CSI 2 Flashcards
What are harm groups made of
Porphyrin ring bound to a ferrous iron ion (Fe2+)
What does oxygen bind to in the harm groups?
The Fe2+ ion
How many functional globin chains are there
8
What are the functional globin chains?
Alpha 1, Alpha 2, Beta, A Gamma, G Gamma, Delta, Epsilon, Zeta
What is the alpha globin cluster made of?
5’ zeta, alpha 2, alpha 1 3’
What is the beta globin cluster made of?
5’ epsilon, G gamma, A gamma, delta, beta 3’
Which globin chains make up HbA?
2 alpha and 2 beta
Which globin chains make up HbA2?
2 alpha and 2 delta
Which globin chains make up fetal haemoglobin?
2 alpha and 2 gamma
What does P50 mean
The value at which 50% of the harm groups have oxygen bound
What is the P50 of myoglobin?
2 torr
What is the P50 of haemoglobin?
26 torr
What is fractional saturation?
The fraction of haem groups that have oxygen bound
What curve is for haemoglobin?
Sigmoidal oxygen dissociation curve
Where is myoglobin found?
Muscles
What is ppO2 in the lungs like?
100 torr -> haemoglobin becomes highly saturated with oxygen
What is the ppO2 in the tissues like?
20 torr -> Haemoglobin releases around 66% of its oxygen
What is positive cooperativity?
Once an oxygen molecule binds to one haem group, it becomes easier for oxygen to bind to the next haem group within the molecule
What is 2,3-BPG?
It’s an allosteric effector that binds to a site in deoxyhaemoglobin and stabilises the structure
What is the effect of 2,3-BPG?
It decreases the affinity of haemoglobin for oxygen. This shifts the curve to the right. This is what allows the haemoglobin to act as an effective oxygen carrier.
Haemoglobin in the absence of 2,3-BGP does not have the sigmoidal shaped oxygen dissociation curve
How is 2,3-BGP generated?
Through the Rapoport-Leubering Shuttle
In the shuttle, some of the 1,3-BGP produced in glycolysis is converted into 2,3-BPG by movement of a phosphate group.
Which enzyme catalyses formation of 2,3-BPG?
Bisphosphoglycerate mutase (BPGM)
What happens in rapidly respiring cells?
Pyruvate is decarboxylated to acetyl coA forming CO2
Pyruvate is converted to lactate so pH decreases
How does pH and CO2 affect haemoglobin?
H+ ions and CO2 are allosteric effectors which bind to sites in haemoglobin and decrease the affinity for oxygen so oxygen is released when RBCs enter the tissues
How does age affect globin gene expression?
A foetus would produce more alpha and gamma goblins.
After birth, there is more production of beta goblins and less production of gamma globins.
A very small amount of HbF persists throughout adult life.
Other than sickling, what can be seen in the blood of people with SCA
More HbF than people without SCA to compensate
How does the gamma chain differ from a beta chain?
One single amino acid substitution
What does the difference in structure of the gamma and beta chain lead to?
Gamma chains have decreased affinity for 2,3-BPG and thus and increased affinity for oxygen
Why does HbF need a higher affinity for O2?
It obtains oxygen from maternal HbA, not the air
What is expected in a sickle cell vs normal haemoglobin electrophoresis?
HbA would run further towards the positive electrode as it is more negatively charged
Why is there a difference in charge between HbS and HbA
Point mutation in Beta chain.
Glutamate in normal (hydrophilic, negatively charged) is replaced by valine (hydrophobic, uncharged)
What causes sickle cell?
HbA contains 2 alpha and beta chains like HbA.
But there is a mutation at codon 6 of the beta globin gene. A glutamate is replaced by a valine molecule at position 6.
Glutamate : negative, polar, hydrophilic, soluble
Valine: neutral, nonpolar, hydrophobic, insoluble
In terms of affinity, how does HbS differ from HbA
Lower affinity than HbA
How does baseline haemoglobin concentration with SCA differ from normal?
About half the concentration compared to normal
How do patients with HbS not experience symptoms such as tiredness, breathlessness and heart palpitations
If people with HbA had the same baseline haemoglobin as HbS, they would experience those symptoms. But HbS has lower affinity in comparison so oxygen is readily supplied so the symptoms are not caused.
What are the main symptoms of sickle cell
1) painful crises that can last up to a week
2) anaemia which can cause tiredness and shortness of breath
3) An increased risk of serious infections
What is sickle cell genetically?
A recessive condition
How would someone know if they have sickle cell disease?
Screening is offered to all pregnant women in England to check the risk of the baby being born with the condition.
All babies are offered screening as part of the newborn blood spot test (heel prick test)
Blood tests can also be carried out at any age to check for the disease or if you are a carrier.
What are the treatments for sickle cell disease?
1) Drinking plenty of fluids and staying warm to prevent painful episodes
2) Painkillers, such as paracetamol or ibuprofen
3) Daily antibiotics and having regular vaccinations to reduce your chances of getting an infection
4) medicine e.g hydroxycarbamide
5) Regular blood transfusions if symptoms continue or get worse; or there are any signs of damage
6) Emergency blood transfusion if severe anaemia develops
What are the cures for sickle cell?
Stem cell or bone marrow transplant
Not done often due to risks involved
How is SCA inherited
It follows an autosomal recessive inheritance pattern
What happens during deoxygenating in haemoglobin?
Haemoglobin S undergoes a conformational change here. The hydrophobic valine residues protrude from the surface and causes hydrophobic interactions between the HbS molecules resulting in the formation of fibres.
What do the fibres formed by valine during deoxygenation do?
They distort the shape of the RBC which results in the sickly form of the cell
What methods can be used to detect sickle cell
1) Electrophoresis
2) Solubility (in presence of sodium dithionite - a reducing agent - the solubility of sickle cells decreases as oxy HB is converted to deoxy Hb: but this does not differentiate AS from SS)
3) Blood film
How long do sickle cells last and why
20 days (compared to 120) because repeated sickling weakens the membrane and causes haemolysis which results in anaemia
What may you see in patients whose sickle cells are undergoing haemolysis
Jaundice and gallstones as bilirubin is released to break down the blood cells and it’s a yellow substance
What is Vaso-occlusion?
It is the blockage of the micro-circulation due to rigid sickle cells getting stuck resulting in the blockage of blood flow to parts of the body.
Adhesion also occurs due to binding of sickle cells to p-selectin on blood vessels.
Sickle cells also tend to aggregate together
This leads to ischaemia and necrosis
What is bone crises
Blockage of blood flow to the bone marrow
Why can sickle cell lead to stroke
Blockage of blood flow to the brain tissue leading to ischaemic stroke. Collateral vessels form to circumvent the blockage but they are very fragile and so are more likely to rupture if there is blockage which would lead to haemorrhaging stroke.
Why can sickle cell lead to vision loss
Blood blockage in the micro vessels in the retina. This damages the vessels and lead to the formation of collateral vessels which are weaker and could rupture leading to haemorrhage.
Triggers of Vaso-occlusive crises
Exercise leading to Hb deoxygenation
Cold temperatures leading to vasoconstriction
Dehydration leading to increased blood viscosity
What genetic makeup causes sickle cell anaemia
HbSS
What genetic makeup causes sickle cell trait?
HbAS
What are reticulocutes
Immature RBCs
What happens to reticulocyte levels in sickle cell disease?
Increased as a compensatory mechanism
What is splenomegaly?
Accumulation of degraded erythrocytes increases the size of the spleen
What is dactylitis?
Pain in hand joints due to blood vessels being occluded and less oxygen delicery
What is splenic sequestration?
Large volume of blood pooled in spleen - sudden drop in Hb
What may multiple blood transfusions lead to?
Hemosciderosis which can damage liver, heart, pancreas and other organs leading to diabetes mellitus
What is wrong with using hydroxycarbamide
Increased risk of infection
How to treat crises
Painkillers (aspirin and ibuprofen)
Severe pain treated using opioid (morphine) daily meds
How can crises symptoms be alleviated?
Keeping warm
Constant hydration
Avoiding locations exposed to high altitudes and low oxygen levels
How does hydroxycarbamide work?
It stimulates synthesis of HbF
Hydroxyurea further suppresses marrow production of reticulocytes and neutrophils which reduces Vaso-occlusion since neutrophils promote vascular adhesion)
What increases sickling?
Acidosis
Low flow blood vessels
What three amino acids make the hydrophobic pocket
Alanine, phenylalanine, leucine
