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31 Cards in this Set
- Front
- Back
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# of Protein Chains in Mb, Hb
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1 in Mb; 4 in Hb (tetramer of 2 alpha & 2 beta)
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Composition of myoglobin
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153 amino acids, 8 alpha helices, 1 heme group
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Composition of hemoglobin
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2 alpha chains & 2 beta chains (names NOT related to alpha helices or beta strands/sheets) Each alpha chain has 141 amino acids and 7 alpha helices. Each beta chain has 146 aa's and 8 alpha helices
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How many hemes per subunit in Hb / Mb
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One heme for each of the four Hb subunits. Mb is a monomer, so there are not subunits but only one heme in the molecule
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3 key physiological points about hemoglobin
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Hb is a tetramer; Hb is an allosteric molecule; Hb exhibits positive cooperativity
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3 key physiological points about myoglobin
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Mb is is a monomer; Mb is not allosteric, Mb does not exhibit positive cooperativity. (Everything Hb is, Mb is not)
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What is allosteric?
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A protein molecule where small molecules can bind at one site and regulate the binding of another molecule at another site.
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What molecules bind to Hb allosteric sites?
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2,3-BPG; CO2; H+
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What is positive cooperativity w/r/t Hb?
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As hemoglobin can hold up to 4 oxygen, the first oxygen is most difficult to bind and each successive oxygen becomes easier. Similarly, as hemoglobin is losing oxygen, the first oxygen lost is the hardest and then next ones become easier to lose.
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Explain what happens to the shape of Hb and ease of successive O2 addtions.
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Going from no oxygen in the T-State, the first oxygen is forced onto the heme and is rather unstable within the unit. The second oxygen is added to the other alpha-beta subunit and increases the level of unstability at first, but then Hb shifts to R-form.
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Myoglobin's primary role
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Storage vesicle for O2, binds tight and releases at very low O2 pressure of 1 to 2 torr. This somehow helps get oxygen to the "deepest crevices" of yoour muscles.
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Hb's role in the lungs
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Primarily a transport protein that binds O2 tightly in the lungs
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Hb's role in the tissue
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Hb has to release O2, so it cannot bind it as tightly.
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What allows Hb to serve it's two different roles?
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There are two different arrangements of the 4 subunits. One binds O2 tightly and the other arrangement releases O2
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What is a pyrole?
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A 5-member ringed molecule with one Nitrogen heteroatom. It is aromatic.
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What is the porphyrin Ring?
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A structure in heme that is 4 pyrole molecules arranged in a circle with the nitrogens facing the middle. A Fe2+ is in the middle of the circle. A histadine-93 is hanging off the Fe2+ somehow.
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What shape is the heme ring when O2 is bound to it?
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Flat
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What shape is the heme ring when O2 is NOT bound to it?
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Puckered
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What is the shape of the Mb O2 saturation vs. pressure (O2) graph?
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A hyperbola, meaning a "simple" equilibrium will establish beteewn the ligand (O2) and the macromolecule (Mb)
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What does the shape of the Mb O2 saturation graph tell us?
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The shape is hyperbolic, indicating that what is already bound to molecule does not affect the finding of anything else (O2 in the case of Mb)
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Where does Mb show saturation?
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At very low PO2
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How would the myoglobin - O2 binding be classified?
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Mb binds O2 very tightly
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P50 values - explain and give the values for Hb & Mb
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This is the pressure of O2 at which the molecule is 50% saturated. For Mb it is 2.8 torr; for Hb it is 26 torr.
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What does a low P50 value mean?
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It means the molecule binds O2 tightly.
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What is the shape of the Hb saturation curve and comment on the meaning
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sigmoidal, meaning there are 2 different regions of the curve - "s- shaped". In one region, Hb is not happy to bind O2, in another region it is more easily bound. This reflects the 2 different states of Hb.
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With oxygen saturation plotted for both Hb and Mb on the same graph, what curve(s) can move and why?
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If a curve shifts, it means that the molecule is saturated at different pressure levels. Mb never shifts. Hb can shift left or right due to allosterics
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What do the two states of Hb allow for?
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The T state (deoxy) and the R state (oxy) provide the structural basis for cooperativity
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What pushes Hb to the T state?
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Allosteric regulators or low PO2
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What stabilizes the R state in Hb?
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High PO2 in the lungs
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What is the molecular basis for cooperativity?
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Cooperativity (found in Hb) is possible because of the shape changes from planar (w/ O2 or "oxy") to puckered (without O2 or "deoxy")
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When does Hb shift from T to R state?
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When there is one O2 in each alpha-beta subunit.
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