Haemoglobin: Allostery and Cooperativity Flashcards
This has a dished haem shape.
Deoxyhaemoglobin
When in the presence of oxygen, this flattens the haem and pulls histidine F8 and helix F towards binding site.
Oxyhaemoglobin
These are shifts in the orientation of protein secondary elements.
Conformational changes
This is an allosteric inhibitor of O2 binding to haemoglobin
BPG
How is BPG produced?
BPG is produced during respiration in peripheral tissues, and promotes oxygen release where needed
This allows efficient unloading of oxygen.
Cooperativity
Sigmoidal binding is an indication of what in hemoglobin?
Cooperativity
This model suggests that subunits must be in the same state.
MWC model
In the MWC model, the _______ stabilise the T form and the ________ stabilise the R form.
Inhibitors, activators
The R form is usually what form of hemoglobin?
The oxygenated form
The T form is usually what form of hemoglobin?
Deoxygenated form
What stabilises the T state of hemoglobin?
Inhibitors
What does unmasking cooperativity mean?
Stabilising the T state - so making sure that O2 will be released
Why do we need inhibitors?
To push the hemoglobin to a T state, that will release O2 and push the hemoglobin back to an R state
What other 2 molecules reduces the affinity of hemoglobin for O2?
elevated CO2 and low pH (so elevated H+)
This effect describes how elevated CO2 and low pH in metabolising tissues reduces the affinity of hemoglobin for O2.
Bohr Effect
This type of hemoglobin includes alternate isoforms with higher affinities for O2.
Foetal hemoglobin.
What type of hemoglobin enables the foetus to capture oxygen in the placenta?
Foetal hemoglobin
What is on the B chain’s binding site for BPG in foetal hemoglobin? (this is what makes it structurally different to adult hemoglobin)
Has a serine instead of a histidine
This is a mutation by which the oxidation of haem Fe2+ to Fe3+ occurs, resulting in a shift of one subunit to the R state conformation without oxygen bound.
Methaemoglobin
How does the methaemoglobin mutation impair the function of hemoglobin? (there are 2 ways)
1) the methaemoglobin subunit doesn’t bind to oxygen despite being in R state (because of Fe3+)
2) 3 other subunits are shifted to the R state - so they don’t release oxygen in tissues
This enzyme is responsible for regenerating hemoglobin by reducing methaemoglbin back to Fe2+ state using transfer of electrons from NADH.
Cytochrome b5 reductase
This is the mutation of His E7 to Tyr E7 - it changes the environment which causes Fe2+ to oxidise to Fe3+
HbM / Boston haemoglobin
This has a gain of function mutation - which enables an abnormal hydrophobic interaction between Hb molecules particularly when in deoxy form
HbS / sickle cell hemoglobin
How does sickle cell hemoglobin distort the red blood cells?
Through polymerisation of Hb into chains
Sickle cell anemia results from _______
haemoglobin polymerisation
What interactions enables BPG to bind to deoxygenated-Hb?
electrostatic interactions
Remember: BPG is an inhibitor so it will stabilise the T state of haemoglobin (reduce O2 affinity) so it will always promote oxygen release where its needed.
How do positively charged side chains like histidine and side chain and amino termini contribute to the binding of BPG?
They signal BPG where to bind (facilitate the binding of BPG)
- positively-charged side chains in B chains make positively charged allosteric sites for allosteric effectors like BPG
What does stripped haemoglobin mean?
The haemoglobin is predominantly in the R state - shows little cooperativity because R-state means that oxygen affinity to haem is quite high.
Why is cooperativity most prominent in the presence of allosteric inhibitors?
Because allosteric inhibitors unmask cooperativity and stabilise the T state.
Remember: stabilising the T state means that the binding affinity of O2 is reduced so haemoglobin will release O2 more efficiently (thus undergo conformational shape change more likely)
What 3 things stabilise the T-state and unmask cooperativity?
Allosteric inhibitors like BPG, CO2, and H+
What is the ideal pH where O2 can bind less tightly to Hb?
Low pH (this means elevated H+) (low pH reduced affinity of haemoglobin for O2)
Why is there an increase in BPG in higher altitudes?
Being in higher altitudes means a decrease in the partial pressure of O2 which increases the rate of respiration - therefore increasing the amount of BPG made. this will deliver more oxygen to the tissues.
Why does foetal haemoglobin bind O2 more tightly?
Because foetal haemoglobin is less sensitive to BPG.
The beta chain (which would usually contain positively charged side chains) is replaced by a gamma chain - which makes foetal haemoglobin less sensitive to BPG
This is the type of haemoglobin where the His E7 is mutated to change the environment, which causes oxidation of Fe2+ into Fe3+.
HbM or Boston Haemoglobin
How does BPG bind to Hb?
BPG binds to the positively charged active site in the Hb through ionic interactions.
BPG has five negative charges
