Ligand linked assembly of Scapharca dimeric hemoglobin
Program in Molecular Medicine and the Department of Biochemistry and Molecular Biology
Animals; Bivalvia; Chromatography, Gel; Hemoglobins; Models, Molecular; Protein Conformation; Water
Life Sciences | Medicine and Health Sciences
The assembly of Scapharca dimeric hemoglobin as a function of ligation has been explored by analytical gel chromatography, sedimentation equilibrium, and oxygen binding experiments to test the proposal that its cooperativity is based on quaternary enhancement. This hypothesis predicts that the liganded form would be assembled more tightly into a dimer than the unliganded form and that dissociation would lead to lower oxygen affinity. Our experiments demonstrate that although the dimeric interface is quite tight in this hemoglobin, dissociation can be clearly detected in the liganded states with monomer to dimer association constants in the range of 10(8) M-1 for the CO-liganded state and lower association constants measured in the oxygenated state. In contrast, the deoxy dimer shows no detectable dissociation by analytical ultracentrifugation. Thus, the more highly hydrated deoxy interface of this dimer is also the more tightly assembled. Equilibrium oxygen binding experiments reveal an increase in oxygen affinity and decrease in cooperativity as the concentration is lowered (in the muM range). These experiments unambiguously refute the hypothesis of quaternary enhancement and indicate that, as in the case of human hemoglobin and other allosteric proteins, quaternary constraint underlies cooperativity in Scapharca dimeric hemoglobin.
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Citation: J Biol Chem. 1997 Feb 28;272(9):5689-94.
The Journal of biological chemistry
Royer, William E. Jr.; Fox, Rachael A.; Smith, Francine R.; Zhu, Dan; and Braswell, Emory H., "Ligand linked assembly of Scapharca dimeric hemoglobin" (1997). Open Access Articles. 790.