Hemoglobin Flashcards

1
Q

What is the structure of hemoglobin?

A

Quaternary structure with 4 subunits: 2 alpha-chains and 2 beta-chains
Donut -> slight hole in the middle

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2
Q

Structure of hemoglobin vs myoglobin

A

Hemoglobin: tetramer, quaternary structure
Myoglobin: Monomer, only secondary structure?

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3
Q

Myoglobin and Hemoglobin bind oxygen in

A

Heme groups, which are protoporphyrin rings (4 pyrrole rings with N facing towards inside) with Fe in middle
Fe2+ has 2 additional coordination sites

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4
Q

Coordination sites of hemoglobin and myoglobin

A

Fe is slightly too big to fit well into the hole of protoporphyrin ring
5th coord site: proximal his
6th coord site: Fe
When oxygen binds to 6th coord site, radius decreases = Fe smaller -> fits into ring better!

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5
Q

Hemoglobin displays cooperative behaviour

A

Sigmoidal curve represents cooperativity in oxygen binding
- Oxygen is transported from the lungers where the partial pressure of oxygen is high (100torr) to the tissues where the pO2 is low (20torr)

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6
Q

What are the two states of hemoglobin?

A

T-state/T form = tense, deoxyhemoglobin

R-state/R form = relaxed, oxyhemoglobin

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7
Q

How does oxygen binding lead to transition from T state to R state?

A
  • Binding of oxygen shifts the proximal histidine (pulls up Fe and proximal his)
  • This results in movement of the corresponding alpha-helix
  • This alters the interface of the alphabeta dimers
  • These structural changes provide a path of communication between subunits, triggering a change from T to R, making it more favourable for oxygen to bind
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8
Q

Allosteric Regulator of Hemoglobin

A

2,3-BPG binds a pocket only found on T form (deoxy) and stabilizes it
- For the T to R transition to happen, 2,3-BPG must be expelled from the binding site

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9
Q

How is oxygen transferred from maternal hemoglobin to fetal hemoglobin?

A
  • Fetal hemoglobin must bind oxygen when it is released from maternal hemoglobin
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10
Q

How is the affinity of fetal hemoglobin for oxygen increased?

A

Decreasing the affinity for 2,3-BPG will increase in oxygen binding

  • Fetal hemoglobin is a tetramer with 2 alpha chains and 2 y chains
  • The y chain is 72% identical to the beta chain- one change is His to Ser in 2,3-BPG binding pocket
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11
Q

Hydrogen ions and CO2 promote the release of oxygen

A

H+ and CO2 regulate oxygen binding to hemoglobin (Bohr effect)
CO2 from tissue diffuses and binds into hemoglobin -> CO2 + H2O < > H2CO3 < > HCO3- + H+
Proton produced lowers pH

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12
Q

How does pH influence oxygen binding?

A
  • Low pH favours formation of a salt bridge that stabilizes (deoxy) form of hemoglobin, favouring release of oxygen
  • At higher pH, His 146 is not protonated
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13
Q

CO2 promotes the release of oxygen

A

Cells most in need of oxygen use the end products of oxygen-dependent metabolism (CO2 and H+) to signal need for more oxygen

  • CO2 reacts with terminal amino groups to form carbamate (neg charge)
  • Carbamate groups form salt bridges at interface of alphabets dimers, stabilize deoxy and favour release of oxygen
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14
Q

Sick-Cell Anemia

A
  • Caused by a mutation in both copies of the hemoglobin gene
  • Sickle-cell hemoglobin (HbS) forms long fibers that deform the cell
  • Sickle-cells rupture easily, leading to anemia
  • HbS has Glu 6 -> Val 6 mutation on beta chains, which reduces the solubility of deoxy but not oxy form
  • When there is high concentration of deoxy, Val 6 interacts with Phe 85 and Leu 88 to form aggregates
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15
Q

Sickle-Cell Hemoglobin

A

A = normal adult hemoglobin
S = sickle cell hemoglobin
Blue triangle = sticky path on deoxy
Pink triangle = sticky path on oxy and deoxy hemoglobin S

Deoxy S aggregation (all the shit sticks together)

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16
Q

Potential treatment strategies for hemoglobin disorders?

A

Activate expression of fetal hemoglobin using gene editing in blood stem cells and inject back into patient to produce functional RBCs