Medical Pharmacology Chapter 14:  Physics and  Chemistry for Anesthesiology

Gas Transport in Blood: Hemoglobin

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Note the nonlinear shape of the hemoglobin curve compared to the oxygen solubility curve above

illustration attribution: Raj Srinivasan, 1996

 

  • The underlying mechanism for these curve shifts is based on the presence of titratable groups in Hb which can accept or release the proton (H+).  The consequence of accepting a proton (or releasing one) is that the entire structure of hemoglobin may be somewhat altered and a consequence of this change is a change in the likelihood (affinity) of oxygen binding.  The dependency of the curve shape on pH is referred to as the Bohr effect, named after Christian Bohr, a Dutch physiologist who first noted the effect of pH on oxygen association from hemoglobin.

Heme Group; Protoporphyrin interactive

 

Hemoglobin subunits:  Deoxy form (left); Oxygenated form (orange) (Right)

  • 2,3 diphosphoglycerate (DPG): Organic phosphates have a significant effect on the oxygenation curve for hemoglobin.  Probably the most significant molecule physiologically is 2,3 diphosphoglycerate (DPG).

  •  2,3 diphosphoglycerate (DPG)

  • The effect of DPG follows from its binding preferentially to deoxyhemoglobin.  Accordingly, DPG will shift the oxygenation curve in favor of the deoxy state.  In order to saturate about 50% of the available DPG binding sites on hemoglobin, a DPG concentration of about 15 micromolar is required.  Since normally the erythrocyte concentration of DPG is many orders of magnitude higher (4 mM), the deoxy form is strongly favored.  

    • Typically, physiological levels of DPG move the oxygen-hemoglobin association curve somewhat to the right; however, this effect is more pronounced during somewhat prolonged hypoxic states during which DPG concentrations increased, shifting the curve further to the right. 

    • The consequence of this curve shift is that oxygen may be released to the tissue at as much as 10mm Hg elevated tissue pO2 than without the DPG effect. It has been suggested that adaptation to hypoxic conditions (including altitude acclimatization) depends in part on alterations in DPG levels.

 
 
 

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