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Oxygen Dissociation Curves Show the Relationship Between Oxygen Levels (As Partial Pressure) and Haemoglobin Saturation

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Oxygen Dissociation Curves Show the Relationship Between Oxygen Levels (As Partial Pressure) and Haemoglobin Saturation EWING CHRISTIAN COLLEGE, PRAYAGRAJ. B.Sc. Semester VI Department of Chemistry Paper – 1 (Inorganic Chemistry) Unit – 5 (Chemistry of Hemoglobin) ---------------------------------------------------------------------------------------------------- Oxygen dissociation curves show the relationship between oxygen levels (as partial pressure) and haemoglobin saturation. The oxyhemoglobin dissociation curve relates oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2), and is determined by what is called "hemoglobin affinity for oxygen"; that is, how readily hemoglobin acquires and releases oxygen molecules into the fluid that surrounds it. Because binding potential changes with each additional O2 molecule, the saturation of haemoglobin is not linear . The oxygen dissociation curve for adult haemoglobin is sigmoidal (i.e. S-shaped) due to cooperative binding . There is a low saturation of haemoglobin when oxygen levels are low (haemoglobin releases O2 in hypoxic tissues) . There is a high saturation of haemoglobin when oxygen levels are high (haemoglobin binds O2 in oxygen-rich tissues) Oxygen Dissociation Curve – Haemoglobin The curve is usually best described by a sigmoid plot. A hemoglobin molecule can bind up to four oxygen molecules in a reversible method. The shape of the curve results from the interaction of bound oxygen molecules with incoming molecules. The binding of the first molecule is difficult. However, this facilitates the binding of the second, third and fourth, this is due to the induced conformational change in the structure of the hemoglobin molecule induced by the binding of an oxygen molecule. Abhinav Lal, Dept. of Chemistry, Ewing Christian College,Prayagraj. Page 1 EWING CHRISTIAN COLLEGE, PRAYAGRAJ. B.Sc. Semester VI Department of Chemistry Paper – 1 (Inorganic Chemistry) Unit – 5 (Chemistry of Hemoglobin) Factors affecting standard deviation curve: The oxygen binding capacity to hemoglobin is affected by several factors. These factors shift or reshape the oxyhemoglobin dissociation curve. A rightward shift indicates that the hemoglobin under study has a decreased affinity for oxygen. This makes it more difficult for hemoglobin to bind to oxygen, but it makes it easier for the hemoglobin to release oxygen bound to it. The effect of this rightward shift of the curve increases the partial pressure of oxygen in the tissues when it is most needed, such as during exercise or hemorrhagic shock. In contrast, the curve is shifted to the left by the opposite of these conditions. This leftward shift indicates that the hemoglobin under study has an increased affinity for oxygen so that hemoglobin binds oxygen more easily, but unloads it more reluctantly. The various factors are as follows: 1. pH: A decrease in pH (increase in H+ ion concentration) shifts the standard curve to the right, while an increase shifts it to the left. This occurs because at greater H+ ion concentration, various amino acid residues, such as Histidine 146 exist predominantly in their protonated form allowing them to form ion pairs that stabilize deoxyhemoglobin in the T state. The T state has a lower affinity for oxygen than the R state, so with increased acidity, the hemoglobin binds less + O2 for a given PO2 (and more H ). This is known as the Bohr effect. A reduction in the total binding capacity of hemoglobin to oxygen due to reduced pH is called the root effect. The binding affinity of hemoglobin to O2 is greatest under a relatively high pH. 2. Carbondioxide: CO2 affects the curve in two ways. First, CO2 accumulation causes carbamino compounds to be generated through chemical interactions, which bind to hemoglobin forming carbaminohemoglobin. CO2 is considered an Allosteric regulator as the inhibition happens not at the binding site of hemoglobin. Second, it influences intracellular pH due to formation of bicarbonate ion. Formation of carbaminohemoglobin stabilizes T state hemoglobin by formation of ion pairs. Only about 5–10% of the total CO2 content of blood is transported as carbamino compounds, whereas (80–90%) is transported as bicarbonate ions and a small amount is dissolved in the plasma. The formation of a bicarbonate ion will release a proton into the plasma, decreasing pH (increased acidity), which also shifts the curve to the right as discussed above; low CO2 levels in the blood stream results in a high pH, and thus provides more optimal binding conditions for hemoglobin and O2. This is a physiologically favored mechanism, since hemoglobin will drop off more oxygen as the concentration of carbon dioxide increases dramatically where tissue respiration is happening rapidly and oxygen is in need. Abhinav Lal, Dept. of Chemistry, Ewing Christian College,Prayagraj. Page 2 EWING CHRISTIAN COLLEGE, PRAYAGRAJ. B.Sc. Semester VI Department of Chemistry Paper – 1 (Inorganic Chemistry) Unit – 5 (Chemistry of Hemoglobin) 3. 2, 3-BPG: 2,3-Bisphosphoglycerate or 2,3-BPG (formerly named 2,3- diphosphoglycerate or 2,3-DPG) is an organophosphate formed in erythrocytes during glycolysis and is the conjugate base of 2,3-bisphosphoglyceric acid. High levels of 2,3-BPG shift the curve to the right, while low levels of 2,3-BPG cause a leftward shift, seen in states such as septic shock, and hypophosphatemia. In the absence of 2,3-BPG, hemoglobin's affinity for oxygen increases. 2,3-BPG acts as a heteroallosteric effector of hemoglobin, lowering hemoglobin's affinity for oxygen by binding preferentially to deoxyhemoglobin. An increased concentration of BPG in red blood cells favours formation of the T, low-affinity state of hemoglobin and so the oxygen-binding curve will shift to the right. 4. Temperature: Increase in temperature will shift the Oxygen dissociation curve to the right. With the increase of temperature for the same O2, the oxygen saturation decreases because the bond between Hb and O2 gets denatured and partial pressure of O2 also increases. 5. Carbonmonooxide: CO binds with hemoglobin 210 times more easily than O2. The reaction HbO2 + CO → HbCO + O2 almost irreversibly displaces the oxygen molecules forming carboxyhemoglobin; the binding of the carbon monoxide to the iron center of hemoglobin is much stronger than that of oxygen, and the binding site remains blocked for the remainder of the life cycle of that affected red blood cell. Oxygen Dissociation Curve – Myoglobin . Myoglobin is an oxygen-binding molecule that is found in skeletal muscle tissue . It is made of a single polypeptide with only one heme group and hence is not capable of cooperative binding . Consequently, the oxygen dissociation curve for myoglobin is not sigmoidal (it is logarithmic) . Myoglobin has a higher affinity for oxygen than adult haemoglobin and becomes saturated at lower oxygen levels . Myoglobin will hold onto its oxygen supply until levels in the muscles are very low (e.g. during intense physical activity) . The delayed release of oxygen helps to slow the onset of anaerobic respiration and lactic acid formation during exercise Abhinav Lal, Dept. of Chemistry, Ewing Christian College,Prayagraj. Page 3 EWING CHRISTIAN COLLEGE, PRAYAGRAJ. B.Sc. Semester VI Department of Chemistry Paper – 1 (Inorganic Chemistry) Unit – 5 (Chemistry of Hemoglobin) Oxygen Dissociation Curve – Myoglobin Abhinav Lal, Dept. of Chemistry, Ewing Christian College,Prayagraj. Page 4 .
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