Nested allosteric interactions in extracellular hemoglobin of the leech Macrobdella decora |
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Authors: | Hellmann Nadja Weber Roy E Decker Heinz |
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Affiliation: | Institute for Molecular Biophysics, University of Mainz, Jakob-Welder-Weg 26, 55128 Mainz, Germany. nadja@biophysik.biologie.uni-mainz.de |
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Abstract: | Hemoglobin from the leech Macrobdella decora belongs to the class of giant extracellular hexagonal bilayer globin structures found in annelid and vestimentiferan worms. These complexes consist of 144 heme-bearing subunits, exhibit a characteristic quaternary structure (2 x (6 x (3 x 4))), and contain tetramers as basic substructures that express cooperative oxygen binding and thus provide a structural basis for a hierarchy in allosteric interactions. A thorough analysis of the isolated tetramer indicates that it functions as a trimer of cooperatively interacting subunits and a non-cooperative monomer rather than as four interacting subunits. A thermodynamic analysis of the whole molecule favors the application of a nested Monod-Wyman-Changeux model with six cooperatively interacting 12-mer allosteric units. In contrast to the isolated tetramers, all subunits of the tetramers seem to be coupled cooperatively within the oligomerized 144-mer. Thus, besides hemocyanins and GroEL, the hexagonal bilayer hemoglobins represent another class of proteins in which the hierarchical quaternary structure provides the basis for nested interaction in their functional properties. |
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