Structure, function and metabolism of hemoglobin

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

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)

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

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

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

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

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.

Convertion of heme to bilirubin green red-orange

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

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.

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

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%).

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

Transport of O2 and CO2 in lungs

Transport of O2 and CO2 in tissues

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

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