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12 22 Sept 2010 - 2 2 Hemoglobin tetramer 1 1(2 2 1 switch...

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α 2 β 2 Hemoglobin tetramer ( α 1 β 1 )( α 2 β 2 ) α 1 α 2 β 1 β 2 A B G G B A A B G G B A C FG C FG C FG C FG switch hinge switch hinge
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The low and high affinity states follow hyperbolic binding Oxygen titrations involve stepwise increases of oxygen partial pressure At low pO2, the T-state low affinity curve is followed As pO2 increases, the Hb shifts to R-state high affinity The resultant curve is sigmoidal Sigmoidal curves are indicative of cooperativity – one subunit influencing the affinity of another subunit through conformational changes
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The binding curve for myoglobin is hyperbolic The binding curve for hemoglobin is sigmodial Higher affinity in the presence of higher oxygen concentrations Lower affinity at lower oxygen concentrations The carrier activity of Mb and Hb are coupled Myoglobin acts as a reservoir to maintain oxygen levels When activity is high, Mb can extract oxygen from Hb
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CO and O2 have similar k a (k on ) but CO has slower kd (k off ) therefore K d is much lower = higher affinity Most people have about 1 – 3 % of Hb sites occupied by carbon monoxide, smokers up to 15% Ligand binding increases affinity, CO locks Hb into the high affinity state The switch to T-state is blocked and O 2 release is limited
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Nomenclature Cooperativity – Changes in one subunit influence properties of adjacent subunits - Cooperativity can be either positive or negative
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