Lectures 7-8 Hb Flashcards
primary structure of haemoglobin
around 150 amino acid-long primary sequence
whats the secondary hemoglobin structures
8 alpha helices, A-H.
whats the tertiary structure of haemoglobin
a globin fold with a hydrophobic pocket
what the quatenary structure of haemoglobin
4 monomeric units interacting to make the heteromeric quaternary structure.
what does the Haem group bind to
His F8
describe the structure of the haem group
a central Fe2+ ion bound to His F8 as well as 4 N pyrrole rings, leaving the one binding site for O2.
what other amino acid is important in O2 binding to haem and explain
the His E7. His E7 distorts the binding of the O2 to the Fe2+, This reduces the binding affinity of oxygen to myoglobin, making it easier to
release oxygen to the muscle cell. reducing the binding affinity through steric hindrance.
what is allosteric hindrance
hindrance of binding but not at the active site of the protein
what is the shape of the myoglobin curve
it is hyperbolic
what does the hyperbolic curve shape of myoglobin mean
Myoglobin is saturated with O2 at low
pO2 only releasing O2
to muscle cells
when the cellular pO2
is very low.
This is shown in a ‘hyperbolic’ curve.
This suits its function as a “back-up”
store of O2 in muscle cells.
what binds oxygen more tightly, the myoglobin or haemoglobin
myoglobin
how are the 4 globin subunits of haemoglobin joint together
via non covalent association bonds
describe T state
tense state and has low affinity for oxygen
describe R state
relaxed state and has high O2 affinity
describe the concerted model for Haemoglobin
Subunits can be in a low-activity,
tense (T) or high-activity, relaxed
(R) conformation.
* All subunits must be in the same
state (either T or R).
Binding each successive substrate
(S) shifts equilibrium in favour of R.
in the concerted model, what do inhibitors and activators do
inhibitors stabilise the T form, activators stabilise the R form
whats the sequential model
One substrate binding induces a T → R conformational
change in only one subunit.
* This conformational change influences the neighbouring
subunits (i.e. cooperativity), changing their affinity for substrate. Many conformations are possible, like 2T and 2R at the same time
what are the haem shapes in deoxy and oxy haemoglobin
in deoxyhaemoglobin the shape is dish shaped and oxyhaemoglobin is more flat
what flattens the haem group
the binding of O2 pulls iron ion into the plane of the haem
what happens when haem binds O2 in terms of shape
the haem group flattens, this pulls His F8 toward the haem binding site, which then causes a conformational change in helix F, which changes C. which then causes conformational changes and cooperativity in the other helices.
T/F, T state means deoxthaemoglobin
F
are the T state and R state and the binding molecules specific to haemoglobin
no, they apply to all multimeric proteins.
describe the conformational changes of haemoglobin binding
The change of position is in helix F, this changes angle between E and F. in Deoxy its more open, in oxy is closed. The binding moves FG in a fair bit.
As the F helix moves the FG loop moves upwards relative to the C helix.
BPG is a what? and what is its charge
its an allosteric inhibitor and it has -5 charge
where does BPG bind
into an allosteric site in the centre between the 2 beta strands, that site has His 2 and 143, and Lys 82
when is BPG produced and what does this mean
BPG is produced during
respiration in peripheral
tissues, so promotes
oxygen release where it
is needed.
what does BPG do
BPG stabilises Hb in the
deoxy T-state, reducing
oxygen affinity.
what do the allosteric inhibitors, H+, CO2 and BPG do
they stabilise the T state, helping haemoglobin undergo cooperativity and unbind O2 in tissues
what shows more cooperativity, stripped Hb or whole blood
whole blood that has CO2 and H+ in it
whats the bohr effect
when CO2 and H+ cause lower affintiy of Hb to O2.
where do the CO2 and H+ come from, and where does it bind
Like BPG, elevated CO2 and lowpH (elevated H+) in metabolising tissues. the CO2 binds to terminal NH3 and H+ binds to polarisable Aa side chains
what happens to O2 binding of Hb at altitude and why
The first substantial adaptation to high altitude is
an increase in BPG. this lowers Hb affinity for O2. which means the O2 that is picked up by Hb is held onto less and thus dumped more effectively in the tissues.
long term at altitude out Hb numbers increase
whats the structural difference between fetal haemoglobin and normal
fetal has 2 alpha and two gamma strands. the gamma strands have Ser 143 instead of His 143. so fetal Hb has less affinity for BPG.
what does lower BPG affinity for fetal Hb mean
This allows the foetus to capture oxygen in the placenta.
what is methemoglobin
when the Fe2+ of the haem group is oxidised to Fe3+ changing one subunit to the R state
what does methaemoglobin cause for the haemoglobin molecule
the shift to Fe3+ changes one of the haemoglobin subunits to the R state permanently. this change to Fe3+ means the haem doesn’t bind O2, despite the R state.
also the three other subunits are made into the R state which means they don’t release the oxygen in the periphery
how is methaemoglobin fixed
The enzyme cytochrome b5 reductase regenerates haemoglobin by reducing methaemoglobin back to Fe2+ state using transfer of electrons from NADH.
what is HbM, or boston haemoglobin
there is a mutation to His E7 changing it toTyr E7, this causes Fe2+ to be oxidised to Fe3+. Haem plane moves slightly, breaking the connection of Fe-His F8 .
HbM remains in ‘T’ state, with low affinity for oxygen
what causes sickle cell anemia
Abnormal haemoglobin is caused by a point mutation of the 6th amino acid in the beta chain. The positive Glu on the surface of the protein is replaced by non polar valine.
what does the mutation of sickle cell anemia do?
This change makes haemoglobin less soluble and in the Deoxy form the valine binds to a hydrophobic pocket of another abnormal haemoglobin, forming a sickle shaped cell.
