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Biochemistry Section 1 - Bitmm Fan var 59(le Page 1 1...

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Unformatted text preview: , Bitmm Fan var 59(le Page 1 1. Please mark A to indicate that this is version E of the exam. “w i A. Please mark this bubble. I O / \ 2. Please mark E to indicate that this is version C of the exam. E. Please mark this bubble. 3. Which statement is TRUE regarding myoglobin and hemoglobin? ,K. The iron in myoglobin and hemoglobin is normally in different oxygen states. B. The three-dimensional structure of myoglobin is unrelated to the three-dimensional structures of either the alpha or beta subunit of hemoglobin. C. Hemoglobin shows positive cooperativity towards oxygen; myoglobin shows negative cooperativity towards oxygen. D. Myoglobin has four identical subunits; hemoglobin has two copies each of two different subunits. (E) The 02 affinity of myoglobin is higher than that of hemoglobin. 4. In going from the T state to the R state of hem 'n the iron ion and the proximal histidine along with its alpha helix move towards the porphyrin. the iron ion moves out of the plane of the porphyrin, while the proximal histidine moves closer to the porphyrin. C02 makes a carbamate with the N-terminus of a subunit. a complex mechanism involving two alpha helices and a beta strand relay changes at the interface of the subunits to the oxygen-binding heme. K In the absence of oxygen, the low affinity state of hemoglobm IS preferred. C -’ @ The position of the iron ion and the proximal histidine with respect to the plane of the porphyrin is solely g 4:” \ determined by the presence or absence of oxygen. .7“ fly The subunits of the R state interact with fewer salt bridges than the subunits of the T state. )3” The principal difference between the T and R states is in the orientation of one alpha-beta dimer with respect to the other. The packing differences at the interface result 1n slight motion of a key alpha helix. 6. In fetal hemoglobin (Hb (12%), the gamma subunit is homologous to the [3 subunit of maternal Hb. A crucial difference between the gamma and beta subunits is that the gamma subunit has a serine at position 143 where the [3 subunit has a histidine. This alters amino acid residues forming a cavity in the center of T state Hb. As a result maternal Hb can transfer 02 to fetal Hb in the placenta. Which statement explains the reason for this facilitated transfer? A. This cavity binds bisphosphoglycerate (BPG). The His for Ser change lowers the affinity of fetal Hb for BPG. . Now, fetal hemoglobin can be in the R state in the placenta, while maternal Hb is in the T state. ,3." The [5 His 143 residues make salt bridges specific to T state Hb. Since fetal Hb cannot make these interactions, its R state is more stable. In the placenta fetal Hb is R and maternal Hb is T. ,9.” Ser in the fetal Hb can make salt bridges in the R state that His' in maternal Hb cannot make. Hence the R state of fetal Hb is more stable in the placenta. D” The His to Ser change alters the heme environment. The His crowds 02 1n the T state of the [5 subunit As a result, T state fetal hemoglobin has a higher 02 affinity than maternal hemoglobin. )3. Beta His l43binds the Bohr protons. Thus, fetal Hb doe not show a Bohr effect. In the acidic placenta, fetal Hb remains in the R state, while maternal Hb switches to the T state. ...
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