What is The oxygen dissociation curve of myoglobin & hemoglobin is different due to? and explain in detail?
Ans. is `b' i.e., Cooperative binding in Hb Cooperative binding is responsible for sigmoid shape of the oxygen-hemoglobin dissociation curve. As myoglobin is monomeric (consists of one polypeptide chain only), it can bind only one molecule of oxygen and for the same reason myoglobin cannot show the phenomenon of cooperative binding. Hence, the oxygen?myoglobin dissociation curve is hyperbola as compared to sigmoid shape of Hb-O2 curve. Hemoglobin - O2 binding Each molecule of hemoglobin can combine with upto four molecules of oxygen. Combination with the first molecule alters the conformation of the hemoglobin molecule in such a way as to facilitate combination with the next oxygen molecule. In light of this, if we look at the curve, as the PO2 stas rising from 0 mm Hg upwards, initially all hemoglobin molecules in blood stas combining with their first oxygen molecule. This is the most difficult molecule to combine with. Hence saturation rises only slowly with initial rise in PO2. As PO2 rises fuher, hemoglobin molecules combine with their second, third and fouh molecules, which are progressively easier to combine with. Hence saturation rises steeply between PO2 of 15 mm Hg and 40 mm Hg. When PO2 rises still fuher, oxygen finds most of the hemoglobin molecules carrying four molecules of oxygen each. Since no molecules of hemoglobin can carry more than four molecules of oxygen, there is not much scope for more O2 combining with hemoglobin. Hence the curve becomes almost flat again beyond the PO2 of 60 mm Hg. Thus, the primary reason for the sigmoid shape of the oxygen-hemoglobin dissociation curve is that out of the four molecules of oxygen that can combine with a hemoglobin molecules, the first combines with the greatest difficulty and binding of an oxygen molecules increases affinity to next O2 molecule. This phenomenon is termed as cooperative binding or cooperativity, i.e., a molecule of O2 binds to a hemoglobin tetramer more readily if other O2 molecules are already bound. Myoglobin O2 binding Myoglobin is present in higher concentration in red (slow) muscle fibers. Myoglobin has greater affinity for oxygen than hemoglobin and its P50 is only 5 mm Hg (as compared to PO2 of hemoglobin which is about 26 mm Hg). Therefore, myoglobin-oxygen dissociation curve is shifted far to the left than Hb-O2 dissociation curve. It has shape of hyperbola as compared to sigmoid shape of Hb-O2 curve because it binds 1 molecule of O2 per mole (in comparison to Hb which binds 4 molecules of O2 per mole). The role of myoglobin is to bind O2 at very low PO2 and release them at even lower PO2, for example in exercising muscles where PO2 close to zero.