Lecture 8: PROTEINS IN ACTION - 2 Flashcards
What does deoxyhaemoglobin have?
A dished haem
What does oxygen in oxyhemoglobin do?
Flatten the haem and pulls histidine F8 and helix F towards the binding site (increasing its binding affinity for oxygen)
What weakens oxygen binding?
Anything that keeps helix F away
What does the oxygen in oxyhemoglobin do to the fe2+?
Moves it into the plane of the haem, draws His F8 down and repositions the helix F
What are shifts in protein secondary elements called?
Conformation changes
What is an example of the conformational change?
Helix F moving relative to helix c
Where is the most change observed?
At the FG loop
What happens to side chain packing in R and T states?
The interaction between side chains of the F helix and C helix change
Positions of side chains in R and T states are both …
stable
The transition state between R and T is …
Unstable so not much time is spent in that position
Describe the changes when going from R to T conformation?
The F helix moves so that the interaction between threonine and aspartic acid is borked and an interaction with a different threonine and asparagine is formed. This makes the binding pocket larger
What is BPG?
2,3-bisphosphoglycerate
What is BPG?
An allosteric inhibitor of oxygen binding to haemoglobin
What is the charge of BPG?
Highly negatively charged
How does BPG bind to deoxyhaemoglobin?
By electrostatic interaction
What does BPG do?
Stabilises haemoglobin in the deoxygenated T state, reducing oxygen affintiy
When is BPG produced?
During respiration in peripheral tissues, so promotes oxygen release where it is needed
When is cooperativity prominent?
Only in presence of allosteric inhibitors of binding
What do allosteric inhibitors BPG, CO2 and H+ do?
Stabilise the t state which unmasks cooperativity
What happens in the absence of inhibitors?
Stripped haemoglobin is predominantly in the r state so shows little cooperativity (acts almost as myoglobin)
What does elevated CO2, and low pH (elevated H+) do?
In metabolising tissues it reduces the affinity of haemoglobin for oxygen, known as the Bohr effect
What can CO2 do?
Bind to the extreme N terminal amino group
What can H+ do?
Protonate certain amino acid side chains which contributes to stabilising the deoxyhaemoglobin conformation
what is the first substantial adaptation to high altitude?
An increase in BPG
What does an increase in BPG do?
Reduces haemoglobin oxygen binding
What does a rightward shift of the curve do?
Deliver more oxygen to the tissues
What do different amino acid sequences of normal haemoglobin subunits in foetus do?
Alter their oxygen binding properties
What does foetal haemoglobin include?
Alternate isoforms (epsilon, gamma and zeta) which have higher affinities for oxygen
What does foetal haemoglobin allow?
The foetus to capture oxygen in the placenta
What does foetal haemoglobin lack?
An amino acid in the BPG binding site so bind BPG less well
What is methaemoglobin?
A damaged form of haemoglobin where oxidation of haem fe2+ to Fe3+ shifts one subunit to the R state conformation without oxygen bound
How does methaemoglobin impair the function?
The methaemoglobin does not bind oxygen despite otherwise being in the R state because of the Fe3+.
The subunits of the tetramer are shifted to the R state so don’t release oxygen in the tissues as they should
What does the enzyme cytochrome b5 reductase do?
regenerates haemoglobin by reducing haemoglobin back to Fe2+ state using transfer of electrons from NADH
What are most haemoglobin abnormalities caused by?
Genetic mutations
What is the mutation in HbM/Boston haemoglobin?
His E7 mutation to Try E7 changes the environment causing Fe2+ to oxidize to Fe3+
What happens as a result of the mutation in HbM/boston haemoglobin?
The haem plane moves slightly, breaking the connection of Fe-HisF8
What state is HbM in after the mutation?
It remains in the T state, with low affinity for oxygen as the larger tyrosine blocks some of the binding space
What does the sick shape do in sickle cell anaemia?
Means the red blood cells get stuck in blood capillaries which causes a range of secondary debilitating effects in the person
What happens to sickled red blood cells?
They become worse at carrying oxygen
What is the mutation in sickle cell anaemia?
A gain of function (Hb Beta EGV)
What does the beta EGV mutation enable?
Abnormal hydrophobic interactions between Hb molecules, particularly when in the deoxygenated form, causing polymerisation of Hb into chains the distort the red blood cells
