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An in-depth analysis of myoglobin and hemoglobin, the primary oxygen-carrying molecules in vertebrates. Discover their structures, functions, and the importance of their allosteric properties in oxygen transport. Learn about the role of the heme prosthetic group, the differences between myoglobin and hemoglobin, and the significance of their cooperative binding and the bohr effect.
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Allosteric Proteins: Myoglobin and Hemoglobin The transition from anaerobic to aerobic life was a major step in evolution because it uncovered a rich reservoir of energy. Fifteen times as much energy is extracted from glucose in the presence of oxygen. Vertebrates have evolved two principal mechanisms for supplying their cells with a continuous and adequate flow of oxygen.
Hemoglobin Oxygen transporter in erythrocytes Four polypeptide chains- 2 (141 residues) chains and 2 chains (146 residues) in adults Embryos and fetuses have distinctive hemoglobins to facilitate diffusion of O 2 from the maternal blood Sub-unit interactions are crucial to the ability of hemoglobin to transport O 2 , CO 2 , and H+ in a physiologically responsive way. Comparison of hemoglobins from different species shows 9 invariant residues; F8 His Proximal heme-linked E7 His Distal CD1 Phe Heme contact F4 Leu Heme contact B6 Gly Allows close approach of B and E helices C2 Pro Helix termination CH2 Tyr Cross-links H and F helices H10 Lys Uncertain C4 Thr Uncertain Invariant residues stabilize oxygen binding site or stabilize helical segments Amino acid residues in the interior vary considerably, but are highly conservative. The non-polar interior is maintained. Exterior residues are highly variable and are not conserved. Hemoglobin has more complex structure and more complicated binding properties. Hemoglobin is an allosteric protein, whereas myoglobin is not. This difference is expressed in three ways:
Capillaries: Low pH; low pO2; protons bind to Hb, cause unloading of O2; causes more CO 2 to be converted to bicarbonate to be transported in the bloodstream.
Lungs: High pH; high pO 2 ; protons released from Hb; enhances O 2 binding ; release of H+ drives release of CO 2 to be exhaled from lungs. Side chains of histidines 146 and 122 and the amino group of the chain account for much of the Bohr effect. Most of the CO 2 is transported as bicarbonate formed by carbonic anhydrase. The remainder binds to the N-terminal groups as carbamates. The carbamates form salt bridges that stabilize the T form. Hence, the binding of CO 2 lowers oxygen affinity of hemoglobin.
BPG -only one molecule is bound to deoxyhemoglobin and stabilizes the structure; creates additional salt links that must be broken for O 2 to bind. High altitudes: after about two weeks, body adjusts by making more erythrocytes; until then, the BPG conc increase to ensure maximum release of O2. BPG is also a glycolytic intermediate; it cannot diffuse out of the cells because of its charge, but it may be utilized for other purposes; fresh blood vs week old blood. Fetal Hemoglobin-higher O 2 affinity at low pO 2 facilitates transfer from maternal Hemoglobin can bind 4 O2, 4H+, 4 CO2, and 1 BPG