title: Nested allosteric interactions in extracellular hemoglobin of the leech Macrobdella decora.
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.
authors.affiliation: Institute for Molecular Biophysics, University of Mainz, Jakob-Welder-Weg 26, 55128 Mainz, Germany. nadja@biophysik.biologie.uni-mainz.de
authors.forename: Nadja
authors.initials: N
authors.lastname: Hellmann
authors.forename: Roy E
authors.initials: RE
authors.lastname: Weber
authors.forename: Heinz
authors.initials: H
authors.lastname: Decker
mesh_terms: D000818:Animals; D001665:Binding Sites; D006454:Hemoglobins / Q000737:chemistry* / Q000378:metabolism; D007865:Leeches / Q000737:chemistry*; D046911:Macromolecular Substances; D010100:Oxygen / Q000378:metabolism; D011487:Protein Conformation; D020836:Protein Structure, Quaternary; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism
chemical_list: D006454:Hemoglobins; D046911:Macromolecular Substances; D021122:Protein Subunits; D010100:Oxygen