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Hopanoids are pentacyclic triterpenoid lipids occurring in bacteria. They are synthesized from isopentenyl units which are
formed in a new biosynthetic route leading to isopentenyl diphosphate. Six C5 units are joined to form squalene, the immediate precursor in hopanoid synthesis. In a highly complex cyclization reaction
that shares considerable similarities with that of oxidosqualene to sterols, the hopane skeleton is formed from squalene by
the squalene-hopene cyclase. Recent elucidation of the X-ray structure of this membrane-bound cyclase has shed some light
on the properties of this unusual enzyme. The active site is located in a cavity within the enzyme. The squalene substrate
diffuses through a channel structure from the membrane into this cavity and is there transformed into hopene. Polar side chains
are attached to hopene resulting in the amphiphilic molecular structure of many hopanoids. These hopanoids are membrane components
involved in regulating membrane fluidity and stability. However, the many structural variants of hopanoids indicate that they
may have other interesting but as yet unknown functions. 相似文献
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Wolf D. Keidel 《Die Naturwissenschaften》1992,79(8):347-357
Teil I s. Heft 7, S. 300 相似文献
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Although the benefits of dam construction are numerous, particularly in the context of climate change and growing global demand for electricity, recent experience has shown that many dams have serious negative environmental, human, and political consequences. Despite an extensive literature documenting the benefits and costs of dams from a single disciplinary perspective, few studies have simultaneously evaluated the distribution of biophysical, socio-economic, and geopolitical implications of dams. To meet the simultaneous demands for water, energy, and environmental protection well into the future, a broader view of dams is needed. We thus propose a new tool for evaluating the relative costs and benefits of dam construction based on multi-objective planning techniques. The Integrative Dam Assessment Modeling (IDAM) tool is designed to integrate biophysical, socio-economic, and geopolitical perspectives into a single cost/benefit analysis of dam construction. Each of 27 different impacts of dam construction is evaluated both objectively (e.g., flood protection, as measured by RYI years) and subjectively (i.e., the valuation of said flood protection) by a team of decision-makers. By providing a visual representation of the various costs and benefits associated with two or more dams, the IDAM tool allows decision-makers to evaluate alternatives and to articulate priorities associated with a dam project, making the decision process about dams more informed and more transparent. For all of these reasons, we believe that the IDAM tool represents an important evolutionary step in dam evaluation. 相似文献