Biomembranes sterols

Tricyclohexaprenol, a possible forerunner of sterols in the evolution of biomembranes, was synthesized by construction of the cyclic network in one step using cation-olefin tricyclization and subsequent stereocontroUed attachment of the Cio appendage to ring C. 

Rohmer, M., P. Bouvier, and G. Ourisson. 1979. Molecular evolution of biomembranes structural equivalents and phylogenetic precursors of sterols. Proc. Natl. Acad. Sci. USA 76 847-851. 

Even though cholesterol (substituted by other sterols in plants) is chemically classified as a lipid, the molecule is better grouped it into a special category, which is distinct from membrane proteins and (phospho- and sphingo-) lipids, when discussing its role in biomembrane stmcture and function. Cholesterol constitutes between 25% and 40% of the total lipid plus cholesterol fraction of most typical cell membranes. These numbers translate into a 33-66% fraction of the noncholesterol lipids, or considering its smaller size, about a 10-20%... 

Cholesterol is the main sterol in tissues of animal origin tissues. Plant cells or bacteria on the other hand do not contain cholesterol in their biomembranes. However, they produce other sterols such as sitosterol, stigmasterol, campesterol etc, while lanosterol and ergosterol are predominant in bacteria. Human skin contains derivatives of cho e.stcrol and cholesterol sulfate has been identified as the main cholesterol... 

The influence of plant sterols on the phase properties of phospholipid bilayers has been studied by differential scanning calorimetry and X-ray diffraction [206]. It is interesting that the phase transition of dipalmitoylglycerophosphocholine was eliminated by plant sterols at a concentration of about 33 mole%, as found for cholesterol in animal cell membranes. However, less effective modulation of lipid bilayer permeability by plant sterols as compared with cholesterol has been reported. The molecular evolution of biomembranes has received some consideration [207-209]. In his speculation on the evolution of sterols, Bloch [207] has suggested that in the prebiotic atmosphere chemical evolution of the sterol pathway if it did indeed occur, must have stopped at the stage of squalene because of lack of molecular oxygen, an obligatory electron acceptor in the biosynthetic pathway of sterols . Thus, cholesterol is absent from anaerobic bacteria (procaryotes). 

A cell cannot divide or enlarge unless it makes sufficient amounts of additional membranes to accommodate the expanded area of its outer surface and internal organelles. Thus the generation of new cell membranes is as fundamentally important to the life of a cell as is protein synthesis or DNA replication. Although the protein components of biomembranes are critical to their biological functions, the basic structural and physical properties of membranes are determined by their lipid components—principally phospholipids, sphingolipids, and sterols such as cholesterol (Table 18-1). Cells must be able to synthesize or import these molecules to form membranes. 

A biological membrane is a structure particularly suitable for study by the LB technique. The eukaryotic cell membrane is a barrier that serves as a highway and controls the transfer of important molecules in and out of the cell (Roth etal., 2000). The cell membrane consists of a bilayer or a two-layer LB film (Tien etal, 1998). Lipid bilayers are composed of a variety of amphiphilic molecules, mainly phospholipids and sterols which in turn consist of a hydrophobic tail, and a hydrophilic headgroup. The complexity of the biomembrane is such that frequently simpler systems are used as models for physical investigations. They are based on the spontaneous self-organization of the amphiphilic lipid molecules when brought in contact with an aqueous medium. The three most frequently used model systems are monolayers, black lipid membranes, and vesicles or liposomes. 

Classification and Structure M.l. can be classified structurally in several different ways The present classification (Fig.l) divides them into phospholipids, glycolipids and Sterols (see). The phospholipids are divided into glycerophospholipids and sphingophos-pholipids, the ycolipids into glyceroglycolipids and sphingoglycolipids each of these subclasses is then divided into subsubclasses Phospholipids and glycolipids can be described as acylated-M.I. because they have at least one fatty acyl residue (R.C = O where R is a hydrocarbon chain) in their structure in this they differ from sterols which occur in biomembranes in non-acylated form. 

Fig. 10. Membrane lipids. Structure of the main membrane sterols and their orientation and dimensions in a monolayer of the lipid bilayer of a biomembrane.

Among the diverse components of biomembranes, lipids are essential in structural aspects. Lipids are defined operationally as derivatives of fatty acids and their metabolites. As lipids are usually amphiphilic molecules with hydrophobic hydrocarbon tails and hydrophilic head groups (as shown in Figure 1), the bilayer structures of biomembranes are held with hydrophobic forces to the tails and heads of lipids. Another major component of biomembranes is proteins. The weight proportion of proteins in biomembranes is often more than that of lipids. As proteins are more rigid than lipid assemblies, specific interactions by biomembranes are often related to proteins. Further, the sterols contained in biomembranes play unique roles in their apolar regions. As the functionality of biomembranes arises from the diversity of their composition, various kinds of molecules have been studied for biomembrane modeling. 

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