Lecture 8 - Gaymagaybloin Flashcards
Haemoglobin evolved to be a ___
Tetramer
What is a Tetramer
Four globin proteins associated together non-covalently
Oxygen binding changes the shape of
haem and haemoglobin
What does binding oxygen do to heme
Deoxyhaemoglobin has a dished haem.
- when oxygen is bound heme is closed to planar
What happens to the heam when oxygen binds
In oxyhaemoglobin, oxygen flattens the haem, and pulls histidine F8 and helix F toward the binding site.
What weakens oxygen binding
Anything that keeps helix F away will weaken oxygen binding.
Oxygen changes…
Haemoglobins shape
O2 binding to oxyhaemoglobin does what?
Compared to deoxyhaemoglobin, O 2
binding to oxyhaemoglobin moves the
Fe2+ into the plane of the haem, draws His F8 down, and repositions helix F.
Binding of O2 to haemoglobin does what to orientation?
Shifts in the orientation of protein secondary elements, such as helix F moving relative to helix C, are called ‘conformational changes’.
Tense T-state =
Deoxyhaemoglibin
Relaxed R state =
Oxyhaemoglobin
When oxygen binds in one area….
The other areas become more susceptible to bind oxygen
T state to relaxed =
Oxygenation
What is 2,3-bisphosphoglycerate (BPG)
an allosteric inhibitor of O2 binding to haemoglobin
- moves it towards T state
Structure Of the allosteric effector 2,3-bisphosphoglycerate
What does the Positively-charged allosteric site include
Positively-charged allosteric site includes histidine and lysine sidechains and the amino termini of the b chains.
Haemoglobin is under allosteric control of
2,3-bisphosphoglycerate (BPG)
BPG binds to deoxy-Hb by..
electrostatic interactions.
What does BPG do in deoxy T- state do
BPG stabilises Hb in the
deoxy T-state, reducing
oxygen affinity.
When is BGP produced and what does it do
BPG is produced during
respiration in peripheral
tissues, so promotes
oxygen release where it
is needed.
Proof that cooperatively allows effeint unloading
Monomeric myoglobin has a hyperbolic binding curve.
Cooperative, tetrameric haemoglobin has a sigmoidal binding curve.
YO2 = fraction of protein bound to O2
How cooperatively allows efficient ‘unloading’
Monomeric myoglobin has a hyperbolic binding curve.
Cooperative, tetrameric haemoglobin has a sigmoidal binding curve.
YO2 = fraction of protein bound to O2
How cooperatively allows efficient ‘unloading’
Monomeric myoglobin has a hyperbolic binding curve.
Cooperative, tetrameric haemoglobin has a sigmoidal binding curve.
YO2 = fraction of protein bound to O2
Cooperatively is prominent only in..
Presence of allosteric inhibitors of binding
In absence of inhibitors, “stripped haemoglobin” is predominately in the
R- state, so shows little cooperativity.
Allosteric inhibitors BPG, CO2and H+ stabilise the
T-state. This unmasks cooperativity.
What does CO2 do?
CO2 reduces O2 affinity, both directly and
via lowered pH of blood
Process of CO2 reducing O2 affinity
Like BPG, elevated CO 2 and low pH (elevated H+) in metabolising tissues both reduce the affinity of haemoglobin for O 2, known as the Bohr effect.
Like BPG, CO 2 can bind to the at lower pH amino-terminal amino group, stabilising deoxy-Hb conformation in T-state.
CO2 with haemoglobin equation
In what case is weaker binding better?
- the first substantial adaption to high altitude is an increase in BPG
- this reduces haemoglobins oxygen binding
- rightward shift of the binding curve delivers more oxygen to the tissues
What alters the oxygen binding properties ?
Different amino acid sequences of normal haemoglobin subunits alter their oxygen binding properties
What aspect of foetal haemoglobin have higher affinities for O2
Alternate isoforms
- this allows the forest us to caplure oxygen in the placenta
How foetuses hold oxygen tighter
Different amino acid sequences of normal haemoglobin subunits alter their oxygen binding haemoglobin haemoglobin properties.
Foetal haemoglobin includes alternate isoforms (, & with )
higher affinities for O2.
This allows the foetus to capture oxygen in the placenta.
How foetal haemoglobin being less sensitive to BPG leads to oxygen being bound more tightly
BPG binds to deoxy-Hb by electrostatic interaction.
The y chain replaces the B chain in foetal Hb. The y chain has serine place of a histidine found on the b chain’s binding
site for BPG.
What does oxygen of haem Fe2+ to Fe3+ do?
Oxidation of haem Fe2+ to Fe3+ shifts one subunit to the R- state conformation, without oxygen bound
Oxidation of haem Fe2+ to Fe3+ shifts one subunit to the R- state conformation, without oxygen bound -METHAEMOGLOBIN . This mutation impairs function two ways:
The methaemoglobin subunit does not bind oxygen despite otherwise being in the R-state, due to the Fe3+.
The three other subunits of the tetramer are shifted to the R-state, so do not release oxygen in the tissues as they should.
The enzyme cytochrome b5 reductase regenerates haemoglobin by reducing methaemoglobin back to Fe2+ state using transfer of electrons from NADH.
His e7 can mutate to
Tyr (H58Y)
His E7 mutation to Tyr (H58Y) changes..
.. the environment, causing Fe2+ to oxidise to Fe3+
-haem plane moves slightly, breaking the connection of Fe-His F8
- HbM remains in ‘T’ state, with low affinity for oxygen
Sickle cell haemoglobin
The ‘sickle’ shape red blood cells get stuck in blood capillaries
This causes a range of secondary debilitating effects in the person
HbS ‘gain of function’ mutation
The Hb b E6V mutation enables an abnormal hydrophobic interaction between Hb molecules, particularly when in the deoxy form, causing polymerisation of Hb into chains that distort the red blood cells. Recent trials of gene therapy are very promising
Oxygen binding is weakened allosteically by…
BPG, CO2 and low pH
When oxygen binding is weakened allostrically by BPG, CO2 and low pH, this can be describes as…
…shifting the tetramer to the T-state
In presence of __,___ and ____, haemoglobin displays _____ binding of _____, evident in a ________ binding curve
In the presence of BPG, CO 2 and low pH, haemoglobin
displays cooperative binding of oxygen, evident in a
sigmoidal binding curve.
How to the R- and T- states differ
The R- and T-states differ in how helix F interacts with the
haem and with the helix C, and spacing between H helices.
Oxygen affinity is tuned in ____ and at ____ ______
Oxygen affinity is tuned in foetuses and at high altitude.
Mutations to haemoglobin impair
Oxygen transport
Sickle-cell anaemia results from
haemoglobin polymerisation.
Haemoglobin function
- 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 BPG, CO 2 and low pH, haemoglobin
displays cooperative binding of oxygen, evident in a
sigmoidal binding curve. - The R- and T-states differ in how helix F interacts with the
haem and with the helix C, and spacing between H helices.
Physiological effects
• Oxygen affinity is tuned in foetuses and at high altitude.
• Mutations to haemoglobin impair oxygen transport.
• Sickle-cell anaemia results from haemoglobin polymerisation.
