circulation)
b) oxidized (in the presence of O2) to pigments urobilin (orange) and stercobilin (yellow) → they are excreted in the stool
Urobilinogen also appears in the urine.

15. Types of hemoglobin Adult Hb (Hb A) = 2 α and 2 β subunits
HbA1 is the major form of Hb in adults and in children over 7 months.
HbA2 (2 α, 2 δ) is a minor form of Hb in adults. It forms only 2 – 3% of
a total Hb A.
Fetal Hb (Hb F) = 2 α and 2 γ subunits
in fetus and newborn infants Hb F binds O2 at lower tension than Hb A → Hb F has a higher affinity to O2
After birth, Hb F is replaced by Hb A during the first few months of life.
Hb S – in β-globin chain Glu is replaced by Val
= an abnormal Hb typical for sickle cell anemia

16. Derivatives of hemoglobin
Oxyhemoglobin (oxyHb) = Hb with O2
Deoxyhemoglobin (deoxyHb) = Hb without O2
Methemoglobin (metHb) contains Fe3+ instead of Fe2+ in heme groups
Carbonylhemoglobin (HbCO) – CO binds to Fe2+ in heme in case of CO poisoning or smoking. CO has 200x higher affinity to Fe2+ than O2.
Carbaminohemoglobin (HbCO2) - CO2 is non-covalently bound to globin chain of Hb. HbCO2 transports CO2 in blood (about 23%).

17. Process of O2 binding to Hb
Hb can exist in 2 different forms: T-form and R-form.
T-form (T = „tense“) has a much lower oxygen affinity than the R-form. The subunits of Hb are held together by electrostatic interactions. The binding of the first O2 molecule to subunit of the T-form leads to a local conformational change that weakens the association between the subunits → R-form („relaxed“) of Hb.
Increasing of oxygen partial pressure causes the conversion of T-form to R-form.
T  R
Hb + ↑pO2  HbO2

18. Transport of O2 and CO2 in lungs

19. Transport of O2 and CO2 in tissues

20. Porphyrias - disturbances of heme synthesis
are hereditary or acquired disturbances of heme synthesis
in all cases there is an identifiable abnormality of the enzymes which synthesize heme
this leads to accumulation of intermediates of the pathway and a deficiency of heme → excretion of heme precursors in feces or urine, giving them a dark red color
● accumulation of porphyrinogens in the skin can lead to photosensitivity
the neurological symptoms

21. Thalassemias – inherited autosomal recessive blood diseases
genetic defect results in reduced rate of synthesis of α- or β-globin chain → it causes the formation of abnormal Hb molecules → anemia
are prevalent in populations where malaria was endemic – people of Mediterranean origin and Asians
genetic counseling and genetic testing is recommended for families that carry a thalassemia trait