Elongation mechanisms

We turn now to the biosynthesis of lipid structures. We begin with a discussion of the biosynthesis of fatty acids, stressing the basic pathways, additional means of elongation, mechanisms for the introduction of double bonds, and regulation of fatty acid synthesis. Sections then follow on the biosynthesis of glyc-erophospholipids, sphingolipids, eicosanoids, and cholesterol. The transport of lipids through the body in lipoprotein complexes is described, and the chapter closes with discussions of the biosynthesis of bile salts and steroid hormones. 

Beryllium is a light metal (s.g. 1 -85) with a hexagonal close-packed structure (axial ratio 1 568). The most notable of its mechanical properties is its low ductility at room temperature. Deformation at room temperature is restricted to slip on the basal plane, which takes place only to a very limited extent. Consequently, at room temperature beryllium is by normal standards a brittle metal, exhibiting only about 2 to 4% tensile elongation. Mechanical deformation increases this by the development of preferred orientation, but only in the direction of working and at the expense of ductility in other directions. Ductility also increases very markedly at temperatures above about 300°C with alternative slip on the 1010 prismatic planes. In consequence, all mechanical working of beryllium is carried out at elevated temperatures. It has not yet been resolved whether the brittleness of beryllium is fundamental or results from small amounts of impurities. Beryllium is a very poor solvent for other metals and, to date, it has not been possible to overcome the brittleness problem by alloying. 

This is the most widely used naturally occurring rubber. The literature search shows that many research groups have prepared nanocomposites based on this rubber [29-32]. Varghese and Karger-Kocsis have prepared natural rubber (NR)-based nanocomposites by melt-intercalation method, which is very useful for practical application. In their study, they have found increase in stiffness, elongation, mechanical strength, and storage modulus. Various minerals like MMT, bentonite, and hectorite have been used. 

The ultimate evidence for the nucleation-elongation mechanism is the effect of imbalanced stoichiometry on the MW of the polymerization. In standard step-growth polymerizations the MW is dependent upon the ratio of the monomer concentrations where almost exact 1 1 stoichiometry is necessary to bring about high MW [90]. An irreversible imine condensation of 67 and 68 was performed under imbalanced stoichiometry ([67]/[68]=0.5) and nonfolding conditions (Fig. 40) [93]. The starting material was completely consumed and... 

In the synthesis of fatty acids the acetyl irnits are condensed and then are reduced to form straight hydrocarbon chains. In the oxo-acid chain elongation mechanism, the acetyl unit is introduced but is later decarboxylated. Tlius, the chain is increased in length by one carbon atom at a time. These two mechanisms account for a great deal of the biosynthesis by chain extension. However, there are other variations. For example, glycine (a carboxylated methylamine), under the influence of pyridoxal phosphate and with accompanying decarboxylation, condenses with succinyl-CoA (Eq. 14-32) to extend the carbon chain and at the same time to introduce an amino group. Likewise, serine (a carboxylated ethanolamine) condenses with... 

Sasaki, N., and Odajima, S. (1996). Elongation mechanism of collagen fibrils and force-strain relations of tendon at each level of structural hierarchy. J. Biomech. 29, 1131-1136. 

After the induction time, chain scissioning became uninhibited and was manifested by loss of elongation. Mechanisms of chain scissioning and stabilization are discussed. 

The most plausible elongation mechanism, however, seems to be the transacylation as described in Figure 14.12, in which the growing polymer chain does not leave the serine during the elongation process (step IV in Figure 14.9). The... 

Liu, J. and Doetsch, P.W. (1996) Template strand gap bypass is a general property of prokaryotic RNA polymerases implications for elongation mechanisms. Biochemistry, 35, 14999-15008. 

Figure 8.11 Proposed chain-elongation mechanism via the formation of racemic antiparallel (ap) 6-sheets comprising alternating oligo-R and oligo-S chains, both with the enantiopure initiator S-Phe-OMe, ft, at their C terminus, as modeled on the basis of...

The collaborative work with Bristol University is continuing, and it is planned in the future to study the isotope ratios of the component sterols and tocopherols, and whether there is any difference in the isotope ratios of an acid located at the triglyceride 2-position in comparison with the same acid at the 1- and 3-positions. It may also be possible to develop views on whether there is any isotopic bias on the desaturation and/or chain-elongation mechanisms involved in the biosynthesis of palmitic, oleic and linoleic acids. 

Incubations of cell suspension cultures of Idesia polycarpa and of seed tissue ofH. anthelminthic a with [l,2- C]a-ketopimelate proved the validity of the Cl chain elongation mechanism. Although in the systems tested this substrate was metabolized to some extent, cyclopentenylglycine was synthesized at a much higher rate from a-ketopimelate than from acetate. In a control experiment, cell suspension cultures of soya were incubated with [l,2- C]a-ketopimelate or [l- C]acetate, but cyclopentenylglycine was not synthesized in either case (Tober and Spener, 1980). 

Itoh, Y. Matsumura, T. Umemoto, S. Okui, N. Sakai, T. Contraction/elongation mechanism of acrylonitrile gel fibers. Polymer Preprints -Japan (Enghsh Editions), 36(5-10) E184-E186 (1987)... 

Y. Itoh, T. Matsamura, S. Umemoto, N. Okui, and T. Sakai, Contraction/elongation mechanism of acrylonitrile gel fibers, Polym. Prepr. J6 E184 (1987). 

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