Session 10-Protein Function-Hb And Myoglobin Flashcards
What does the Haem group consist of?
Protoporphyrin ring and Fe atom bound to four N atoms of the ring, Fe2+ can make two additional bonds to O2, one on either side of the plane
How is the Fe atom bound to the protein?
Via a histidine residue (proximal histidine) on the other side of the ring
What are three features of the myoglobin structure?
- 153 AA
- Compact
- 75% alpha-helical
Binding of O2 to myoglobin shows what kind of dependence on O2 concentration?
Hyperbolic
What is the shape of the Hb oxygen dissociation curve?
Sigmoidal
What is the structure of Hb? (3)
- Four polypeptide chains: alpha chain (141 AA), beta chain (146 AA) in alpha2 beta2 tetramer
- Each chain contains essential haem prosthetic group
- Conformation of polypeptide chain very similar to that of myoglobin
True or false: Hb undergoes structural change on binding to O2
TRUE
What are the two states in which deoxyhaemoglobin can exist?
- Low affinity T state (tense state)
2. High affinity R state (relaxed state)
Oxygen binding to Hb promotes the stabilisation of which state?
R state - AA residues change conformation and Hb molecule changes to the high affinity state
Why is the O2 binding curve sigmoidal for Hb?
Cooperative binding of O2 (cooperativity):
Binding affinity for O2 increases as more O2 molecules bind to Hb subunits
Binding of first O2 molecule to the first subunit is hard (low affinity)
Binding of the last O2 molecule to the fourth subunit is easy (high affinity)
What is an allosteric effect?
Binding of a ligand to a site on a protein molecule in such a way that the properties of another site on the same protein are affected
Define allosteric
Alteration of the activity of an enzyme by means of a conformational change induced by a different molecule
How is oxygen binding regulated?
2,3-bisphosphoglycerate (BPG)
What does BPG do?
Changes the affinity of Hb for O2
How much BPG is present in RBCs?
~5mM
Complete the sentence:
BPG concentration increases at ____ altitudes, promoting O2 _______ at tissues. Hb therefore has a ______ affinity for O2 as the curve shifts to the ____
High
Release
Lower
Right
What does the binding of H+ and CO2 to Hb do?
Lowers the affinity of Hb for O2
What is the Bohr effect?
Ensures delivery of O2 coupled to demand
Why is CO poisonous?
Combines with ferromyoglobin and ferrohaemoglobin and blocks O2 transport
What does partial CO binding do?
Increases affinity for O2 for unaffected subunits
Does HbF (foetal Hb) have a higher or lower affinity for O2 than HbA?
Higher, allows the transfer of O2 to foetal blood supply from the mother
Sickle cell anaemia is the mutation of what?
Glutamate to valine in beta-globin
What are thalassaemias?
Group of genetic disorders where there is imbalance between the number of alpha and beta globin chains
What happens in beta-thalassaemia?
Decreased or absent beta-globin chain production
Alpha-chains are unable to form stable tetramers
What happens in alpha-thalassaemia?
Decreased or absent alpha-globin chain production
Beta-chains can form stable tetramers with increased affinity for O2
