Hemoglobin Flashcards
1
Q
Heme Group
A
- Comprised of protoporphyrin IX attached to an Fe2+ atom
- 4 ring nitrogens in the center used to coordinate iron atom
- Proximal histidine (F8) provides the 5th
- Oxygen can occupy the 6th
- Distal histidine (E7) aids in positioning of the atom
2
Q
Methemoglobin
A
- The ferric state (Fe3+) binds to water and cannot accomodate oxygen → methemoglobin (metmyoglobin).
- Globin protein forms a “hydrophobic shield” preventing water from oxidizing heme’s iron.
- Change in protein which causes a break in the shield can be caused by environmental or inherited mutations.
- Defect in NADH methemoglobin reductase which normally reduces Fe3+ to Fe2+ causes methemoglobinopathy.
3
Q
Myoglobin
(Mb)
A
- Primarily found in muscle and heart tissues
- Single polypeptide chain
- Heme sits in a crevice flanked by nonpolar residues
- Proximal histidine F8 binds directly to the heme group
- Distal histidine E7 stabilizes binding of oxygen
4
Q
Hemoglobin
(Hb)
A
- Hemoglobin A (HbA) is the major form in adults
- Tetramer of two identical alpha/beta dimers
- Each subunit similar to Mb
- In D=deoxy form, α and β subunits bind tightly through salt bridges
- When first O2 binds subunits, first α/β pair rotates relative to the second α/β pair ⇒ T (deoxy) or R (oxy) forms
- Shows cooperative binding of oxygen
- O2 site poorly exposed in deoxy state
- Site becomes more revealed with each subsequent O2 binding making it easier to bind each one
- Has ability to transport H+ and CO2
5
Q
Oxygen Binding
Mb vs Hb
A
- Mb can only bind 1 oxygen (O2)
- Higher affinity for oxygen than Hb
- P50 = 1 torr
- Must bind oxygen at low O2 tension present in the muscles
- Hb can bind between 1-4 oxygens
- Amount of O2 bound expressed as saturation value (designated Y)
- Amount of O2 bound depends on local oxygen concentration expressed as pO2
- P50 value = pO2 at which 50% of the sites are occupied by oxygen
- P50 = 26 torr
6
Q
Hemoglobin
Cooperative Binding
A
- Interface between α and β subunits stabilizied by interchain hydrogen bonds
- In deoxy state: H-bonds pull the proximal histidine F8 downward from the heme group and out of plane.
- Enter input needed to break and realign H-bond to create R-state before oxygen can bind
- Each oxygen which is bound causes subunits to shift so that F8 of other subunits more in plane and can bind oxygen more easily
7
Q
Hemoglobin
Oxygen Delivery
A
All allosteric effectors work to stabilize the deoxy (T) state making it easier for Hb to deliver O2.
- Last oxygen binds to Hb with a 300x greater affinity
- Relatively small change in pO2 needed for Hb to release it’s oxygen
- Intra-chain and inter-chain salt bridges between acid and basic residuals stablize deoxy state making it easier for Hb to release oxygen
- Induction of salt bridge formation shifts Eq from R state to T state
- CO2, H+, and Cl- are all allosteric effectors which make it easier for Hb to release oxygen
8
Q
Bohr Effect
A
H+ and CO2 released by metabolizing tissues reduce the affinity of O2 for Hb by stabilizing the deoxy state of the protein.
Bohr Effect is the observation that lowering of the pH or elevation of the pCO2 stabilizes the T-form and pushes the release of O2 from Hb.
Results in a shift to the right of the dissociation curve.
The opposite is also true: higher pH or lower pCO2 increases oxygen affinity of Hb.
Amino groups on α-chain and C-terminal His on β-chains bind H+.
9
Q
Effects of CO2
A
- CO2 binds to the amino termini of the 4 subunits of Hb forming a carbamino group
- Hb carries CO2 to the lungs
- ~10% of bodies CO2 expelled by Hb
- When CO2 is expelled from the lungs, Hb reverts to the R-state and binds O2
- CO2 acts as a signal for metabolic activity
- Majority of CO2 brought to the lungs as bicarbonate:
- RBC’s w/ anion exchanger that brings in Cl- while neg. charged bicarbonate’s exit
10
Q
Effects of H+
A
- Protons picked-up by free amino groups on the protein
- Induces a positive charge
- Drives induction of salt bridges
- Net effect to push Eq towards deoxy state
- Thus, change in pH of the environment leads to a more efficient release of oxygen
11
Q
2,3-BPG
A
2,3-BPG is the most important allosteric effector of Hb.
- Normally present in RBC’s at Eq concentration of 4.5 mM
- Binding of 2,3-BPG to the β-chains of Hb dramatically stabilizes the deoxy state, reducing affinity for oxygen
- When oxygen binds, BPG is released
- In the absence of BPG, Hb would have difficulty releasing oxygen to the tissues (P50 = 1 torr without)
- Under pathological conditions such as emphysema, high altitudes, or chronic anemia the body increases levels of BPG through increase in glycolysis (hypoxic response pathway)
- PK deficiency results in increased production of 2,3-BPG
12
Q
Altitude Effects
A
- Higher altitude = lower pO2
- Induction of 2,3-BPG production
- Causes a rightward-shift of the Hb binding curve
- Results in better oxygen delivery to tissues
13
Q
Temperature Effects
A
- Increases in body temperature = decrease in affinity of Hb for oxygen
- Oxygen release facilitated
- Beneficial during prolonged exercise
- Hypothermia causes the opposite effect
14
Q
Carbon Monoxide Effects
A
- CO binds tightly to the iron in Hb
- Forms carbon monoxyhemoglobin aka carboxyhemoglobin
- CO binding to one heme in Hb pushes Eq towards R form
- Hb binds oxygen more tightly and does not allow for normal release to the tissues.
15
Q
Nitric Oxide Effects
A
- Hb carries NO bound to specific thiol groups in the globin protein to form an S-nitrosothiol
- NO dissociates into free nitric oxide and thiol again as Hb release oxygen from heme site
- Hb carries NO to stimulate vasodilation in hypoxic areas
16
Q
Hb Effectors
Summary Table
A
17
Q
Hb Genes
A
Hb is synthesized from 8 different genes:
- Developmental gene expression of Hb generally goes from 5’ expression towards 3’ expression
- Zeta → Epsilon → Gamma → Alpha
- Beta expression after birth
- Small amount of delta also made
18
Q
Fetal Hemoglobin
A
- HbF is the predominant Hb in the fetus from 3rd through 9th months of gestation
- Persists at low level throughout life
- Subunit structure α2γ2
- γ-chain of HbF has a much lower affinity for 2,3-BPG
- Has higher affinity for oxygen than HbA (adult form)
- pO2 low in the placenta so HbF needs to bind oxygen at lower concentrations
- Left shift of dissociation curve
- HbF increased in thalassemia and other disorders
- HbF restricted to RBC’s called F-cells
22
Q
Sickle Cell Anemia
A
- Single missense mutation: Glu-6 → Val-6 on β-subunit
- Autosomal recessive
- Normal amounts of abnormal Hb made
- Qualitative hemoglobinopathy
- HbS with reduced solubility of deoxy-Hb
- Allows formation of fibrous polymeric filaments of doxy-Hb that precipitate in RBCs
- Causes “sickle” shape of RBC leading to decreased flexibility, mobility, and thus accumulation in capillaries
- Upon exposure to oxygen in the lungs HbS filaments immediately dissolve
- Hypoxia, dehydration, and acidosis can lead to sickle cell crisis
- Heterozygotes with sick cell trait with malaria protection
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