Term
| A single chain with a single O2 binding site is called ??? |
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| What consists of 4 chains, each of which has an O2 binding site? |
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| Transports O2 to tissue. Transports CO2 from tissue to lungs. |
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| Accepts O2 from hemoglobin in muscle. |
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| True or False: Hemoglobins are globular proteins, found in blood, that binds oxygen in lungs and transport oxygen to tissues. Hemoglobins also transports CO2 and protons from tissues to lungs. |
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| True or False: A hemoglobin molecule consists of four polypeptides, two each of different AA sequences. The major form of human adult hemoglobin, HbA1 consists of two alpha and two beta chains. Each chain has one O2 – binding site, so that the heme protein has a total of four sites. |
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| True or False: Myoglobin is a single polypeptide chain with a single O2 – binding site. Myoglobin binds and releases O2 with changes in the oxygen concentration in the sarcoplasm of skeletal muscle cells. Myoglobin is a model for what occurs when a single protomer acts alone without the inter-chain interactions displayed by the hemoglobin tetramer. |
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| Each of the globin polypeptide subunits contain a ???. Without this group, the protein is an apoprotein. With this, the protein is a holoprotein. |
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| A prosthetic group that is responsible for the actual O2 transport. |
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| True or False: O2 is tightly bound to hemoglobin. |
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| Heme contains protopophyrin IX that has a ferrous iron atom in the center. The Fe++ ion has the d6 configuration, and can therefore bind as many as 6 atoms. Four of the ligand atoms are the pyrrole N- atoms. The fifth is the proximal histidine residue. In the case of oxyhemoglobin, Hb(O2)4, molecular O2 forms the sixth ligand. It is placed between the Fe++ ion and the distal histidine |
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| True or False: The conformation of the hemoglobin and myoglobin molecules have the result that the Fe(II)- O2 complex is buried deeply within the hydrophobic region of the molecule. If this were not the case, the Fe(II) would be readily oxidized by air to Fe(III), which could not carry O2 . |
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| True or False: Color of hemoglobin is determined by its absorption spectrum. This spectrum is highly sensitive to degree to which oxygen or other molecules (e.g. CO) is bound to iron. |
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| The binding of a ligand (read “O2”) has no effect on the binding of a second ligand. This is automatically the case with myoglobin. What kind of binding is this? |
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Definition
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Term
| The binding of a ligand to one binding site influences the binding of a second ligand. This is the case with hemoglobin. |
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Definition
| Cooperative binding (Cooperativity) |
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Term
| The association of oxygen to myoglobin is characterized by a simple equilibrium. Since only one oxygen molecule is bound per molecule of myoglobin, the binding of a second O2 molecule to a second (non-interacting) chain does not depend on whether or not the first chain is bound. This is ??? |
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Definition
| Simple, non-cooperative binding. |
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Term
| Hemoglobins, on the other hand, have four interacting monomeric subunits. The binding of O2 to one subunit facilitates the binding of a second O2 to the other subunits. This is positive cooperativity. (In cases where the binding of a ligand to one interacting site hinders the binding of a second ligand, we have negative cooperativity). |
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| True or False: Oxygen saturation curve for myoglobin is hyperbolic, and the hemoglobin is sigmoidal. |
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In the Hill Equation:
Y is the fraction of sites bound
pO2 is the oxygen tension
P 50 is oxygen tension where Y = 0.5
nH is a cooperativity parameter (n = 1 for noncooperative systems) |
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Definition
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| Cooperative binding of O2 to hemoglobin. Initially, the four subunits are in the T (tense) state, in which they resist the binding of O2. As O2 binds, the subunits change their conformation, increasing the likelihood of additional O2 binding. This example of positive cooperativity results in a sigmoidal curve. As O2 binds, the conformation of hemoglobin changes from the T state to the R (relaxed) state. |
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| Most (~ 80%) of the CO2 is hydrated and subsequently transported as HCO3- (bicarbonate). However, some CO2 is carried as, on free amino groups in hemoglobin |
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| CO binds avidly to ???. When CO binds to one or more of the 4 sites on hemoglobin, the hemoglobin reverts to the R state, which causes the remaining oxygen to be bound more tightly. Consequently, the bound oxygen is unavailable for tissues. Fortunately, CO binding is reversible, although it takes heroic measures to reverse this effect. |
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| True or False: Hemoglobin has a greater affinity for protons than does deoxyhemoglobin. |
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| True or False: Oxygen has a much greater affinity for myoglobin than for hemoglobin |
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| This equation indicates that deoxyhemoglobin has a greater affinity for protons than does hemoglobin. At pH > 6, appreciable H+ dissociation occurs when deoxyhemoglobin is converted to oxyhemoglobin. The release of oxygen is enhanced when the pH is lowered or when there is an increase in pCO2. This is known as the ???. An increase in protons shifts the equilibrium to the right; an increase in pO2 (or decrease in protons) shifts the equilibrium to the left |
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True or False: The following represents the Bohr Effect.
HbO2 + H+ = HbH + O2
Deoxyhemoglobin has a greater affinity for protons than does hemoglobin.
The release of oxygen is enhanced when the pH is lowered or when there is an increase in pCO2. |
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True or False: An increase in H+ or an increase in pCO2 means that hB would have less affinity for oxygen and it will release it easier.
An increase in [H+], results in an increased amount of oxygen is dissociated from the Hb molecule, and therefore made available to the cells. |
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| Bisphosphoglycerate (2,3 – BPG) is an important regulator of the binding of O2 to hemoglobin. It is synthesized as intermediate in glycolysis, and is as abundant as hemoglobin in the red blood cell. It binds to hemoglobin, but not deoxyhemoglobin. The binding stabilizes the ??? of deoxyhemoglobin. |
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| True or False: A single molecule of 2,3-BPG binds in a positively charged cavity formed by the beta-chains of hemoglobin in the doexy form. |
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One molecule of 2,3 – BPG binds to the center of the deoxyhemoglobin tetramer formed by the two beta - globin chains. This pocket contains several positively – charged amino acids which can interact with the negatively charged phosphates. 2,3 – BPG is expelled on oxygenation of the hemoglobin.
Higher concentrations shift the dissociation curve to the right, by lowering the oxygen affinity. This makes more oxygen available for tissues. On the other hand, stripping the blood of 2,3 – BPG increases the affinity of oxygen for hemoglobin. Such hemoglobin acts as an “oxygen trap” rather than as an oxygen transport system. |
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