Term
| How many sub-units does myoglobin and hemoglobin have, respectively? |
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Definition
| Myoglobin - 1, Hemoglobin - 4 |
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Term
| How many alpha and beta sub-units does hemoglobin have? |
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Definition
| 2 - alpha, 2 - beta. The major contacts occur between alpha-1/beta-1 subunits, and alpha-2/beta 2 subunits. |
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Term
| Structurally, what are the names of the deoxy and oxygenated forms of the hemoglobin respectively? |
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Definition
| T or tense state for deoxy-Hb (low affinity state) and R or relaxed state oxygenated-Hb (high affinity state) |
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Term
| What type of protein-protein interaction is disrupted in hemoglobin when oxygen binds? |
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Definition
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Term
| What type of co-operativity is involved in the binding of oxygen by hemoglobin? |
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Definition
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Term
| Describe positive co-operativity? |
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Definition
| The binding of a ligand to one binding site of a protein, increases the affinity for binding the next ligand binding site such that the last binding site has the highest affinity. |
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Term
| How does D-2-3 Bisphosphoglycerate (BPG) affect the affinity of hemoglobin for oxygen? |
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Definition
| D-2-3 Bisphosphoglycerate decreases the affinity hemoglobin for oxygen by binding to and stabilizing the deoxy-hemoglobin state. BPG binds weakly to the oxy hemoglobin (relaxed state) |
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Term
| What role does BPG play in high altitude adaptation? |
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Definition
| At high altitude, there is an increase of BPG production. As a result the affinity for oxygen by hemoglobin is decreased. The lower affinity results in faster release of oxygen to the tissues. |
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Term
| What role does BPG play in transfer of oxygen from the maternal hemoglobin (HbA) to the fetal hemoglobin (HbF)? |
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Definition
| Unlike adults who have alpha and beta chains in the hemoglobin, a foetus expresses a zeta chain instead of alpha and epsilon, then later gamma instead of the HbA beta chain. The gamma chain of fetal hemoglobin binds BPG with a lower affinity relative to the adult hemoglobin. This means that the fetal hemoglobn has a higher affinity for oxygen than the adult hemoglobin and transfer of oxygen from the low affinity HbA to the higher affinity HbF is thus enabled. |
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Term
| What mutation in hemoglobin causes sickle cell? |
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Definition
| Mutation of glutamate (Glu) 6 to a valine (Val). Glu 6 is located on the surface of the beta chain of HbA. |
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Term
| How does the mutation of Glu6 to Val6 affect the charge properties of Hb? |
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Definition
| The protein loses a negative charge, and becomes more hydrophobic by gain of a hydrophobic amino acid. |
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Term
| How does the Glu6 to Val (E6V) mutation affect the Hb oxygen affinity? |
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Definition
| There is no change in affinity. |
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Term
| How does E6V lead to sickling? |
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Definition
| E6V exposes a hydrophobic patch in DEOXY-Hb (not OXY-Hb) leading to aggregation and formation of fibres running through the entire length of the cells leading to sickling |
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Term
| What are the consequences of formation of sickled hemoglobin or HbS? |
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Definition
| Formation of HbS makes the RBC's sticky and makes it difficult for the cells to travel through the vessels. This leads to acute episodes of intense pain, anemia, and decreased red blood cell viability. |
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Term
| Why are individuals who are heterozygous for sickle cell asymptomatic? |
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Definition
| Manifestation of symptoms for sickle cell trait is dependent on the concentration of HbS (which determines the rate of fiber formation). The concentration of the HbS in a heterozygous individual is about half of that in a homozygous individual. |
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Term
| Why do individuals homozygous for the sickle cell trait not have sickling occur constantly? |
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Definition
| Sickling occurence depends on the concentration of deoxy-Hb. If the rate of sickling is slower than the rate of flow of deoxy-blood from the tissues to the lungs for oxygenation, then sickling does not occur. |
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Term
| What factors promote sickling? |
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Definition
| Injury/dehydration/other factors that increase amount of deoxy-Hb increases the amount deoxygenated blood and formation of HbS |
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Term
| True or False? HbS causes resistance to malaria |
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Definition
| True. HbS promotes resistance to malaria and has evolutionary been selected for in malaria endemic regions. |
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Term
| Hydroxyurea is used in the treatment of sickle cell. How does it work? |
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Definition
| Hydroxyurea induces the expression of the fetal gamma chain and increases the level of HbF. The gamma chain contains Glu87 which prevents aggregation of HbS. |
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Term
| What electrophoresis method can be used to distinguish between HbA and HbS? |
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Definition
| Cellulose acetate electrophoresis |
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Term
True or false?
Cellulose acetate electrophoresis is run under acidic conditions |
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Definition
| False. the electrophoresis is done in basic conditions. |
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Term
| In one cellulose acetate electrophoresis experiment, native HbA and HbS proteins loaded on the gel and they migrate toward the anode (+). Which form of Hb migrtes faster? |
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Definition
| HbA. Mutation of E6V makes HbS less negative and it migrates slower. |
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Term
| Name the 3 variables that affect the rate and extent of polymer formation of HbS red cell? |
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Definition
1. The cell's degree of deoxygenation
2. The presence or absence of HbF
3. The intracellular Hb concentration. |
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Term
| What ion transport mechanisms have an impact on the hydration status of HbS RBC's? |
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Definition
1. Potassium-Chloride co-transport
2. Ca2+ activated K+ efflux |
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Term
| Name 3 approaches to therapy for sickle cell disease. |
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Definition
1. Inhibition of HbS polymerization (still a challenge)
2. Reduction of intracellular Hb concentration by preventing K+ and water loss (using clotrimazole which inhibits transport through the Gardos channel - Ca2+ activated K+ efflux channel).
3. Induction of HbF (using hydroxurea) |
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