Hemoglobin & Myoglobin Flashcards
Myoglobin vs Hemoglobin similarities and differences
Same for both:
- Globular structure (all-alpha globulin fold)
- Heme prosthetic group (non-covalently bound)
- Oxygen binding function
Different for both:
- Location (Hb in RBC, Mb in muscle cells for O2 storage)
- Structure (Hb is tetramer, Mb is monomer)
- # of O2 bound (Hb is 4, Mb is 1)
- Binding other things (Hb does like CO2 and NO, Mb doesn’t)
Hb function
Mb function
What is NO needed for?
Why do we need Hb?
- Hb binds O2 in lungs, transports it to tissues and releases in capillary beds
- Hb carries CO2 and H+ from tissues to lungs for elimination
- Hb carries and releases NO, potent vasodilator & inhibitor of platelet aggregation, in periphery
- Mb binds, stores & releases O2 in muscle myoplasm based on changes in O2 conc.
- NO needed to help release O2 and pick up CO2, helps with gas exchange in arterial wall by vasodilation
- Hb delivers O2 75-100x better than blood alone
Hb & Mb Structure
What is oxidation state of iron in heme? How many coordination valencies and where do they bind?
what is secondary structure and how many of them?
Where is heme located?
What are the components of heme? What is ring character?
Where does O2 bind heme?
- Must be 2+ (ferrous) to bind O2! 6 coordination valencies, 4 bind heterocyclic N, 2 bind proximal/distal histidine. Fe3+ has only 4 coordination valencies so can’t bind oxygen
- Alpha helix, 8 (A-H)
- Hydrophobic pocket, apolar amino acids interact w/ heme to keep it in globin fold
- protoporphyrin IX ring + Fe2+, ring is apolar with polar carboxylate groups facing out
- O2 replaces the distal histidine since it binds weaker than proximal
When are different Hb genes expressed during development?
Why switch from gamma to beta?
- Adults have
1. HbA1 (alpha2beta2) 98%,
2. HbA2 (alpha2delta2) 2%
3. HbF (fetal, alpha2gamma2), <1%
Embryonic (zeta2episilon2 and zeta2gamma2) disappear after 3-4 weeks
Gamma binds O2 better, so needed to grab O2 from mother, but once born, want to be better able to deliver O2 so switch to beta. Is replaced 12-18 weeks after birth.
Perutz Model: T and R states
How does it lead to cooperativity?
How are interactions between alpha-beta chains? between dimers?
T state is deoxy form- repulsive forces keep Fe2+ out of the plane
R state is oxygenated form- O2 binding pulls Fe2+ back into plane, moving proximal His, F helix, whole globin
- This forces all of the other subunits to follow and convert to R conformation (and vice-versa)
- Strong hydrophobic interactions between alpha/beta
- Weak ionic & H-bonds between dimer pairs in deoxygenated state, get broken in oxygenated state
Mb oxygen saturation curve (shape, pO2 in lungs, tissue, P50 value, when is O2 released?)
Hb oxygen saturation curve (shape, pO2 in lungs, tissue, P50 value, when is O2 released?)
- Hyperbolic curve, fully saturated in lungs/tissues, release O2 only when O2 partial pressure in tissues drop, P50=1 torr
- Sigmoidal curve, fully saturated in lungs, half saturated in tissue (releases half of oxygen), positive cooperativity in O2 binding and release, P50=26 torr
Effect of BPG on Hb
In hypoxia due to anemia, smoking, or high altitude what happens to BPG?
What happens to BPG while storing blood?
What about BPG binding to HbF?
- BPG has high affinity for deoxy T state, so helps Hb release O2 at higher partial pressures
- Increases b/c in all cases, not as much O2 is transported (anemia has fewer RBCs, smoking has blocked iron on Hb, altitude has Hb not fully saturated, also make more RBC and Hb)
- Decreases due to presence of anticoagulant, called “oxygen trap”
- BPG binds with less affinity to HbF, facilitating flow of O2 from mom to fetus
Bohr Effect- proton & CO2 transport by Hb
Acidic Bohr Effect
Alkaline Bohr Effect
Effect of pH on O2 binding
How does Hb transport CO2 (3 ways)?
What happens when a person hyperventilates?
- When Hb is dexoygenated, some ionic & H-bonds reform and take up protons
- When Hb is oxygenated, some ionic & H-bonds are broken, releasing protons
- Decrease in pH helps Hb release O2 in tissue at higher partial pressures and bind H+ instead
- Transport CO2 by:
1. bind to free amino terminus as carbamate (stabilizes T form, help unload O2 at higher partial pressure)
2. Couple to proton transport in blood (HCO3- buffer)
3. Dissolved in blood - Lowers CO2 conc. in blood, elevating pH, shifting O2 saturation curve left so O2 not released as much. Leads to dizzines, muscle cramps, passing out
NO Transport by Hb & Temp effects
- What form does it bind to?
- What happens to NO in blood?
- What is effect of high temperature (i.e. fever) on Hb?
Binds to the R form, and released when Hb release O2 and goes to T form
- NO in blood is eliminated so conc. is very low, therefore needs facilitated transport to be sequestered from rapid destruction
- High temp weakens Hb O2 binding, so need more partial pressure to get Hb to fully bind (in additional to more O2 to meet metabolic requirements of fighting infection)
4 issues w/ blood transfusions
- Infection (blood-born pathogens)
- Low BPG (cannot release all O2)
- Low NO (gas exchange inefficient)
- Extra Hb and Fe (transfusional hemosiderosis, can lead to liver failure)
Genetic hemoglobinopathies
Where in Hb can mutations occur?
What kind of change could cause a problem?
Where would the mutation cause a problem?
What is changed in Hb structure?
What are consequences of mutant Hb?
HbS
HbC
Hb M-Boston/Hb M-Saskatoon
Thallasemia
- Anywhere
- Change in protein structure (i.e. alpha helix breaks), change in quality of AA @ strategic locations, incapable of synthesizing or folding Hb subunits
- “Hot spots” (places where any change in AA changes function), heme binding pocket, Fe2+ binding AA’s, subunit interactions, Pro can be a helix breaker
- Instable, incr/decr O2 affinity, oxidation of Fe2+ to Fe3+, Hb precipitate, Hb eliminated
- Hb precipitate, hemolysis (RBC death), RBC elimination (leads to anemia, reticulocytosis, splenomegaly, urobilinuria)
- Beta6 Glu-Val (polar to non polar), sickle cell from exposing a hydrophobic patch in beta chains, leading to aggregates and sickling
- Beta 6 Glu-Lys (- to +), mild anemia from HbC crystals
- Affect proximal or distal histidine, lead to methemoglobin (Fe3+ oxidization, either reducing enzyme methemoglobin reductase or diaphorase [recessive] or globin is mutated [dominant])
- Autosomal recessive disorder, one of globin genes not expressed properly, can get some abnormal quaternary structural aggregates
Acquired hemoglobinemias
CO poisoning
Acquired methemoglobinemia
- CO binds Fe2+, displacing O2 (1000x stronger), forming carboxyhemoglobin and lowering max O2 saturation (need 100% O2 to treat)
- Hb and reductase are normal, but not enough reducing agent present due to consuming an oxidizing agent (like nitrate/nitrite in water)
