Chapter 7

What are some characteristics of hemoglobin?
-A protein on red blood cells
-Carry oxyen and CO2 from the lungs and tissues.
-Composed of 4 polypeptide chains
-Uses about 90 percent of it's capacity
-cooperatively binds to oxygen (O2 binding to 1 chain increases binding on other chains)
What are some characteristics of myoglobin?
-A protein in muscle cells
-Uses about 7% of it's capacity
-Used in a time of need
-1 polypeptide chain
-Uncooperative in binding to O2
What allows hemoglobin and myoglobin to bind to oxygen?
-Heme allows the binding of oxygen
-provides the red color in blood and muscles
-Composed of an organic part (protoporphyrin) and a central iron atom in the ferrous form.
There are two other binding areas for heme, what are the names and what do they bind?
The 5th and 6th binding coordination sites.
5th-binds to imidazole ring of histidine
6th-binds to oxygen
What happens to the iron atom after the 6th coordination site binds to oxygen?
The iron is normally too large to exist in the center of the atom at 0.4 A outside the porphyrine plane. When it binds to oxygen, the electrons rearrange and the iron becomes smaller allowing it to get to the center of the protein.
When oxygen is bound to myoglobin, how does the resonance appear?
The resonance is between the ferrous form of iron, and dioxgen. The other form is the ferric form of iron and superoxide ion.
What is metmyoglobin?
If the superoxide ion were to leave heme, then the iron would be in the ferric form F^3+ and this form does not bind oxygen.
What stabilizes the iron to prevent it from releasing superoxide and creating metmyglobin?
The distal histidine of myoglobin creates a hydrogen bond that helps stabilize the oxymyoglobin.
How does hemoglobin appear? Hint: it is similar to myoglobin.
2 identical alpha helices (form a globin fold)
2 identical beta chains
How is the binding strength of hemoglobin and myoglobin measured?
This is measured by looking at the oxygen binding curve: fractional saturation (the possible binding sites that contain oxygen already "0-1" and the oxygen concentration (measured in oxygen partial pressure).
Describe the oxygen binding curve for myoglobin.
The oxygen binding curve sharply rises when the partial pressure of oxygen reaches 2 torr. This shows that it easily is occupied by oxygen. The graph appears like a hyperbolic shape.
Describe the oxygen binding curve for hemoglobin. Why is the half saturation much higher than myoglobin?
The oxygen binding curve rises slowly and reaches to half saturation at 26 torr. This is much higher because the hemoglobin interacts with 2,3-bisphosphoglycerate, this lowers affinity to oxygen. The shape is sigmoidal.
How does the coperativity of hemoglobin affect it's binding of oxygen?
-lungs have high partial pressure of oxygen allowing it to bind easily
-tissues have lower partial pressure allowing oxygen to be released.
-hemoglobin is able to contribute 66% of oxygen bound to others
-myoglobin would contribute only 7% of binding capacity.
Explain the mechanism observed during oxygen binding in the T and R states that contribute to the cooperativity of the hemoglobin.
Recent studies have shown that when hemoglobin is absent of oxygen the bonds are tense (T) in the two alfa-beta dimers. When the oxygen is bound, there is more flexibility (R state) allowing the other subunits to easily bind to the oxygen.
How much do the alfa-beta dimers rotate when bound to oxygen?
They locally rotate approximately 15 degrees form T state to R state.
Describe the concerted model of cooerativity.
This model shows that the hemoglobin can only exist in a T or R state. When the hemoglobin is bound to one oxygen it slightly favors the R state. If 3 segments are O2 bound the remainder is in R form. All or nothing
Describe the sequential model of cooperativity.
This shows that the subunits only turn into R state if and only if the O2 is bound to them, but it still causes them to be slightly more favorable to R state. So if one is O2 bound the other 3 are in T state.
How would the graphs look like if the hemoglobin was exclusively in T-state or the R-state binding curve?
The binding curve for the T state woul dnot even reach half saturation at 200 torr. The R state would reach saturation at an unusually low O2 pressure.
How does the movement of one structure affect the other structures in the segments?
When oxygen binds to one segment, the iron moves in the plane. The proximal histidine shifts with the a-helix segment. This movement causes the others to adapt the R state.
What is the role of 2,3-BPG in allosteric effects?
This small segment is bound to the hemoglobin, and it stabilizes the T-state. When the O2 bound to the protein reaches threshold, the 2,3-BPG. This allows there to be a small delay in binding oxygen so it can be an efficient transporter.
How is fetal hemoglobin different from maternal red blood cells?
Fetal hemoglobin binds oxygen more tightly because it does not bind 2,3-BPG as tightly. This is so that oxygen can be transferred from the mother to the baby more effectively.
What occurs during CO poisoning?
CO binds so tightly to iron on hemoglobin that it stabilizes the R state so that it does not change into T state. This means that the hemoglobin does not transfer the oxygen to the peripheral tissues.
What is the Bohr effect when it comes to binding to oxygen.
CO2 and H+ is produced by respiring tissues, and this lowers the pH in our tissues allowing salt bridges to form that stabilize the T state.
What happens in sickle cell anemia?
-gene mutation causes valine substitution for glutamate at position 6 of the B-chain.
-valine interacts with other deoxy HbS to form aggregates and blockage.