Chapter 7 Possible Essay Question

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  1. 1)      What is the difference in structure between myoglobin and hemoglobin?
    a.       Myoglobin exists as a single polypeptide, whereas hemoglobin comprises four polypeptide chains. The four chains in hemoglobin bind oxygen cooperatively, meaning the binding of oxygen to a site in one chain increases the likelihood that the remaining chains will bind oxygen. Furthermore, the oxygen-binding properties of hemoglobin are modulated by the binding of hydrogen ions and carbon dioxide in a manner that enhances oxygen-carrying capacity. The similarities between them is the heme group, which is a molecule that contains an organic molecule and an iron ion that will allow the hemoglobin to bind to oxygen. 
  2. 1)      What does the binding of the oxygen at the sixth coordination site do? 
    a.       Binding of oxygen at the sixth coordination site is going to transfer electrons, allowing the iron to fit into the plane of the porphyrin ring. This movement of the iron will cause the proximal to shift as well, since the proximal histidine is bound to the fifth coordination site of the iron. Since the proximal histidine is also attached to the alpha helix, the alpha helix will also shift, which causes the movement to be transmitted between the dimers since the carboxyl terminal end of the helix is located in the interface of the a1B1-a2B2 dimers. This small movement at the surface has therefore caused a transmittance of the signal to the dimers, which will enable them to rotate by 15o. These localized conformational shifts allow them to move more freely with respect to one another in the oxygenated state than in the deoxygenated state. Frthermore, they will cause the transition from the T to the R state. 
  3. 1)      Why is it important that oxygen, when released, leaves as dioxygen rather than superoxide?
    a.       The reason for this because superoxide and other species generated from it are reactive oxygen species that can be damaging to many biological materials and also because the release of superoxide would leave the iron ion in the ferric state, which is termed metmyoglobin and does not bind oxygen.
  4. 1)      What features stabilize the oxygen complex in myoglobin?
    a.       What stabilizes the oxygen complex in myoglobin is the distal histidine, which will donate an extra hydrogen bond to the superoxide molecule, which is strengthening this interaction. The distal histidine sits in the binding pocket.
  5. 1)      What is the structure of hemoglobin?
    a.       Hemoglobin is a tetramer with four subunits—two alpha chains and two beta chains. Each of the subunits consists of alpha helices arranged in a similar way that myoglobin is, called the globin fold. It is described as a pair of identical alpha-beta dimers that associate to form the tetramer. In deoxyhemoglobin, the alpha-beta dimers are linked by an extensive interface, which includes the carboxyl terminus of each chain. 
  6. 1)      Explain the oxygen-binding curves.
    a.       Oxygen binds with high affinity to myoglobin. Therefore, the curve immediately rises sharply as the pO2 increases and then levels off. The hemoglobin, on the other hand, is a sigmoid curve, that demonstrates the cooperative properties of hemoglobin. At low oxygen levels, the curve of hemoglobin demonstrates an inability to become fully saturated. However, as the concentration increases, hemoglobin, by the curve, shows a stronger affinity for the oxygen, rising steadily, until eventually, it levels off at its maximum saturation. It gets half saturated at p26. When the pH decreases, such as in actively metabolizing tissues, the pO2 is lower; and so, oxygen is released and the saturation level drops. 
  7. 1)      Why wouldn’t myoglobin be an efficient oxygen transporter?
    a.       It would release only 7% of the oxygen it carries due to its high affinity and tight binding of the oxygen. 
  8. 1)      Explain hemoglobin during exercise.
    a.       Under resting conditions, the oxygen concentration in muscles is approximately 40 torr, but during exercise, it drops to 20 torr. In the decrease from 100 torr in the lungs to 40 torr in resting muscle, the oxygen saturation of hemoglobin is reduced from 98% to 77%, allowing 21% of oxygen to be released. In a decrease from 40 torr to 20 torr in actively metabolizing tissue, the oxygen is released 45% over a drop of 20 torr. Thus, because more oxygen is released in actively metabolizing tissue than in resting tissue, oxygen is effectively delivered to tissues in need. 
  9. 1)      What is 2,3-BPG and its function?
    a.       It is a molecule that lowers hemoglobin’s oxygen affinity. This molecule stabilizes the T state, which is usually so unstable it pushes the equilibrium to the R state. This is a highly anionic compound present in red blood cells at approximately the same concentration as that of hemoglobin. It lowers oxygen’s affinity of hemoglobin by binding int eh center of the tetramer, in a pocket present only int eh T form. From T to R, this pocket collapses and 2,3-BPG is released; and, the bonds between hemoglobin and 2,3-BPG must be broken for this to occur. In the presence of 2,3-BPG, more oxygen binding sites within the hemoglobin tetramer must be occupied in order to induce the T to R transition; and so, hemoglobin remains in the lower affinity T state until higher concentrations of oxygen are reached. 
  10. 1)      Explain why fetal hemoglobin has a higher oxygen affinity than maternal hemoglobin. ‘
    a.       Fetal hemoglobin is made of two alpha and two gamma chains. Because of the gamma chains, two positive charges are lost in the binding site of hemoglobin, causing the affinity for 2,3-BPG to decrease. With this decrease of affinity for 2,3-BPG, the fetal hemoglobin will occupy the R state more often than the T state, causing the oxygen to transfer from the maternal hemoglobin to the fetal hemoglobin. 
  11. 1)      What is the effect of hydrogen ions and carbon dioxide on oxygen transport? 
    a.       Drops in pH and carbon dioxide affect oxygen transport by stabilizing the T state. 
  12. 1)      What are the effects of carbon monoxide on oxygen transport? 
    a.       It competes with oxygen for binding to the hemoglobin; and, it is 2oo times more effective at binding. It also can bind at one binding site on a subunit and induce the rest of the subunits to convert to their R state, thus causing the affinity of oxygen to increase but be unable to be released. 
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Chapter 7 Possible Essay Question
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