Lecture 12: Haemoglobin 2 Flashcards
What is an allosteric regulator? Include examples for haemoglobin
A substance that binds to a site on an enzyme or receptor distinct from the active site
- Allosteric inhibitors BPG, CO2 and H+ stabilise the T-state.This unmasks cooperativity.
- In absence of inhibitors, “stripped haemoglobin” is predominately in the R-state, so shows little cooperativity.
Haemoglobin is under allosteric control by which molecule?
2,3-bisphosphoglycerate (BPG)
How does BPG modify the O2 binding characteristics of haemoglobin?
- BPG binds to deoxy-Hb by electrostatic interactions.
- BPG stabilizes Hb in the deoxy T-state, reducing oxygen affinity.
- BPG is produced during respiration in peripheral tissues, so promotes oxygen release where it is needed.
Why is the O2 saturation curve for haemoglobin sigmoidal?
The sigmoid or S-shape of the curve is due to the positive cooperativity of hemoglobin. In the pulmonary capillaries, the partial pressure of oxygen is high allowing more molecules of oxygen to bind hemoglobin until reaching the maximum concentration.
How do the shapes of haem and haemoglobin change on O2 binding?
- Deoxyheaemoglobin has a dish/dome shape haem.
- Oxyhaemoglobin - oxygen flattens the haem, pulling His F8 And helix F towards the Binding site
- Shifts in the orientation of protein secondary elements, such as helix F moving relative to Helix C are called conformational changes.
- Conformational changes (especially affecting helix F) can change the balance of T ⇆ R state.
How does shifting from the R-state to the T-state affect how globin interacts with the haem molecule?
- In R state the haem is more planar so the F helix shifts closer to E helix which allows for higher oxygen affinity
- In T state the haem is more dish shaped so the F helix is further away from E helix which lowers oxygen affinity
Name two factors that facilitate the release of O2: (other than BPG)
CO2 and Low pH (lower affinity for O2 in Hb (Bohr effect)
- good for gas exchange at tissues (Hb gives up O2 easier)
- lower pH favours cooperativity and protonation of histidine and stronger ionic reactions in the T-state
- CO2 can bind to the amino-terminal amino group, stabilising deoxygenated Hb conformation in T-State
How does a change in BPG concentration aid acclimation to altitude?
The quantity of BPG in the blood increases at high altitude because of its role in increasing the quantity of oxygen that hemoglobin can release and provide to cells. This allows the body to make better use of the reduced oxygen that is available in the atmosphere.
How are the oxygen binding properties of foetal haemoglobin different to that in adult haemoglobin?
Fetal hemoglobin is composed of two α chains and two γ chains, in contrast to adult hemoglobin, which contains two α and two β chains. Fetal hemoglobin is less sensitive to BPG (y chain has a serine instead of a histidine in the allosteric binding site). This difference results in fetal hemoglobin having a higher oxygen affinity than adult hemoglobin.
How does the structure of HbS (sickle cell) differ to that of HbA?
Hb B E6V variant (valine)
(substitution of the polar β6-glutamic acid by the neutral amino acid, valine)
- this causes an abnormal hydrophobic interaction between Hb tetramers
- results in sickle shape RBC that get stuck in capillaries
Briefly describe the current treatment options for HbS. What are the pros and cons of each?
CRISPR-based therapy (CASGEVY™)
- Approved therapy actually up regulates foetal haemoglobin
- very expensive (not applicable)
Voxelotor (sold as Oxbryta) is the first approved hemoglobin oxygen-affinity modulator
- Stabilises the oxygenated state, increases oxygen affinity
- cause severe skin reactions and serious allergic reactions
