W9.3_Quaternary Structure of Proteins

Question 1

What is a quaternary structure of protein? Contrast homomultimeric and heteromultimeric proteins. Describe the structure and compositions of haemoglobin.

Answer 1

• Quaternary structure: combining subunits in a multi-unit protein
• Homomultimeric protein: identical subunits
• Heteromultimeric protein: different subunits
• Complexes of polypeptide chains non-covalently bind in precise ratio -> precise 3D structure
• Haemoglobin: found in blood inside RBC, carries O2
• Has 4 subunits (tetramer), with each subunit having a haem group
• Subunits are different from myoglobin but they all have proximal and distal histidines
• Two types of subunits: α/ß (differ in sequence)

Question 2

Describe the differences between myoglobin and haemoglobin. How does it influence their oxygen carrying capacities? Relate that to the sigmoid curve and the oxygen saturation levels in lungs and tissues.

Answer 2

• Mb & Hb: similar 3D folds but very different sequences
• ∵ Mb binds O2 more strongly than Hb in any pO2 (lower affinity for O2 in Hb)
• ∴ O2 goes from Hb to Mb in muscle (low pO2)
• Saturation is very sensitive for middle part of sigmoid curve (Hb) but less so for hyperbolic curve (Mb)
• Saturation is very sensitive for low pO2 at Mb curve but less so for Hb curve
• Reason for sigmoid curve in Hb: cooperativity (when first O2 bind/unload to/from Hb, it makes it easier for next one to do so)
• Larger difference in saturation level of haem between pO2 in lungs and tissues in Hb -> more O2 can be bind/unloaded

Question 3

Describe the subunit interactions of haemoglobin during oxygen binding.

Answer 3

• Binding of first O2 -> dimer α1ß1 and α2ß2 rotates with respect to each other by 15o -> changes the quaternary structure from T-state (deoxyhemoglobin, unstable) to R-state (oxyhaemoglobin, stable)
  ≈ information transferred to next subunits’ haem to ease O2 binding of second haem site
• Helix F moves EF & FG corners -> reaches αß interface -> transmitted by helix movement in next haem -> eases O2 binding in second haem
• Actual proportions of R/T states decide fractional saturation (Y) in observed Hb curve

Question 4

Explain how the allosteric factor 2,3-BPG influence the binding of oxygen in haemoglobin.

Answer 4

• Regulates function by binding to sites other than active site (central cavity of subunits for BPG)
• Bind only in T state, stabilises it to weaken O2 binding -> released when O2 binds/R state
  -> larger difference in saturation level between pO2 in lungs and tissues in haemoglobin with 2,3-BPG -> stronger/more binding/offloading for O2

Question 5

Explain how the structural differences in foetal and maternal haemoglobins are going to allow oxygen transfer.

Answer 5

• Foetal Hb: ß chains replaced by γ chains -> lower electrostatic forces between BPG & HbF (∵ His replaced by Ser -> loss of +ve charges) -> weaker affinity with BPG -> higher fractional saturation of O2 in HbF in same pO2 -> O2 is transferred from mother to the foetus at same pO2/[O2]

Question 6

Explain how carbon dioxide concentration and allosteric enzymes can affect the binding affinity to oxygen in haemoglobin.

Answer 6

• Others: CO2 concentration
• Tissues: 0.2 units lower pH than lungs -> extra 10% release of O2
• ∵ His (ß146) protonation -> salt bridge formation with Asp (ß94) -> stabilise T state -> less degree of sigmoid curve
• Others: allosteric enzymes (binding of substrate at one site -> alter binding of substrates at another in same molecule -> cooperativity, can be regulated by effectors binding at sites)