CHEM3040

• Porphyrins are nature's favourite macrocycles, differing in ring substituents (EWG or EDG) which allows for a wide variety of properties.
• Porphyrins usually lose two H+ to become a dianionic ligand.
• The basic structure of porphyrin consists of four pyrrole units linked by four methene bridges.
• The porphyrin macrocycle is aromatic with 22 π electrons, but only 18 are involved in any one delocalization pathway.
• Fe-containing porphyrin macrocycles bind and transport/store O2.
• Haemoglobin exhibits a cooperative mechanism when binding O2.
• The binding of O2 is regulated allosterically to control release in Haemoglobin.
• Myoglobin is found in muscle tissue and stores O2.
• CO binds more tightly to Hb and can disrupt the O2 transport process.
• Porphyrins obey Hückel's rule of aromaticity (4n+2)π electrons where n=4) and have been shown by X-ray crystallography to be planar.
• The aromatic character of porphyrins can also be seen by NMR spectroscopy.
• Metalloporphyrins are ideal for complexation of a 1st row M2+ transition metal due to the good fit into the central cavity.
• Once coordinated, the ring is very rigid due to delocalization of the π electrons.
• Haemoglobin has two functions: bind O2 molecules and transport them from the lungs to the muscles, and help transport CO2 to the lungs.
• Myoglobin is contained in the muscles where it receives O2 from haemoglobin and stores it.
• Haemoglobin has four subunits (2a2b), each with one Fe-haem group.
• Conformational changes in haemoglobin (Hb) switch it from T (low O2 affinity) to R (high O2 affinity).
• Myoglobin has one Fe-haem group with a molecular weight of 64,500 Da.
• Binding of O2 to Hb is a complex equilibrium.
• The T/R switch in Hb is controlled by other allosteric effectors.
• O2 reacts with water to form carbonic acid (mediated by carbonic anhydrase), leading to a decrease in pH.
• At low pH, Hb becomes protonated, stabilising the T state.
• CO2 also reacts with terminal lysine residues to form carbamate groups, stabilising the T state.
• The Bohr effect states that haemoglobin's O2 binding affinity is inversely related both to acidity and to the concentration of CO2.
• The T state of Hb has an internal cavity that can bind diphosphoglycerate, stabilising the T state.
• Expression of diphosphoglycerate leads to a reduction in Hb O2 affinity.
• Diphosphoglycerate is a product of glucose metabolism, hence concentrations are typically high in "working" tissue (like CO2 and H+).
• The O2 molecule binds to Fe(II) with "bent" geometry.
• CO is a π acceptor ligand, binding strongly to low oxidation state metals like Fe2+.
• O2 is a weaker π acceptor, hence linear binding modes are not as stable.
• KCO/KO2 ratio in haem model system is 25000, in myoglobin it is 200.
• KH = [HbO2]/[Hb][O2] is 2.8 for haemoglobin.
• Haemoglobin can accept 4 molecules of O2 but the binding of the 4 is not independent.
• Affinity for binding the 4th O2 molecule is approximately 300 times more favourable than the 1st O2.
• These factors favour O2 transport.
• Myoglobin (Mb) has a greater affinity for O2 than Haemoglobin (Hb) in order to effect transfer of O2 to the cell.
• KM = [MbO2]/[Mb][O2] is 0.0001 for myoglobin.
• Myoglobin is largely converted to oxymyoglobin even at low O2 concentration, as in cells.
• Hb has a sigmoidal O2 binding curve with high affinity for O2 at high pO2 (lungs) and low affinity for O2 at low pO2 (tissues).
• Affinity of Hb is altered by allosteric effectors such as CO2, H+, and 2,3-bisphosphoglyceric acid.