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Hemoglobin: Difference between revisions

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Both α and β chains of Hb resemble [[myoglobin]] (the single-chain O2-binder in muscle), both in overall tertiary structure and in using an Fe atom centered in a heme group as the site where oxygen is reversibly bound.  The heme is surrounded by a hydrophobic pocket, which is necessary in order for it to bind oxygen reversibly without undergoing oxidation or other undesirable reactions.   
Both α and β chains of Hb resemble [[myoglobin]] (the single-chain O2-binder in muscle), both in overall tertiary structure and in using an Fe atom centered in a heme group as the site where oxygen is reversibly bound.  The heme is surrounded by a hydrophobic pocket, which is necessary in order for it to bind oxygen reversibly without undergoing oxidation or other undesirable reactions.   


Here we see some of the hydrophobic sidechains that form the heme pocket. They actually surround the binding site so thoroughly that O2 cannot get in or out without parts of the protein moving out of the way a bit, so that its dynamic properties are essential to have any O2 binding at all; this restrictive process also increases the specificity of ligand binding.
The heme binding pocket contains mostly <scene name='32/32/Heme_binding_pocket_apo/1'>hydrophobic residues</scene>, shown in grey. They actually surround the binding site so thoroughly that O2 cannot get in or out without parts of the protein moving out of the way a bit, so that its dynamic properties are essential to have any O2 binding at all; this restrictive process also increases the specificity of ligand binding.


The shift between R and T state requires subunit interactions and does not occur in myoglobin, or in isolated α or β chain monomers.  These monomers bind O2 quite tightly, which would work well for loading O2 in the lungs but would not allow unloading it for delivery to the tissues.  Therefore, the central critical feature of hemoglobin function is how it achieves, uses, and allosterically controls cooperativity between the 4 binding sites in the tetramer to tune O2 binding for satisfying physiological needs.
The shift between R and T state requires subunit interactions and does not occur in myoglobin, or in isolated α or β chain monomers.  These monomers bind O2 quite tightly, which would work well for loading O2 in the lungs but would not allow unloading it for delivery to the tissues.  Therefore, the central critical feature of hemoglobin function is how it achieves, uses, and allosterically controls cooperativity between the 4 binding sites in the tetramer to tune O2 binding for satisfying physiological needs.
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