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• Hemoglobin


  Present in red blood cells and responsible for oxygen transport in animals
• Hemoglobin contains 65% of iron in the human body
• Myoglobin


  Present in muscle cells and transports and stores oxygen across the muscle cells
• Structure of myoglobin


  • Tertiary structure composed of a globin chain (secondary structure that is made up of primary chain) and a haem group (contains Fe(II) and Protoporphyrin IX)
• Globin chain


  Contains 153-160 amino acid residues
• Protoporphyrin IX


  Belongs to the family of tetradentate planar ligands called porphyrins, contains 4N donor atoms and is heteroaromatic in nature, contains 4 pyrrole like rings joined by CH groups, also contains peripheral substituents
• Myoglobin molecule


  • Has a compact shape with a protein chain folded about the haem group, globin chain is linked to the haem group through a histidine residue (proximal histidine), has 8 major helical regions and non-helical loops/regions, interior is hydrophobic and the exterior is hydrophilic, contains two hydrophilic histidine residues that are essential for biological activity
• Haem iron coordination sphere

  The FeII is coordinatively unsaturated (CN = 5) as it contains less than its maximum coordination number, the vacant coordination site is used to complex with oxygen, the metal ion is prevented from getting oxidized as the haem group is placed into a non-polar crevice
• Forms of myoglobin


  • Myoglobin (Fe2+ - purplish red)
  • Oxymyoglobin (Fe2+ - bright red. O2 ligand)
  • Metmyoglobin (Fe3+ - brownish red. Water ligand)
• Myoglobin/Hemoglobin


  • Iron II, d6 is high spin and paramagnetic, has one empty coordination site which can complex with oxygen and lies outside the porphyrin plane, is coordinatively unsaturated, contains 4 unpaired electrons
• Oxymyoglobin


  • Low spin and diamagnetic, smaller in size (electrons can be concentrated into 3 orbitals), coordinatively saturated and fits in the porphyrin plane
• Hemoglobin structure


  • Nearly spherical shape with haems being in non-polar crevices
• The amino acids in only 24 positions of both Hemoglobin and myoglobin 3D structure are identical
• Hemoglobin


  Has 4 subunits (myoglobin like) - 2 alpha and 2 beta chains which are linked by salt bridges, HbA have a tetrameric structure (alpha has 141 a.a and beta has 146 a.a) and each alpha chain is in contact with both beta chains
• Salt bridges


  The most important form of interactions between hemoglobin subunits, can take part in various forms in the protein, terminal backbone groups can also take part in salt bridges formation
• Hemoglobin oxygenation is reversible