Hemoglobin

• • hemoglobin has quaternary structure: 2 alpha subunits and 2 beta subunits
  • Myoglobin does not have quaternary structure: only one subunit with one oxygen binding site
  • both myoglobin and hemoglobin use heme groups to bind to oxygen
• Iron can form two additional bonds to the Heme in its 5th and 6th coordination sites.

  • 5th site= bound to proxmimal histidine
  • 6th site = binds to oxygen
  • oxygen binding changes the position of Fe so it fits in the porphyrin ring
• oxygen binds to hemoglobin cooperatively

  • can be seen by sigmoidal curve

  A) Hemoglobin

  
• Cooperativity enhances oxygen deliver in hemoglobin

  • hemoglobin: 66% of potential oxygen binding sites release oxygen in the tissues
  • myoglobin: 7% of potential oxygen binding sites release oxygen in the tissues

  
• Oxygen binding leads to change in structure, and transitions T state (deoxyhemoglobin) active sites to R states (oxyhemoglobin)


  • binding of oxygen shifts the proximal histidine up
  • this results in movement of the corresponding alpha helix and alters the interface of alpha-beta dimers of hemoglobin: transitions T state to R state
• Allosteric regulation of hemoglobin

  2,3-BPG is a negatively charged molecule that binds to a pocket on deoxyhemoglobin (T-form) and stabilizes it. Thus, for T to R transition, 2,3-BPG must be expelled.
• 2,3-BPG binds the central cavity of deoxyhemoglobin, interacting with positively charged residues, therefore stabilizing the structure of the T state
• How is oxygen transferred from maternal hemoglobin to fetal hemoglobin?

  affinity of fetal hemoglobin for oxygen needs to be greater than the affinity of maternal hemoglobin for oxygen

  • fetal hemoglobin has reduced affinity for 2,3-BPG which increases affinity for oxygen
• Decrease in pH promotes release of oxygen (part of bohr effect)

  • low ph favours formation of a salt bridge that stabilizes the T-state form of hemoglobin, thus favoring the release of oxygen
  • high ph is when His 146 is deprotonated, disrupting the salt bridge and favouring oxygen binding
• carbon dioxide promotes release of oxygen (bohr effect)

  • hemoglobin responds to carbon dioxide with a decrease in oxygen affinity
  • CO2 reacts with terminal amino groups to form carbamate (neg charge)
  • carbamate forms salt bridges (stabilizing T form), thus decreasing oxygen affinity
• Explain why sickle-cell anemia results from the difference of asingle amino acid residue of hemoglobin

  sickle cell hemoglobin (HbS) has Glu 6 to Val 6 mutation on β chains

  • Reduces solubility of deoxyhemoglobin
  • when there is a high conc' of deoxyhemoglobin, Val 6 interacts with Phe 85 and Leu 88 to form aggregates (clusters)