Lecture9 - Lecture 9 10/12/11 Background reading: Berg, et...

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Lecture 9 10/12/11 Background reading: Berg, et al.: pages 195 – 204 Garrett and Grisham: Chapter 15: Bottom of Page 491- 500 Segel: Pages 86 - 90 Outline: ± Comparison of hemoglobin with myoglobin ± Conformational changes occur when hemoglobin binds to oxygen. ± Allosteric behavior of hemoglobin. ± Hemoglobin as a blood buffer.
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± Comparison of hemoglobin with myoglobin (continued): Oxygen binding: Binding curve of hemoglobin for oxygen is sigmoidal. Sigmoidal binding curves suggest that the binding of oxygen at one site on the tetramer increases the likelihood that oxygen binds at the remaining unoccupied sites.
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Fe NN N N Fe ++ 5th 6th Iron in heme group Deoxymyoglobin Oxymyoglobin Proximal histidine binds to the 5 th coordination site. When oxygen binds the iron moves into the plane of the ring. O 2 binds to 6 th coordination site.
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Resonance occurs between the two structures when oxygen is bound to iron. If superoxide were to be released it could cause damage and could leave the iron in a ferric state where it could no longer bind oxygen. When oxygen is bound to myoglobin, another histidine, called the distal histidine , forms a hydrogen bond with the bound oxygen. This bond stabilizes the oxygen complex so that superoxide is less likely to be released. To prevent this problem: Thus the protein portion of myoglobin controls the reactivity of the heme and makes it more suitable for reversible oxygen binding.
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The structure of deoxyhemoglobin is stabilized by specific hydrogen bonds and salt linkages, which include eight salt bridges linking the polypeptide chains. The carboxyl-terminal groups of all four chains are anchored in salt linkages. Β 2 -chain carboxyl terminal end H146 α 1 -chain amino-terminal end α 2 -chain β 1 -chain amino-terminal end carboxyl terminal end K40 H146 K40 Two of these linkages (shown with red dashed lines) are between K40 of the α -chains and H146 of the β -chains.
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Lecture9 - Lecture 9 10/12/11 Background reading: Berg, et...

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