Proteins in Action: Oxygen Transport and Storage by Haemoglobin and Myoglobin Flashcards
Describe how oxygen changes haemoglobin’s shape
It is a conformational change that shifts haemoglobin between the T state and the R state. It is a shift in the orientation of secondary structural elements that does this.
Describe the different methods haemoglobin uses to do the conformational changes
Shifting between T and R states os key for physiological Haemoglobin function.
Conformational changes (especially affecting helix F) can change the balance of equilibrium of T and R states.
Mechanisms to do this:
- Allosteric regulation
- pH
- Physiological changes/genetic variants
Describe how co-operativity is prominent only in the presence of allosteric inhibitors binding.
Allosteric inhibitors BPG, CO2 and H+ stabilise the T-state. This unmasks cooperativity.
In the absence of inhibitors haemoglobin is predominantly in the R state.
Describe how haemoglobin is under allosteric control of BPG
BPG binds deoxy-hameoglobin by electrostatic interactions
- BPG stabilises haemoglobin in deoxy-T-state which reduces oxygen affinity
- BPG is produces during respiration in peripheral tissues, so promotes oxygen release where it is needed.
Describe how CO2 reduces O2 affinity
CO2 reduces O2 affinity by both directly and via lowering pH of blood.
Elevated CO2 and low pH in metabolising tissues both reduce the affinity of haemoglobin for O2 known as the Bohr affect.
- Lowers the pH which favours protonation of histamine residues - promoting ionic interactions
- CO2 can bind directly to N-termini of the beta subunit which stabilises the T state
Describe how foetuses capture oxygen across the placenta
- Foetal haemoglobin includes alternate isoforms (gamma, epsilon and zeta) with higher affinities for O2.
- A gamma subunit replaces a beta chain of haemoglobin
- Different amino acid sequences of normal haemoglobin subunits alter their oxygen binding properties
- foetal haemoglobin also holds oxygen tighter because it is less sensitive to BPG (the gamma chain has serine residues replacing two of the histidine residues at the BPG binding site)
Describe Sickle Cell Hameoglobin
- It is a variant in which the glutamate at AA 6 has been swapped to a valine, which causes large polymers to be formed.
- The ‘sickle’ shape red blood cells get stuck in blood capillaries and this causes a range of secondary debilitating effects (but confers resistance to malaria).
- This variant enables an abnormal hydrophobic interaction between haemoglobin tetramers (in the partially exposed deoxy-T-state of the haemoglobin)
What are some treatment options for sickle cell disease?
- Remove the variant or change the biochemistry
CRISPR based therapy:
- Upregulates foetal haemoglobin by changing a transcription factor (has the different gamma gene. this is translated so there are some cells that are working properly to replace some of the ones that aren’t functioning)
- Very expensive
Voxelotor (sold as Oxygryta) is a haemoglobin oxygen-affinity modulator:
- Stabilises the oxygenated state of haemoglobin - so increases the oxygen affinity (by pushing it into the R state - as the polymers are only formed in the T state)
Briefly sum up the affects of BPG, CO2 and low pH
- Oxygen binding is weakened allosterically by BPG, CO2 and low pH
- This is described as shifting the tetramer to the T state
- In the presence of BPD, CO2 and low pH, haemoglobin displays cooperative binding of oxygen, evident in a sigmoidal binding curve
Briefly sum up the physiological effects
- Oxygen affinity is tied in foetuses
- Sickle-cell anaemia results from haemoglobin polymerisation
what is the key structural feature that works against oxygen binding?
anything hat keeps alpha helix F away works AGAINST oxygen binding
- haemoglobin is flattened when oxygen binds, which pulls HisF8 TOWARDS the binding site
Does haemoglobin have both allostery and co-operativity even though its not an enzyme?
Yes
Allostery: BPG and CO2 binding to other sites
Co-operativity: shift between T and R states
