The deletion of two residues in the central helix of CaM does not distort or cause a bending of the linker alpha helix. The main consequence of the mutation is a change in the relative orientation of the two globular calcium-binding domains, causing the hydrophobic patches in these domains to be closer and much less accessible to interact with the target enzymes. This may explain why this mutant of CaM shows a marked decrease in its ability to activate some enzymes while the mutation has little or no effect on its ability to activate others.
The crystal structure of a mutant calmodulin (CaM) lacking Glu-84 has been refined to R = 0.23 using data measured to 2.9-A resolution. In native CaM the central helix is fully extended, and the molecule is dumbbell shaped. In contrast, the deletion of Glu-84 causes a bend of 950 in the linker region of the central helix at Ile-85. However, EF-hand domains 1 and 2 (lobe 1,2) do not touch lobe 3,4. The length, by a-carbon separation, of des-Glu5"-CaM is 56
In order to evaluate fully the meaning of small-angle X-ray diffraction data from collagen fibers in terms of the distribution of molecular substance along fibrillar axes, it is necessary to have some means of determining the phase angles of the several components of the axial diffraction series for combination with measured amplitudes in the formulation of a Fourier series expressing the fibrillar electron density profile. This investigation has developed strip models for fibrillar axial structure based on reported electron micrographic descriptions of how stainable bands and molecular overlap zones ("backgrounds") are located along the fibrils. These models permit the calculation of phases for use with the experimental amplitudes. Once band descriptions (identical widths and density heights plus relative locations) were fixed, three parameters dealing with background width, height, and location were varied to refine the models until they were reasonably capable of accounting theoretically for the observed diffraction amplitudes. Further minor adjustments, indicated by the initial results, finally produced models and profiles for dry and moist kangaroo tail tendon (KTT). The results show that the X-ray and electron optical conclusions regarding collagen fibrillar axial structure are in essential agreement down to a resolution of about 45 A.
Earlier small-angle X-ray diffraction studies have indicated that central and peripheral nerve myelins may be significantly different structurally, although relatively few examples for each system and for individual species have been examined. In order to understand better the intra- and inter-system relationships, this study has developed more extensive information for a single species: six cases centrally and ten peripherally, featuring cranial nerves and a few others of the human nervous system. Peripheral myelin spacings (membrane pair thicknesses) are relatively similar, 184.4 +/- 1.40 A, and the ratios of diffraction peak height intensities of the second to fourth orders are also closely bunched: 1.85 +/- 0.216. Central myelin spacings and intensity ratios are distinctly different and more variable: spacings 160.3-165.8 A and intensity ratios 2.81-4.46. It appears that within a given species or between closely related (e.g., mammalian) species peripheral myelins possess relatively invariant structures, though significant spacing declines are encountered for both systems as phylogenetic relationships become more distant. The observed greater variability of CNS structures within a single species may correspond to known compositional differences between CNS regions or result from observational difficulties. In any case there is a marked discontinuity between the myelin structures of CNS and PNS nerves.
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