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Molecular Association (molecular + association)

Distribution by Scientific Domains

Chemistry	100%

Selected Abstracts

Structural and compositional changes in very low density lipoprotein triacylglycerols during basal lipolysis

FEBS JOURNAL, Issue 24 2002
Jyrki J. Ågren
Triacylglycerols secreted by liver and carried by very low density lipoprotein (VLDL) are hydrolysed in circulation by lipoprotein and hepatic lipases. These enzymes have been shown to have positional and fatty acid specificity in vitro. If there were specificity in basal lipolysis in vivo, triacylglycerol compositions of circulating and newly secreted VLDL would be different. To study this we compared the composition of normal fasting VLDL triacylglycerol of Wistar rats to that obtained after blocking lipolysis by Triton WR1339, which increased plasma VLDL triacylglycerol concentration about 4.7-fold in 2 h. Analyses of molecular species of sn -1,2- and sn -2,3-diacylglycerol moieties and stereospecific triacylglycerol analysis revealed major differences between the groups in the VLDL triacylglycerol composition. In nontreated rats, the proportion of 16:0 was higher and that of 18:2n-6 lower in the sn -1 position. The proportion of 14:0 was lower in all positions and that of 18:0 was lower in the sn -1 and sn -3 positions in nontreated rats whereas the proportions of 20:4n-6, 20:5n-3, 22:5n-3 and 22:6n-3 were higher in the sn -1 and lower in the sn -2 position. These results suggest that the fatty acid of the sn -1 position is the most decisive factor in determining the sensitivity for hydrolysis of the triacylglycerol. In addition, triacylglycerol species with highly unsaturated fatty acids in the sn -2 position also favoured hydrolysis. The in vivo substrate specificity followed only partly that obtained in in vitro studies indicating that the nature of molecular association of fatty acids in natural triacylglycerol affects its susceptibility to lipolysis. To conclude, our results indicate that preferential basal lipolysis leads to major structural differences between circulating and newly secreted VLDL triacylglycerol. These differences extend beyond those anticipated from analysis of total fatty acids and constitute a previously unrecognized feature of VLDL triacylglycerol metabolism. [source]

Features of homotetrameric molecular association in protein crystals

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 1 2009
Uma V. Katre
The crystal structures of proteins showing homotetrameric association, a common feature observed in many lectins, have been analyzed in order to understand the characteristics of tetrameric association in terms of the arrangement of subunits and their biological significance. The analysis could group the tetramer units into the following four categories. (i) Tetrahedral molecules, in which the four monomers form a nearly perfect tetrahedral arrangement. The angle between the axes of any two monomers is ,109°. (ii) Molecules that form a sandwiched dimer of dimers in which the two dimers are arranged perpendicular to each other, one upon the other. (iii) Planar molecules, in which the four monomers lie in one plane and the corresponding sides of adjacent monomers face in opposite directions. This can be considered as a flattened tetrahedral shape. (iv) Planar closed molecules, in which all four monomers lie in one plane arranged in a head-to-tail fashion in a square. The first group and its variant, the third group, are the most commonly found arrangements in crystal structures. Each arrangement has its own importance for biological function. Some tetrameric assemblies that deviate from the majority described above also have relevance to their biological function. [source]

Crystallization pathways and kinetics of carbamazepine,nicotinamide cocrystals from the amorphous state by in situ thermomicroscopy, spectroscopy, and calorimetry studies

JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 5 2007
K. Seefeldt
Abstract The work presented here was motivated by the premise that the amorphous state serves as a medium to study cocrystal formation. The molecular mobility inherent to amorphous phases can lead to molecular associations between different components such that a single crystalline phase of multiple components or cocrystal is formed. Cocrystallization pathways and kinetics were investigated from amorphous equimolar phases of carbamazepine and nicotinamide using hot-stage polarized microscopy (HSPM), hot-stage Raman microscopy (HSRM), differential scanning calorimetry (DSC), and X-ray powder diffraction (XRPD). Nonisothermal studies revealed that amorphous phases generate cocrystals and that thermal history affects crystallization pathways in significant ways. Two different pathways to cocrystal formation from the amorphous phase were identified: (1) at low heating rates (3°C/min) a metastable cocrystalline phase initially nucleates and transforms to the more stable cocrystalline phase of CBZ,NCT, and (2) at higher heating rates (10°C/min) individual components crystallize, then melt and the stable cocrystalline phase nucleates and grows from the melt. Isothermal studies above the Tg of the amorphous equimolar phase also confirm the nucleation of a metastable cocrystalline phase from the amorphous state followed by a solid phase mediated transformation to the stable cocrystalline phase. Cocrystallization kinetics were measured by image analysis and by thermal analysis from small samples and are described by the Avrami,Erofeev model. These findings have important implications for the use of amorphous phases in the discovery of cocrystals and to determine the propensity of cocrystallization from process-induced amorphization. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 96: 1147,1158, 2007 [source]

T cell adhesion mechanisms revealed by receptor lateral mobility,

BIOPOLYMERS, Issue 5 2008
Christopher W. Cairo
Cell surface receptors mediate the exchange of information between cells and their environment. In the case of adhesion receptors, the spatial distribution and molecular associations of the receptors are critical to their function. Therefore, understanding the mechanisms regulating the distribution and binding associations of these molecules is necessary to understand their functional regulation. Experiments characterizing the lateral mobility of adhesion receptors have revealed a set of common mechanisms that control receptor function and thus cellular behavior. The T cell provides one of the most dynamic examples of cellular adhesion. An individual T cell makes innumerable intercellular contacts with antigen presenting cells, the vascular endothelium, and many other cell types. We review here the mechanisms that regulate T cell adhesion receptor lateral mobility as a window into the molecular regulation of these systems, and we present a general framework for understanding the principles and mechanisms that are likely to be common among these and other cellular adhesion systems. We suggest that receptor lateral mobility is regulated via four major mechanisms,reorganization, recruitment, dispersion, and anchoring,and we review specific examples of T cell adhesion receptor systems that utilize one or more of these mechanisms. © 2007 Wiley Periodicals, Inc. Biopolymers 89: 409,419, 2008. This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com [source]