BCH210H1 Lecture Notes - Lecture 22: Pyrrole, Ferrous, Myoglobin
25 May 2018
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Lecture 22: Hemoglobin
Hemoglobin
• A classic example of allostery – regulation by oxygen binding
• Hemoglobin and myoglobin are oxygen transport and storage proteins
• Compare the oxygen binding curves for hemoglobin and myoglobin
• Myoglobin is monomeric (one polypeptide chain); hemoglobin is tetrameric
• Mb: 153AA, 17,200 MW (17kDal)
• Hb: 2 a chains of 141 residues, 2 B chains of 146 residues
Heme Prosthetic Group
• Mb has one heme prosthetic
• Hb has 4 heme prosthetic
• Fe2+ - ferrous state, can bind oxygen
• Protoporphyrin (without Fe) – 4 pyrrole rings linked
together though methanyl bridges
Fe2+ is coordinated by His
• Iron interacts with six ligands in Hb and Mb (covalent)
• 4 of these are N atoms of the porphyrin
• A 5th ligand is donated by the imidazole side chain of AA residue His
• When Mb or Hb bind oxygen, the O2 molecule adds to the heme iron as the 6th ligand
• The O2 molecule is tilted relative to a perpendicular to the heme plane
Myoglobin
• Is a monomeric heme protein (6 a helices)
• M polypeptide radles the heme group (resides in hydrophobic cleft)
• Fe in Mb is Fe2+ - ferrous iron, the form the binds oxygen (oxy-myoglobin)
• Oxidation of Fe yields 3+ charge – ferric iron
• Mb Fe3+ is called metmyglobin and dose NOT bind oxygen
Hemoglobin
• In red blood cells in high concentration (more than 90% of intracellular protein in RBC)
• Go to lungs, pick up O2 and transfer to tissues
• Non covalent interactions
• Central cavity
• Hemoglobin can bind 4 O2
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Document Summary
Heme prosthetic group: mb has one heme prosthetic, hb has 4 heme prosthetic, fe2+ - ferrous state, can bind oxygen, protoporphyrin (without fe) 4 pyrrole rings linked together though methanyl bridges. In red blood cells in high concentration (more than 90% of intracellular protein in rbc: go to lungs, pick up o2 and transfer to tissues, non covalent interactions, central cavity, hemoglobin can bind 4 o2. Myoglobin has higher affinity for o2 (lower p50) In capillaries, po2 is about 40 torr and oxygen is released from hb subunits more favorable (sigmoidal) They physiological significance of the hb:o2 interaction: hb must be able to bind oxygen in the lungs, hb must be able to release oxygen in capillaries. If hb behaved like mb, very little oxygen would be released in capillaries: sigmoid, cooperative oxygen binding curve of hb, makes its physiological actions possible. R-state (high affinity, relaxed, oxy form) curve is like mb hold on to o2.
