2.  Heme proteins meaning.  Structure and function of myoglobin.  Structure and function of hemoglobin.  Types of hemoglobin.  Oxygenation & deoxygenation of hemoglobin.  Oxygen dissociation curves for myoglobin and hemoglobin.  Allosteric effectors affecting o2 binding affinity of hemoglobin.  Hemoglobinopathies.

3.  Heme proteins are a group of specialized proteins that contain heme as a tightly bound prosthetic group.  Hemoglobin and myoglobin are examples of heme proteins that maintain a supply of oxygen essential for oxidative metabolism.

4.  Present in heart and skeletal muscle.  Functions as a reservoir for oxygen within the muscle cell.  It consists of a single polypeptide chain connected to a heme group.

5.  The main function of red blood cell: – Transfer of O2 from lungs to tissue. – Transfer of CO2 from tissue to lungs.  To accomplish this function red blood cell has hemoglobin (Hb).  Hemoglobin (Hb),is the major protein of red blood cells.  Hemoglobin is the red pigment of RBCs.

7.  Hemoglobin is a tetramer composed of two parts:  Globin: 4 protein chains (subunits) (2α and 2β).  Heme: 4 heme groups each attached to globin chain. Porphyrin ring with central iron.

8. It is a globular protein with a quaternary structure. According to sequence of amino acids in the primary structure of each chain, there are four types of chains; α, β, γ and δ.

9.  Heme is a metaloporphyrin.  Heme is a derivative of the porphyrins.  Porphyrins are cyclic tetrapyrrole consisting of 4 pyrrole rings linked by methylene bridges.  one ferrous group in tetrapyrrole rings.  This iron atom is the site of oxygen binding.

10. Heme is a complex of porphyrin IX and ferrous iron (Fe 2+ ) Pyrrole ring Heme

11.  The iron is held in the center of the heme molecule by bonds of the four nitrogens of the porphyrin ring.  It can form two additional bonds, one on each side of the planar porphyrin ring.  One of these positions is coordinated to the side chain of a histidine amino acid of the globin molecule, whereas the other position is available to bind oxygen.

12.  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 (The oxygen atom binds to the Fe atom perpendicular to the porphyrin ring).

13.  Human produce several types of hemoglobin: Hb A or HbA1: is the major hemoglobin in humans, it is composed of 2 α and 2 β chains. HbA2:is the minor adult Hb, first appearing about 12 weeks after birth, composed of 2 α and 2 δ chains. HbF(fetal Hb): is the normal fetal hemoglobin, composed of 2α and 2 γ chains. Hb F has greater affinity for O2 than HbA so ensure O2 transfer from maternal circulation to fetus RBCs through placenta. HbA1C: has glucose residues attached to β-globin chains.

14.  Some of Hb A is glycosylated.  Has glucose residues attached to β- globin chains. HbA1c  Increased amounts of HbA1c are found in RBCs of patients with diabetes mellitus (DM).  HbA1c could be used as a monitor for the control of the blood glucose level during the last 2 months for diabetic patients.

15.  The hemoglobin tetramer is composed of two identical dimers, (αβ)1 and (αβ)2. tightly  The two polypeptide chains within each dimer are tightly held together, by hydrophobic interactions.  In contrast, the two dimers are able to move with respect to each other, being held together primarily by ionic and hydrogen bonds.

16. deoxyhemoglobin oxyhemoglobin.  The weaker interactions between these mobile dimers result in the two dimers occupying different relative positions in deoxyhemoglobin as compared with oxyhemoglobin.

17. T- Form - “T” = tense form (taut) - It is the deoxy form -- the low oxygen affinity form R- Form - “R” = relaxed form - It is the oxygenated form -- the high oxygen affinity form

18.  The binding of the first O 2 molecule to subunit of the T-form leads to a local conformational change that weakens the association between the subunits → R-form of Hb.

19. Myoglobin can bind only one molecule of oxygen. Hemoglobin can bind four oxygen molecules. – The oxygen binding is cooperative: As each O 2 binds to hemoglobin, the molecule undergoes a conformational change increasing the O 2 affinity for the remaining subunits. This creates the sigmoidal oxygen dissociation curve (S-shaped (sigmoidal) saturation curve of Hb).

20.  The curves for myoglobin and hemoglobin show important differences: 1- Myoglobin has a higher oxygen affinity than does hemoglobin. 2- The pO2 needed to achieve 50 % saturation (P50) is approximately 1 mm Hg for myoglobin and 26 mm Hg for hemoglobin.

21.  a. Myoglobin (Mb): The oxygen dissociation curve for myoglobin has a hyperbolic shape. Mb + O2 MbO2 Note: Myoglobin is designed to bind oxygen released by hemoglobin at the low pO2 found in muscle. Myoglobin, releases oxygen within the muscle cell in response to oxygen demand.

22.  b. Hemoglobin (Hb): The oxygen dissociation curve for hemoglobin is sigmoidal in shape. - The binding of an oxygen molecule at one heme group increases the oxygen affinity of the remaining heme groups in the same hemoglobin molecule. = heme-heme interaction. The affinity of hemoglobin for the last oxygen bound is 300 times greater than its affinity for the first oxygen bound.

23. Allosteric effectors: -1. Heme-heme interactions -2. Bohr effect (pCO 2 & pH) -3. Effect of 2,3-bisphosphoglycerate on oxygen affinity -These are collectively called allosteric (“other site”) effectors, because their interaction at one site on the hemoglobin molecule affects the binding of oxygen to heme groups at other locations on the molecule.

24.  Loading and unloading oxygen: In the lung: [O2 ] is high & Hb becomes saturated (or “loaded”) with O2. In the peripheral tissues: oxyhemoglobin releases (or “unloads”) much of its oxygen for use in the oxidative metabolism of the tissues.

25.  When pH is lowered, or CO2 is increased, both result in decreased oxygen affinity of hemoglobin.(both stabilize T state). = Shifts the curve to the right.  Raising pH or low CO2 results in a greater affinity for oxygen (both stablize R state). = Shifts the curve to the left.

26.  Source of the protons that lower the pH In the tissues: CO 2 is converted by carbonic anhydrates to carbonic acid: CO 2 + H 2 O H 2 CO 3 which spontaneously loses a proton, becoming bicarbonate (the major blood buffer): H 2 CO 3 HCO 3 – + H + Protons are allosteric effectors of hemoglobin.

27.  2,3-BPG Is synthesized from an intermediate of the glycolytic pathway.  Its an important regulator of the binding of oxygen to Hb.  When 2,3-BPG binds to the Hb will reduces the affinity of Hb for oxygen.  This enables Hb to release its O2 efficiently to tissues (low pO2).  Only binds to deoxyHb (β-chains) → deoxyHb is thus stabilized.

28.  Hb chains move closer when oxygenated, When oxygenated 2,3- DPG is pushed out.  Hb chains are pulled apart when O2 is unloaded, permitting entry of 2,3-DPG resulting in lower affinity of O2

29.  Most of the CO2 produced in metabolism is transported as bicarbonate ion (HCO 3 – ).  some CO2 is carried as carbamate bound to the N-terminal amino groups of hemoglobin (forming Carbaminohemoglobin). Hb – NH2 + CO2 Hb – NH – COO– + H+  The binding of CO2 stabilizes the T- form (deoxy form) decrease the affinity of Hb to O2. right shift in the oxygen dissociation curve.  What will happen for CO2 in lungs? It dissociates from the hemoglobin and is released in breath.

30.  Mutations in hemoglobin.  It is defined as a family of genetic disorders caused by production of a structurally abnormal hemoglobin molecule, synthesis of insufficient quantities of normal hemoglobin, or both.

31.  It is a genetic disorder of blood that is characterized by the formation of hard, sticky, sickle-shaped red blood cells, in contrast to the biconcave-shaped red blood cells (RBCs) found in “normal” individuals.

32.  This disease is caused by a mutation in hemoglobin β-globin chain resulting in the formation of Hb S.  The mutation occurs in 6 th position of β-chain where polar glutamic acid is replaced by non polar valine.

33.  Hb C is a mutant Hb in which glutamic acid in 6 th position of β- chain is replaced by lysine.  RBCs will be large oblong and hexagonal.

34.  It is a genetic defect result from the partial or total absence of one or more α or β chains of hemoglobin.  Types: α- thalassemia. β- thalassemia.

35.  (Ferrous) Fe 2+ (Ferric)Fe 3+ (oxidation of heme) cannot bind O 2  Due to: - drugs (nitrates) - certain mutations - reactive O2 intermediate (O ● )  It is characterized by “chocolate cyanosis” (a brownish-blue coloration of the skin and mucous membranes) and chocolate-colored blood, as a result of the dark-colored Methemoglobin.

36.  Oxyhemoglobin (oxyHb) = Hb with O 2  Deoxyhemoglobin (deoxyHb) = Hb without O 2  Methemoglobin (metHb) contains Fe 3+ instead of Fe 2+ in heme groups  Carbonylhemoglobin (HbCO) – CO binds to Fe 2+ in heme in case of CO poisoning or smoking. CO has 200x higher affinity to Fe 2+ than O 2.

37.  Carbaminohemoglobin (HbCO 2 ) - CO 2 is non-covalently bound to globin chain of Hb. HbCO 2 transports CO 2 in blood (about 23%).  Glycohemoglobin (HbA1c) is formed spontaneously by nonenzymatic reaction with Glc. People with DM have more HbA1c than normal (› 7%). Measurement of blood HbA1c is useful to get info about long-term control of glycemia.

39. Why does CO (carbon monoxide) considered as severe toxic gas??? 39

40. Thank you