Hemoglobin

Hemoglobin

1. Oxygen Binding Protein

2. Heme

   • Carrier of Oxygen
     • Mb
       • Conformational changes with oxygen binding
         • deoxy-Mb: Fe is out of the plane of polyphorin ring and is pulled toward the histidine
         • Oxygen binding causes Fe to move back into the plane of the porphyrin ring
           • Small shift in position on the F helix.
         • No effect on biological properties of Mb
     • Hb
       • Sigmoidal kinetics for oxygen binding
       • 4 binding sites
       • O2 binding (4 binding sites) is positively cooperative
       • Same conformational change as Mb
         • Explains cooperativity in Hb
       • Structure
         • Symmetric a2B2 tetramer
         • Four heme groups
           • Long way apart
             • Therefore, oxygen binding cooperativity cannot be explained by direct heme-heme interactions
         • Oxygen binding
           • Small changes in tertiary structure in monomer
           • Large change in quaternary structure
           • Each aB dimer moves as rigid body. Two dimers rotate through 15 degrees with respect to each other
     • Oxygen binds to 2+ form to generate oxy-Mg or oxy-Hb
   • Iron ion + Four nitrogens of porphyrin ring
     • Fe can exist in 2+ (ferrous) and 3+ (ferric) oxidation states
       • Fe likes to have 5 or 6 ligands depending on oxidation state
         • Oxy form: four ligands come from porphyrin ring, one is a histidine side chain, and the sixth is oxygen
         • deoxy-Mb and deoxy-Hb (where iron is 2+ (ferrous) and oxygen is not bound) there is no sixth ligand
         • met form: iron exists as 3+ (ferric) state and can't bind oxygen (sixth ligand is water)

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