Amphipathic molecules sterols

Introduction - Liposomes are vesicles composed of one or more lipid bilayers completely surrounding an internal aqueous space. They are usually composed of phospholipids either in pure form or In combination with other amphipathic molecules such as sterols, long chain bases or acids, or membrane proteins. The structure of liposomes varies from large (0.5->5y) multllamellar vesicles to small ( 300 A) unilamellar vesicles.2,3 More recently, new methods have been reported describing the formation of unilamellar vesicles of intermediate size. >5.6 xhe general properties of liposomes and their interaction with various macromolecules have been described in several reviews. ... 

Fig. 1.4 (a) Generalized membrane structure. Hydrophobic heads of amphipathic molecules are shown in black and hydrophilic tails as wavy lines. Membrane reinforcers are shown as boxes and can be likened to nails spanning half the membrane (e.g.sterols and hopanoids) or rivets spanning the entire membrane (e.g. carotenoids). Where they are absent, in archaebacterial thermophiles, the amphipathic molecules span the entire membrane and act as struts , (b) Possible steps in the self-assembly of primitive membranes (M2+ are metal ions). 

Table 6.2). Almost all membrane lipids are amphipathic molecules (section 6.5) such as phospholipids or glycolipids neutral lipids such as triacylglycerols have little part in membrane structure. In organisms which make or utilize sterols, these tend to be concentrated in the external surrounding membrane (compare chloroplast or mitochondria with erythrocyte or myelin membranes in Table 6.2). 

Cell membranes are bilayers of amphipathic acids, for example phospholipids and sterols, which contain globular proteins. The structure is governed by the essential requirement for stability in an aqueous environment, that is, the hydrophobic tails of the lipid molecules point towards each other, leaving the outer surfaces composed of polar, hydrophilic groups. 

Many biomolecules are amphipathic proteins, pigments, certain vitamins, and the sterols and phospholipids of membranes all have polar and nonpolar surface regions. Structures composed of these molecules are stabilized by hydrophobic interactions among the non-... 

Cholesterol ((3(3)-cholest-5-en-3-ol) is a major non-phospholipid component of animal membranes and is the principal sterol of animals. Cholesterol is also amphipathic, the 3-hydroxy being polar and the rest of the molecule hydrophobic. Cholesterol can insert into phospholipid bilayers, lowering membrane permeability and lowering the melting point of membranes (i.e. making the membranes less ordered and more fluid). 

The most recent definition of lipids was provided by a group of lipid chemists who formed the consortium of lipid metabolites and pathways strategy (Lipid MAPS). They defined lipids based on the origin of the lipid structures as hydrophobic or amphipathic small molecules that may originate entirely or, in part, by carbanion-based condensations of thioesters (fatty acids, polyketides, etc.) and/or by carbocation-based condensations of isoprene units (prenols, sterols, etc.). In this book, this definition, its classification (see the following), and its recommended nomenclature are largely accepted. 

Ergosterol is the primary sterol of the yeast plasmic membrane. In lesser quantities, 24 (28) dehy-droergosterol and zymosterol also exist (Figure 1.7). Sterols are exclusively produced in the mitochondria during the yeast log phase. As with phospholipids, membrane sterols are amphipathic. The hydrophilic part is made up of hydroxyl groups in C-3. The rest of the molecule is hydrophobic, especially the flexible hydrocarbon tail. 

Simple micelles, composed only of bile salts, have a limited capacity to solubilize cholesterol. With taurocholate for example, one molecule of cholesterol is solubilized by 25 molecules of bile salt, while addition of a monoglyceride to the preparation essentially doubles the solubilization of cholesterol[17]. Obviously, the participation of additional amphipaths and amphiphiles, such as phospholipids and fatty acids, in the micellar structure causes dramatic increases in sterol solubility (from 100-1000 fold)[48]. 

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