Hemoglobin and Myoglobin Flashcards
Heme
Iron-chelated porphyrin prosthetic group in hemoglobin and myoglobin. Heme B is principle heme in oxygen-binding proteins.
What is Heme B composed of?
Protoporphyrin IX (contains 4 linked pyrrole groups) and ferrous iron (Fe2+)
Hemoglobin structure
Globular, tetrameric protein (4 non-covalently linked subunits: 2 _ type and 2 _ type) with hydrophobic residue patches which facilitate quarternary structure. Eight helices connected by _ turns-E and F helices bind to heme group. 2 polar carboxylic groups interact with Fe2+
What parts of the hemoglobin chain are invariant?
Proximal His (stronger bond than distal His because closer), distal His, and hydrophobic heme binding residues
Myoglobin structure
Single polypeptide chain protein and one O2 binding site. Eight _ helices connected by _ turns (just like hemoglobin).
Apoprotein form of hemoglobin/myoglobin
Does not contain heme and cannot bind oxygen
What happens to the amino acid stucture when oxygen molecule binds?
Oxygen molecule binds to the ferrous ion replacing the distal His
Which chromosomes contain the genes for hemoglobin?
3 _ genes are on chromosome 16 and 4 _ genes are on chromosome 11
_-type subunits
_1, _2, _
_-type subunits
, , _, G, A
HbF chain composition
_2_2
HbA1 chain composition
_2_2
HbA2 chain composition
_2_2
HbGrowler chain composition
_2_2
HbPortland chain composition
_2_2
Hemoglobin T-state (Perutz model)
Tense state where ferrous ion out of plane of heme due to steric hinderance
Hemoglobin R-state (Perutz model)
Relaxed state where ferrous ion moves to the center of the porphyrin ring. Energetically more favorable state.
Steps involved in T-state to R-state transition
- Oxygen binds 2. Iron atom moves 3. Proximal His moves 4. F helix moves 5. Entire globin tertiary structure moves
BPG (sometimes called DPG)
2,3-biphosphoglycerate synthesized from glucose in glycolysis. Has high affinity for T-state Hb. Produced in high quantities in RBCs. Binding of BPG to Hb shifts equilibrium towards T-state promoting release of oxygen. One molecule of BPG binds to positively charged cavity formed by _ chains.
Hypoxia
Can be caused by anemia, smoking, or high altitude. BPG levels increase, promoting release of oxygen to tissues. In anemia RBC levels lower than normal so oxygen transport is low. Smoking/CO poisoning blocks binding of oxygen to Hb (CO will displace oxygen with 1000% higher affinity). In high altitude oxygen pressure is lower and Hb will not saturate 100%–body responds by making more Hb and BPG.
“Oxygen trap”
Storage of blood in presence of anticoagulant causes lower levels of BPG in RBCs so poor oxygen release.
HbF effects on mother
HbF has higher oxygen binding affinity than HbA. _ subunits do not bind BPG well so oxygen binding curve shifts to the left of the mother’s. This facilitates flow of oxygen from mother to fetus.
Where is the oxygen dissociation curve the steepest?
At concentrations in the tissues so oxygen delivery to respond to small changes in oxygen concentration.
Myoglobin oxygen dissociation curve
Hyperbolic curve (saturation=Y). Because of this myoglobin would be a poor carrier of oxygen, but good for storage. Fully saturated in tissue.
Hemoglobin oxygen dissociation curve
Sigmoidal curve. This is because of cooperative binding of the 4 subunits.
Alkaline Bohr Effect
When Hb binds to oxygen with 100% saturation in the lungs, pH is higher than periphery and switches to R state. Ionic interactions between subunits break and release protons. Opposite happens in tissues.
Ways that Hb can transport CO2
- Chemically bound to free amino terminus of globin subunit in carbamate form (this stabilizes T-state and helps unload oxygen) 2. Coupled to proton transfer in the bicarbonate buffer system
Hyperventilation
Causes decrease in CO2 concentrations in blood which results in left shift in the oxygen dissociation curve and less oxygen delivered (via Bohr effect).
Blood transfusion problems
Stored blood loses BPG and cannot release all of its oxygen, quickly loses NO so inefficient gas exchange, delivers extra Hb thus iron so risk of transfusional hemosiderosis
Ways that Hb can transport NO
- Reversibly binds to Hb and gets delivered to receptors of cells of the blood vessel wall. This facilitates transfer of gases between blood and tissues. 2. NO bound to R-state and released when Hb releases oxygen and changes to T-state. This saves NO from rapid destruction in blood.
Nitric oxide (NO)
Very potent vasodialator
HbS mutation
_6 (Glu–>Val) causes sickle cell anemia
HbC mutation
_6 (Glu–>Lys) causes mild anemia and crystals in RBCs
HbM mutation
_58 (HbM-Boston) or _63 (HbM-Saskatoon) (His–>Tyr) causes methemoglobin, cyanosis
Methemoglobinemia
Most frequent genetic mutation occurs at distal or proximal His. If genetic either the enzyme methemoglobin reductase or the globin is mutated. If acquired not enough reducing agent (excess of oxidizing agent). Ferrous iron gets oxidized to ferric and cannot bind oxygen. Treat with IV ascorbate and methylene blue.
Thallasemia
Autosomal recessive disorder where one of the globin genes not expressed properly, leads to anemia. Abnormal quarternary structures result depending on mutation.
Effects on oxygen binding of 1. oxygen concentration 2. CO2 concentration 3. low pH 4. BPG concentration 5. temperaure
- High O2 promotes oxygen binding 2. High CO2 weakens oxygen binding 3. Weakens oxygen binding 4. High BPG weakens oxygen binding 5. High temp weakens oxygen binding
