Browsing by keyword "Oligochaeta"
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Crystallographic Studies of Rrythrocruorin from Lumbricus Terrestris: a DissertationThe viability of multicellular aerobic organisms requires the binding and transport of molecular oxygen from the atmosphere to sites of metabolism. In the earthworm, Lumbricus terrestris, erythrocruorins are freely dissolved multi-subunit protein complexes that serve the same functions as red blood cells The aims of this study were to 1) determine the overall arrangement of hemoglobin chains and non-hemoglobin chains in Lumbricus erythrocruorin, 2) determine the stereochemical determinants specifying erythrocruorin's hierarchical symmetry, and 3) investigate the molecular and chemical basis for the remarkable cooperative binding of ligands to earthworm hemoglobin. Erythrocruorin is a highly cooperative oxygen-carrying protein with Hill coefficients measured at some pH's as high as n = 7.9. Crystallographic analysis of the whole erythrocruorin molecule structure to 5.5 Å resolution reveals a hierarchical organization of 144 oxygen-binding polypeptides and 36 non-hemoglobin linker polypeptide chains. The hemoglobin chains are arranged in a novel dodecameric substructure at the periphery of the complex, whereas 36 linker chains comprise the inner core and projected triple-stranded, helical coiled-coil spokes towards the center of the complex. Interdigitation of these spokes appears crucial for stabilizing the complex. Crystallographic analysis of crystals from isolated hemoglobin chains provides greater detail (resolution = 2.6 Å) and complete atomic models for the hemoglobin polypeptides. Comparison of these models with other hemoglobins reveal unique features in the distal heme pocket, including large aromatic residues at the B10 position in three of the four hemoglobin chains. Aromatic residues at this position have been implicated in other hemoglobins to confer resistance to oxidation. Molecular interactions across each subunit include pH-dependent interactions that are consistent with the observed Bohr effect on oxygen binding. Specifically a π-cation interaction between an arginine of one subunit to a histidine of the opposing subunit is likely an important molecular switch in the allosteric transition from a low to high affInity ligand-binding state.
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Low resolution crystal structure of Arenicola erythrocruorin: influence of coiled coils on the architecture of a megadalton respiratory proteinAnnelid erythrocruorins are extracellular respiratory complexes assembled from 180 subunits into hexagonal bilayers. Cryo-electron microscopic experiments have identified two different architectural classes. In one, designated type I, the vertices of the two hexagonal layers are partially staggered, with one hexagonal layer rotated by about 16 degrees relative to the other layer, whereas in the other class, termed type II, the vertices are essentially eclipsed. We report here the first crystal structure of a type II erythrocruorin, that from Arenicola marina, at 6.2 A resolution. The structure reveals the presence of long continuous triple-stranded coiled-coil "spokes" projecting towards the molecular center from each one-twelfth unit; interdigitation of these spokes provides the only contacts between the two hexagonal layers of the complex. This arrangement contrasts with that of a type I erythrocruorin from Lumbricus terrestris in which the spokes are broken into two triple-stranded coiled coils with a disjointed connection. The disjointed connection allows formation of a more compact structure in the type I architecture, with the two hexagonal layers closer together and additional extensive contacts between the layers. Comparison of sequences of the coiled-coil regions of various linker subunits shows that the linker subunits from type II erythrocruorins possess continuous heptad repeats, whereas a sequence gap places these repeats out of register in the type I linker subunits, consistent with a disjointed coiled-coil arrangement.