Agents that affect oxygen binding Flashcards
What kind of protein is haemoglobin?
Allosteric protein that displays cooperativity in oxygen binding and release
Is the binding of oxygen by myoglobin cooperative?
No because it is a monomer (only one subunit)
What is oxygen binding measured as a function of?
The partial pressure of oxygen
What is the iron bound to in haemoglobin?
4 nitrogens
What is the 5th coordination site of iron occupied by?
Imidazole ring of a histidine (proximal histidine)
What binds to the 6th coordination site of iron?
Oxygen
What happens to the iron when oxygen binds to it?
It moves into the plane of the protoporphyrin ring which pushes amino acids to different position
What stabilises the bound oxygen?
The distal histadine
Why can fMRI distinguish the relative amounts of oxy- and deoxyhemoglobin?
Because the magnetic properties of the heme iron changes when it moves into the plane of the protoporphyrin ring
What is the concept called which makes heme have higher affinity for oxygen once one is bound?
Cooperativity
What kind of oxygen binding curve does myoglobin display?
Hyperbolic
What kind of oxygen binding curve does haemoglobin display?
Sigmoid curve which indicates that O2 binding and release is cooperative
When partial pressure of oxygen decreases, what happens to the affinity?
It also decreases
Is haemoglobin exposed to superoxide ions?
No
Why is it critical that oxygen is only released in the O2 and not superoxide state?
Superoxide is very reactive and will bind and harm other molecules
Iron ion would be left in the ferric (Fe3+) state, preventing additional oxygen binding (Myoglobin in this state is called metmyoglobin)
What is myoglobin called when iron ion is left in the ferric state (Fe3+), preventing additional oxygen binding?
Metmyoglobin
What feature of myoglobin help prevent superoxide release?
Distal histidine which donate a hydrogen bond to the oxygen molecule
What will exercise do to the partial pressure of oxygen in a tissue?
Oxygen will deplete so the partial pressure goes down in the tissue
2 models that describe how haemoglobin goes from T state to R state?
Concerted model
Sequential model
Which model is most likely? Concerted or sequential
Both, they most likely both happen
Which interface is the structural changes at the heme groups transmitted to?
Alpha1 Beta2
Alpha2 Beta1
Where do allosteric effector bind on haemoglobin?
At a site separate from the active site
What is the activity of an allosteric protein regulated by?
Allosteric effectors
What is oxygen? A homotropic of heterotrophic allostery?
Homotropic allosteric modulator because it is itself the ligand, not only a molecule that modifies the structure of haemoglobin but also the molecule that binds haemoglobin
What is a homotropic allosteric effector?
A molecule that affects its own binding to the protein (at other site)
Like how binding of O2 increases the affinity for binding on other side of Hb
What os a heterotropic allosteric effector?
When the allosteric effector is different from the ligand whose binding is altered
Like the effect og H+ on the P50 for oxygen binding
What shift in the O2 binding curve do interactions with homotropic and heterotrophic allosteric effectors lead to?
Horizontal shifts
Examples of homotropic allosteric effectors
CO2
2,3-bisphosphoglycerate
H+
Do heterotrophic allosteric effects increase or decrease the affinity for O2?
Decrease (thus deliver more oxygen to the tissue)
What is 2,3-bisphosphoglycerate?
A byproduct of glycolysis
Negatively charged small molecule which binds to the T-state and stabilises it by binding in positively charged pocket at the interface of each subunits (between the beta subunits)
2 subunits does foetal haemoglobin not have?
The beta subunits
2 subunits does foeatal haemoglobin have?
The gamma subunits
What does the gamma subunit defer from the beta subunit?
1 amino acid
Why does 2,3-bisphosphoglycerate not bind to foetal haemoglobin?
Because it has gamma subunits instead of beta subunits so no positively charged pocket
At the same concentrations of 2,3-bisphosphoglycerate, which has a higher affinity for oxygen, maternal or foetal haemoglobin?
Foetal
What is the role of distal histidine regularly?
Increases the affinity for oxygen by making the space better fit for oxygen by bending the angle of binding to fit oxygen’s angle
Why is CO so toxic?
Because it saturates the haemoglobin so oxygen can’t bind and you die. It does so very quickly
What does distal histidine do?
Keeps oxygen (and CO) at an angle
What can destabilise the affinity for CO?
The distal histidine, but CO still binds better
What does CO act as when bound to heme?
An irreversible ligand
Is the affinity for O2 greater than CO?
No the affinity for CO is 2000 times greater
What is hypoxia?
Low availability of oxygen in a tissuwe
How does temperature effect the affinity for oxygen?
The higher the temperature the less affinity for oxygen
Examples of hemoglobinopathies?
Sickle cell disease
Thalassemia
What are hemoglobinopathies?
Genetic disorder affecting the production or structure of haemoglobin molecule
What gives sickle cell diseased RBC their sickle shape?
The fibers of haemoglobin being misshaped
What is thalassemia caused by?
An imbalanced production of haemoglobin chains
What is alpha-thalassemia?
When there is not enough alpha chains
What can cytoglobin protect against?
Hypoxia
What can glycated haemoglobin tell you about glucose levels?
Levels from the past 3 months
Is the binding of oxygen in myoglobin cooperative?
No
What forms the 5th coordination bond with iron?
Proximal histidine
What forms the 6th coordination bond with iron?
Oxygen
What happens to the magnetic properties of the heme iron when it moves into the plane of the protoporphyrin?
It changes
Why can fMRI distinguish the relative amounts oxygenated and deoxygenated haemoglobin?
Because the properties of the heme iron changes
2 reasons why it is critical that oxygen is released only in the oxygen and not the superoxide state
Superoxide is very reactive and can harm other cellular molecules
The iron would be left in the ferric state, preventing it from binding additional oxygen
What are the main factors that affect the binding of oxygen to haemoglobin?
pH
CO2
2,3biphosphoglicerate
Which way does the saturation curve shift when an allosteric effector is added?
To the right
What does the shift to the right of the saturation curve indicate?
Decreased affinity for oxygen
Which way does the saturation curve shift when an allosteric effector is removed?
To the left
Does a decrease in pH increase or decrease the affinity for oxygen?
Decrease
Is 2,3-bisphosphoglycerate a negative or positive regulator of haemoglobin?
Negative
What is 2,3-bisphosphoglycerate produced from?
An intermediate in glycolysis
Where on the haemoglobin does 2,3-bisphosphoglycerate bind?
The positively charged central cavity
Why does the 2,3-bisphosphoglycerate bind to the positively charged cavity?
Because it is negatively charged because of the phosphate
In which state of haemoglobin does 2,3-bisphosphoglycerate bind better?
T state because it is closed in R state
What happens to oxygen delivery at high altitudes?
It declines by 1/4th to a max of 30%
What does 2,3-bisphosphoglycerate facilitate?
Unloading of oxygen in tissues
Where is the 2,3-bisphosphoglycerate in competition with oxygen for the binding site?
On the beta subunit
Does fetal haemoglobin have a high or low 2,3-bisphosphoglycerate affinity?
Low
What is the Bohr effect?
Carbon dioxide and H+ stimulating the release of oxygen
Example of something that can induce the Bohr effect?
Lowering the pH as it decreases the affinity for oxygen
Which amino acid is a big contributor to the Bohr effect?
His146 of the beta subunit
Which enzyme catalyses the reaction of Oxygen binding by haemoglobin?
Carbonic anhydrase
What is carbonic anhydrase important to keep constant?
pH
How does CO2 effect eh affinity of haemoglobin to oxygen?
It decreases it
2 signals to to haemoglobin that we need more oxygen
Release of hydrogen and 2,3-DPG
In what form is 15-20% of CO2 exported?
Carbamate
What does the formation of carbamate yield?
A proton and saltbridges
How does the formation of carbamate contribute to the Bohr effect?
By yielding a proton
How does the formation of carbamate stabilise the T state?
By forming additional salt bridges
Which binds better, carbon monoxide or oxygen?
Carbon monoxide
In what shape does carbon monoxide bind? Linear or bent?
Linear
How does temperature affect the oxygen affinity?
Higher temperature means lower oxygen
What are hemoglobinopathies?
Haemoglobin mutants
What mutation causes sickle cell disease?
Substitution of Valine for glutamate
When is sickle cell anaemia fetal?
When both alleles of the beta chain are mutated
What does an individual with sickle cell disease present clinically?
Intermittent episodes of haemolytic anaemia resulting from chronic lysis of red cells and painful vasoocclusive crisis
What kind of trait is sickle cell disease?
Heterozygous recessive
What does it mean that haemoglobin can be glyciated?
That it can bind to glucose
