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L11-Protein_Function_II-Hb - Lecture 11 Hemoglobin Overview...

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38 Lecture 11 Hemoglobin Overview function is to transport oxygen found in erythrocytes red blood cells small (6-9 μm in diameter) biconcave disks formed from stem cells called hemocytoblasts contain no nucleus or organelles survive about 120 days in humans function is to carry hemoglobin in lungs and through the heart Hb is about 96% saturated in tissues it is about 64% saturated circulatory system was developed to transport oxygen in blood plasma (the fluid portion of blood) small organisms don't need hemoglobin. organisms with < 1mm thickness can obtain oxygen through diffusion; no circulatory system is needed the solubility of molecular oxygen is low in such a medium, so a transporter is needed normal blood contains about 150 g of Hb/l and carries oxygen in concentrations of approximately 10 mM, which is about the same concentration as in air. in contrast, blood without Hb would accept oxygen at a concentration of approximately 100μM (0.1 mM) Structure solved in 1968 by Max Perutz Mr of 64,500 roughly spherical with a diameter of 5.5 nm Hb molecule dimensions of 64 x 55 x 50 Å a tetrameric protein contains 4 heme groups 1 per polypeptide contains two types of globin, α and β the α -c hains have 141 residues the β -chains have 146 residues < half of the residues in the α and β c hains are identical, but their global structures are similar to each other and to myoglobin same helix naming pattern that is used for myoglobin is used for hemoglobin strong interactions between the α 1 - β 1 pair and the α 2 - β 2 pair of monomers involves >30 residues mild treament with urea will separate the αβ protomers from one another but not the individual subunits from each pair
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39 weaker interactions betwee n α 1 - β 2 and α 2 - β 1 involves 19 residues primarily hydrophobic interactions with many H-bonds and some salt bridges (ion pairs) Structural Changes in Hemoglobin oxyhemoglobin: hemoglobin with oxygen bound deoxyhemoglobin: hemoglobin with no oxygen bound when O 2 binds the α 1 - β 2 and a 2 - β 1 contacts shift changes quaternary structure change is so significant that crystals of deoxyhemoglobin will shatter when exposed to O 2 one αβ d imer rotates ~15° with respect to the other
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L11-Protein_Function_II-Hb - Lecture 11 Hemoglobin Overview...

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