Protein Structure and Folding Lectures 2 - Oxygen Transport Proteins PDF

PROTEIN STRUCTURE AND FOLDING OXYGEN TRANSPORT PROTEINS Dr. Adnan Anwer M.B.Ch.B; F.I.B.M.S (Pathology, Hematology) March 10, 2022 LECTURE 2: LEARNING OUTCOMES 1. Explain the physiological roles of myoglobin and haemoglobin. (LO2.1) 2. Contrast the oxygen-binding properties of myoglobin and haemog...

PROTEIN STRUCTURE AND FOLDING OXYGEN TRANSPORT PROTEINS Dr. Adnan Anwer M.B.Ch.B; F.I.B.M.S (Pathology, Hematology) March 10, 2022 LECTURE 2: LEARNING OUTCOMES 1. Explain the physiological roles of myoglobin and haemoglobin. (LO2.1) 2. Contrast the oxygen-binding properties of myoglobin and haemoglobin and explain why haemoglobin is most suited to its role as an oxygen transporter. (LO2.2) 3. Describe the major structural differences between oxygenated and deoxygenated haemoglobin and the molecular basis of cooperativity. (LO2.3) 4. Describe the effects of CO2,H+,2’3 bisphosphoglycerate and carbon monoxide on the binding of oxygen by haemoglobin, and the physiological significance of these effects. (LO2.4) 5. Appreciate that mutations in globin genes can give rise to diseases such as sickle cell anemia or thalassemia. (LO2.5) IT IS IMPORTANT TO KNOW THAT: § Ligand (L): binding molecule to a protein § Binding site: site of a protein that a ligand binds § Mostly they are specific § Proteins are flexible: changing in conformation § Induced fit: ligand result in a conformational change on protein’s binding site so it fits the binding site more tightly. § Interactions may be regulated. § Enzyme has a catalytic or active site (binding site)- substrate (ligand) REVERSIBLE BINDING OF LIGAND TO PROTEIN & OXYGEN TRANSPORT § O2 Essential for cellular respiration. § O2 is poorly soluble in plasma. § Impossible to transport by simple diffusion. § Free iron dangerous Reactive oxygen(OH) Damage of DNA & other macromolecules § O2 is transported using two proteins (hemoglobin and myoglobin) That contains essential prosthetic group. (heme: contains Fe atom) MYOGLOBIN AND HEMOGLOBIN Myoglobin Hemoglobin § Single polypeptide chain (153 § Four heteromeric polypeptide LO 2.1 chains (4 subunits) in HbA (Adult hemoglobin) 2α-chains (141 residue each), and 2β-chains (146 residue each). § 3D structure of α& β is very § 3D tertiary structure is similar to both α& similar β § Found in RBCs. § In heart and skeletal muscle. § Function to transport O2 from the § Function both as a reservoir for O2 lungs to the capillaries of the tissues. It transports H+ and CO2 and to transport it with in the muscle too - cells. The polypeptide chain of myoglobin is structurally similar to the residue). individual subunit of polypeptide of hemoglobin molecule. - This homology makes myoglobin a useful model for interpreting some of the more complex properties of hemoglobin. STRUCTURE OF HEMOGLOBIN b2 b1 a1 a2 Hemoglobin Iron binding heme group Hemoglobin consists of : globin and heme pigment - Globin Consists of two a and two b subunits -Each subunit binds to a heme group . Each heme group bears an atom of iron, which binds reversibly with one molecule of oxygen HEME LO 2.1 & 2.2 Consists of: 1. An organic part, protoporphyrin, made up of 4 pyrrole rings, 2. Fe+2 atom, which binds to the 4N atoms of the protoporphyrin ring. • • The Fe can form 2 additional bonds, one on either side of the plane. The ferrous(+2) state binds the oxygen, HbFe+3 Ferric state (3+)/ (methemoglobin) can not bind Oxygen, MYOGLOBIN MOLECULE IS SIMILAR TO HEMOGLOBIN SUBUNITS LO 2.1 & 2.2 MYOGLOBIN AND HEMOGLOBIN BINDING TO OXYGEN § Hemoglobin (Hb) subunits and myoglobin are similar in structure and in heme binding pocket. § Tetrameric hemoglobin molecule is structurally and functionally more complex than myoglobin. (Hb) is a dimer of dimers , in which two αβ halves of the heterotetramer are held together at their interface by noncovalent interactions. § Hemoglobin § However body. § It , the Hb can carry 4 O2 from lungs to the cells of the also can transport H+ and CO2 from the tissues to the lung. § The oxygen-binding properties of Hb are regulated by interaction with allosteric effectors. LO 2.3 OXYGEN DISSOCIATION CURVES FOR MYOGLOBIN AND HEMOGLOBIN § The degree of saturation (Y) of O2 binding sites on all myoglobin or hemoglobin molecules can vary between zero (all sites are empty) and 100%(all sites are full). § A plot of Y measured at different partial pressures of O2 is called Oxygen Dissociation Curve. § P50 Expresses the Relative Affinities of Hemoglobin and myoglobin for Oxygen LO 2.2 & 2.3 OXYGEN DISSOCIATION CURVES FOR MYOGLOBIN AND HEMOGLOBIN IMPORTANT DIFFERENCES. § Myoglobin has a higher oxygen affinity than does hemoglobin. § The partial pressure of oxygen needed to achieve half- saturation of the binding sites (P50) is approximately 1 mmHg for myoglobin and 26 mmHg for hemoglobin. § The higher the oxygen affinity (that is the more tightly oxygen binds), the lower the P50 § A hyperbolic relationship between Y and pO2 for myoglobin. § A sigmoidal relationship between Y and pO2 for Hb (cooperative). § The affinity of hemoglobin for the last oxygen bound is approximately 300 times greater than its affinity for the first oxygen bound. LO 2.2 ALLOSTERIC EFFECTS LO 2.4 § The ability of hemoglobin to reversibly bind oxygen is affected by the following parameters: § pH § pO2 § pCO2 § the availability o f 2,3-bisphosphoglycerate (BPG). 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. Heme Fe Hemoglobin Fe Globin with central heme REGULATION OF OXYGEN BINDING LO 2.4 The highly anionic 2,3-BPG is present in red blood cells at~2mM. It binds to haemoglobin (one molecule per tetramer) and decreases the affinity for O2, promoting release in the tissues. The physiological adaptation to high altitude involves increased tissue concentrations of BPG, leading to more efficient O2 release to compensate for the reduced O2 tension. BINDING OF 2,3-BPG TO DEOXYHAEMOGLOBIN 2,3-BPG binds in the central cavity of the tetramer, interacting with three positively charged groups (2His,1Lys) on each beta chain. The oxygenated haemoglobin has a smaller central gap and excludes 2,3-BPG. LO 2.4 DIFFERENTIAL OXYGEN AFFINITY OF FETAL AND MATERNAL RED BLOOD CELLS § Fetal haemoglobin contains a variant of the β chain , called Y. § The fetal haemoglobin thus has a reduced affinity for 2,3-BPG, resulting in an enhanced O2-binding affinity that allows transfer of O2 from the maternal to the fetal red blood cells LO 2.4 THE BOHR EFFECT H+ IONS AND CO2 PROMOTE THE RELEASE OF O2 LO 2.4 § Rapidly metabolizing tissues, such as contracting muscle , have a high need for O2 and generate large amounts of H+ and CO2. Both of these species interact with haemoglobin to promote O2release. § The O2 affinity decreases as the pH decreases from the pH 7.4 found in the lungs. § Increased CO2 concentrations also lead to a decrease in O2-affinity. § This regulation of O2-affinity by pH and CO2 is called the Bohr effect after its discoverer, Christian Bohr(1904). Factors enhance unloading of Oxygen at the tissue and shift curve to the right Include: -High H+ -High CO2 -High Temp -High Altitude BINDING OF CARBON MONOXIDE TO HEMOGLOBIN § Carbon monoxide is toxic even at low concentration, § Binds tightly (but reversibly) to the Hb iron forming carboxyhemoglobin. § Affinity of Hb for CO is 220 times greater than for O2. § Binding of CO to one or more Hb unit Hb high affinity to O2 HbR hyperbolic curve § Treated by 100% oxygen therapy Oxygen Dissociation Curve in Anemia In anemia, the curve is still sigmoidal, but the saturation is reduced. LO 2.4 LO 2.5 LO 2.5 Which variables increase sickling? Any variable that increase HbS deoxystate • Low O2 as a result of high altitude or flying • High CO2 • Low pH • Dehydration • High BPG Questions A 14-year-old girl was admitted to a children’s hospital in coma. Her mother stated that several days before admission the girl began to complain of thirst and also started to get up several times during the night to urinate. She diagnosed as Diabetic ketoacidosis (DKA). 1. Predict the effect of DKA on the patient's oxygen dissociation curve (ODC). 2. Briefly explain the molecular bases of such change in (ODC). 3. Suppose this girl has sickle cell anemia, how can the DKA affect her hemoglobin structure? 4. Will this relief or amplify her illness? Why?