Scientists studying the principles behind oxygen-hemoglobin dissociation have discovered a way to successfully separate hemoglobin tetramers into individual alpha and beta subunits. During an experiment, a solution is created that contains only monomeric beta-hemoglobin subunits under physiologic conditions. If measured, the oxygen dissociation curve of the dissolved beta subunits will most likely resemble which of the following lines?
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Hemeproteins such as hemoglobin and myoglobin contain heme groups that are used to reversibly bind oxygen for transportation and storage. Hemoglobin A (the major form of hemoglobin in adults) is a tetramer consisting of 2 alpha and 2 beta chains. Each hemoglobin subunit is associated with a heme moiety, so each hemoglobin molecule has 4 heme groups. After binding to 1 oxygen molecule, the oxygen affinity of other heme molecules increases; this heme-heme interaction is responsible for the characteristic sigmoid shape of the oxygen-hemoglobin dissociation curve.
In contrast to hemoglobin, myoglobin is a monomeric protein and the primary oxygen-storing protein in skeletal and cardiac muscle tissue; it is only found in the bloodstream after muscle injury. The partial pressure of oxygen at which 50% of myoglobin molecules are oxygen saturated (P50) is only 1 mm Hg, which is much lower than the P50 of hemoglobin (26 mm Hg). Myoglobin also has only a single heme group and so does not experience heme-heme interactions; therefore, its oxygen-dissociation curve is hyperbolic.
The secondary and tertiary structures of myoglobin and the hemoglobin beta subunit are almost identical (the α-subunits are also very similar to myoglobin). Because individual hemoglobin subunits are structurally similar to myoglobin, their oxygen-binding behavior is also similar. That is, if a hemoglobin molecule is dissociated, the individual subunits will have a hyperbolic oxygen-dissociation curve (Choice A).
(Choice B) Line B is left-shifted compared to normal hemoglobin A, which could indicate a high oxygen affinity hemoglobin (eg, fetal hemoglobin). Hemoglobin A will also undergo a left shift in the presence of factors such as increased pH, decreased temperature, and decreased 2,3-diphosphoglycerate.
(Choice C) Line C depicts the normal oxygen-hemoglobin dissociation curve, with the typical sigmoidal relationship of pO2 to hemoglobin O2 saturation.
(Choices D and E) Lines D and E are shifted to the right, indicating a low oxygen affinity hemoglobin. Hemoglobin A has low affinity for oxygen under conditions such as decreased pH, increased temperature, and increased 2,3-diphosphoglycerate.
Educational objective:
The individual subunits of the hemoglobin molecule are structurally analogous to myoglobin. If separated, the monomeric subunits will demonstrate a hyperbolic oxygen-dissociation curve similar to that of myoglobin.