O 2 O 2 O 2. Haemoglobin

Transcription

1 O 2 O 2 O 2 Haemoglobin

2 O 2 O 2 O 2 98% travels in oxyhaemoglobin (in red blood cells) 2% is dissolved in plasma (compared to carbon dioxide, oxygen is relatively insoluble in plasma) O 2 is not very soluble thus needs a carrier!

3 Myoglobin, Haemoglobin, Cytochromes bind O 2 Oxygen is transported from lungs to various tissues via blood in association with haemoglobin In muscle, haemoglobin gives up O 2 to myoglobin which has a higher affinity for O 2 than heamoglobin Cytochromes participate in redox reactions and are components of the electron transport chain

4 Haemoglobin (Hb or Hgb) is the primary constituent of RBCs This molecule gives the characteristic red colour to erythrocytes and to the blood The primary function of haemoglobin is to transport oxygen (O 2 ) from the lungs to the tissue cells of the body and to carry carbon dioxide (CO 2 )

5 Hemoglobin (tetramer) is composed of the protein globin, made up of two alpha chains (141 a.a) and two beta chains (146 a.a), each bound to a heme group Alpha and beta are similar but not identical in a.a. sequence Each heme group bears an atom of iron, which can bind to one oxygen molecule Each hemoglobin molecule can transport 4 molecules of oxygen

6 Abundant in skeletal muscles Consists of one heme and globin consists of single polypeptide chain (monomeric: 153 aa; 17,200 MW)

7 1. Hb A: o o Makes up about 95%-98% of Hb found in adults; contains two alpha (α) protein chains and two beta (β) protein chains 2. Hb A2: o o Makes up about 2%-3% of Hb; Has two alpha (α) and two delta (δ) protein chains

8 3. Hb F: o o o o o Makes up to 2% of Hb found in adults; Has two alpha (α) and two gamma (γ) protein chains; The primary haemoglobin produced by the fetus during pregnancy, its production usually falls to a low level shortly after birth Foetal Hb has a higher affinity for oxygen than adult haemoglobin This means that the fetus can receive oxygen from the mother across the placenta.

9 Responsible for the O 2 - binding capacity of Hb Consists of an iron (Fe) ion held in a heterocyclic ring, known as a porphyrin The protoporphyrin made up of four pyrrole rings linked by methane bridges

10 A Fe atom in its ferrous state (Fe +2 ) is at the center of protoporphyrin Fe +2 has 6 coordination bonds o 4 bonded to the 4 pyrrole N atoms (The nucleophilic N prevent oxidation of Fe +2 ) o The 2 additional binding sites are one on either side of the heme plane: One of these is occupied by the imidazole group of His The second site can be reversibly occupied by O 2

11 When Hb is bound to O 2, it is called oxyhb. This is the relaxed (R ) state The form with a vacant O 2 binding site is called deoxyhb and corresponds to the tense (T) state If iron is in the oxidized state as Fe +3, it is unable to bind O 2

12 R state has a higher affinity for O 2 T state is more stable in the absence of O 2 conformational change The subunits slide and rotate making the central cavity smaller

13 O 2 -binding curves show Hb saturation as a function of the partial pressure for O 2 4 subunits, so 4O 2 -binding sites: If one heme group has a bound O 2, it increases the ability of the other heme groups to bind O 2 (last O 2 affinity is 300 times greater than its affinity for 1 st O 2 ) Cooperative binding Segmoidal curve

14 Myoglobin has a higher O 2 affinity than Hb Myoglobin O 2 dissociation curve is hyperbolic

15 A number of factors reduce the affinity of Hb for O 2 so that more O 2 is released to tissues As the curve shifts from A to B (to right) the affinity for O 2 decreases H +, PCO 2, and BPG modify the structure of Hb and alter its affinity for oxygen: Increases of these factors: Decrease hemoglobin s affinity for oxygen Enhance oxygen unloading from the blood Decreases act in the opposite manner

16 A number of factors reduce the affinity of Hb for O 2 so that more O 2 is released to tissues Increasing temperature also shift the curve to the right

17 CO 2 in blood present in 3 forms: 1. 7% dissolved in plasma In red blood cells 2. 70% travels as HCO 3- ions (hydrogencarbonate ions) 3. 23% travels as carboamino compounds CO2 = waste product of cellular metabolism (the end-product)

18 CO 2 reacts directly with Hb to form the carboaminohb; reversible reaction Small quantity of CO 2 reacts with plasma proteins - less significant (quantity of proteins 1/4 th that of Hb) R N H H + CO 2 R N H COO - + H + Protein Carboamino compound

19 Dissolved CO 2 in blood reacts with water to form Carbonic Acid CO 2 + H 2 O H 2 CO 3 H + + HCO 3 - Carbonic Anhydrase present inside RBCs (but not plasma) catalyzes this reaction Carbonic acid rapidly dissociates into ion H + and bicarbonate ion

20 Bohr Shift The relationship between the binding of O 2, H +, CO 2 to hemoglobin (allosteric site), is knowing as Bohr effect Tissues H 2 CO 3 (carbonic acid) HCO - 3 Leaves the red blood cell With high level of activity CO 2 enters the blood H + Trapped in cytoplasm acidification in the red blood cell Hb affinity with O 2 Bohr Shift Release of O 2

21 The dissociation curve moves to the right at higher concentration of carbon dioxide. This shows that carbon dioxide lowers the affinity of Hb for oxygen. Hb tends to give up O 2 in area of high CO 2 such as the respiring tissues that need it most.

22 The build up of hydrogen carbonate ions causes them to diffuse out of the RBC leaving the inside of the RBC positively charged In order to balance this electric charge chloride ions diffuse into the RBCs from the plasma this is known as the chloride shift

23 When blood gets to the lungs, all the reactions are reversed The hydrogen carbonate and hydrogen ions recombine releasing CO 2 The chloride shift is reversed Carbamino-haemoglobin breaks down to release CO 2

24 Figure 27 7

25 Hemoglobin acts as a buffer in blood by picking up CO 2 or H + In tissues: Hemoglobin becomes more basic when it is deoxygenated, i.e. it binds H + more tightly In the lung: Hemoglobin is oxygenated, becomes more acidic, (i.e. it is a more powerful H + donor), and releases its H +

26 Senescent RBCs Hemoglobin Heme Biliverdin Globin O 2 Heme oxygenase CO NADPH Biliverdin reductase Stercobilin excreted in feces INTESTINE Glucuronic acid is removed and bilirubin is converted to reabsorbed urobilinogen which is then into blood oxidized by intestinal bacteria (Portion of urobilinogen) via bile duct to intestine Bilirubin diglucuronide (water-soluble) Urobilin excreted in urine KIDNEY NADP + Bilirubin (water-insoluble) 2 UDP-glucuronic acid Bilirubin (water-insoluble) via blood to the liver (complexed with albumin) LIVER Catabolism of hemoglobin

27 Unconjugated bilirubin Toxic to tissues Not soluble in aqueous solutions Tightly complexed to albumin Cannot be excreted in the urine even when blood levels are high Conjugated bilirubin Water-soluble Non-toxic Loosely bound to albumin Excreted in urine (bilirubinuria)

28 Jaundice describes the yellowing of sclera, skin and mucosal membranes due to increased circulating bilirubin in the plasma This becomes clinically evident when serum bilirubin reaches about mol/l.

29 Figure : Examples of hyperbilirubinemia A. Hemolytic anemia B. Hepatitis C. Biliary duct stone excess hemolysis unconjugated bilirubin (in blood) conjugated bilirubin (released to bile duct) unconjugated bilirubin (in blood) conjugated bilirubin secreted by liver into bile unconjugated bilirubin (in blood) conjugated bilirubin (in blood) Hemolytic Jaundice Hepatic jaundice Obstructive jaundice

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31 What is anemia? Anemia is due to deficiency of Hb in blood due to lack of erythrocytes and/or their Hb content Normal Hb concentration o Adult male =14g/dl (14-17) o Adult female not pregnant = 12g/dl (12-14) o Adult female pregnant = 11g/dl (11-12)

32 The most common symptom of anemia is tiredness. Other signs and symptoms of anemia include: 1. Weakness, 2. pale skin, 3. brittle nails, 4. Dizziness, 5. irritability.

33 Excess blood loss due to bleeding Undernutrition: deficiencies of several vitamins and minerals like vitamins A, B2, B6, B12, C, iron, calcium and folic acid along with protein all of which can cause anaemia. Pregnancy Others causes: include worm infestation and chronic disease like AIDS, cancer or kidney disease, cancer treatment, and hereditary diseases

34 Hemolytic anemia is a disorder in which the red blood cells are destroyed prematurely RBCs are destroyed faster than the bone marrow can produce them

35 Extrinsic: Red blood cells are produced healthy but are later destroyed by becoming trapped in the spleen, destroyed by infection, or destroyed from drugs that can affect red blood cells

36 intrinsic: o The destruction of the red blood cells due to a defect within the red blood cells themselves o Intrinsic hemolytic anemia is often inherited, such as sickle cell anemia and Glucose-6-Phosphate Dehydrogenase deficiency cells

37 Sickle Cell anemia is a hereditary disease which causes the body to make abnormally shapes red blood cells ( C form) Causes complications because the blood cells are not able to reach certain parts of the body

38 The α chains in mutant Hb (HbS) are the same as in normal Hb (HbA) A point mutation in the Hb β gene is responsible for the sickling of RBCs seen in sickle cell anemia Substitution of non polar valine for a charged Glu

39 Substitution of non polar valine for a charged Glu Normal hemoglobin Sickle Cell hemoglobin No Oxygen No oxygen No Oxygen: Separate No Oxygen: stick together

40 Causes tissue anoxia (Interruption in O 2 supply) This blocking can produce micro vascular occlusions which can cause necrosis (death) of the tissue and pain

41 Do not express the disease symptoms (25%) (50%) (25%)

42 During electrophoresis, HbS moves slowly towards anode than HbA at alkaline ph

43 Lysine replaces glutamic acid at position 6 of the β globin gene. Mild chronic haemolytic anaemia

44 Mixture of Sickle hemoglobin (Hb S) + (Hb C)

45 Thalassemia is inherited disorders characterized reduced or absent amounts of hemoglobin Two major types of thalassemia: 1. Alpha (α): Caused by defect in rate of synthesis of alpha chains (usually caused by gene deletion) 2. Beta (β): Caused by defect in rate of synthesis in beta chains (usually caused by mutation)

46 Absence of 1 α gene (silent carrier): no symptoms, may be slightly anemia, does not require therapy Absence of 2 α gene (α Thalassemia trait): no serious symptoms, except slight anemia Absence of 3 α genes (Hb H disease): microcytic anemia (small RBC), splenomegaly Absence of 4 α genes (Hydrops fetalis): most serious form, death before birth

47 Usually caused by point mutations and short insertions or deletions limited to a few nucleotides Two situations have clearly to be distinguished: 1. β o thalassemia: No β-globin chain is made 2. β + thalassemia: decreased β-globin chain is made Disease results in an overproduction of α-globin chains, which precipitate in the cells

48 Inadequate absorption Inadequate dietary intake of foods high in Fe Excess loss of iron due to bleeding, some parasites, menstrual loss and gastrointestinal bleeding In pregnancy iron is taken from mother by growing fetus, so iron supplement must be taken by pregnant women

49 Folic Acid (also known as vitamin B9) Deficiency causes megablastic anemia (RBCs that are large and fewer in number) Deficiency can be due to: 1. Poor dietary intake 2. Malabsorption syndromes 3. Drugs that inhibit absorption 4. Alcohol abuse 5. Hemodialysis 6. Increased requirement (pregnancy)

50 Vitamin B12 is a water soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood Dangerous anemia It is a type of megablastic anaemia due to malabsorption of Vit B12 (decreased gastric intrinsic factor IF which is needed for absorption of vit B12)

51 The primary defect is a reduction in or depletion of hematopoietic precursor stem cells with decreased production of all cell lines.

52 Aplastic anemia is a severe, life threatening syndrome in which production of erythrocytes, WBCs, and platelets has failed. Aplastic anemia may occur in all age groups and both genders.

53 Incidence (acquired) 2/ rare < 1 year; plateaus yrs; increase > 60 yrs The disease is characterized by peripheral pancytopenia and accompanied by a hypocellular bone marrow.

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