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Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002.
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Lipids differ markedly from the other teams of biomolecules taken into consideration for this reason far. By interpretation, lipids are water-insoluble biomolecules that are extremely soluble in organic solvents such as chlorodevelop. Lipids have a range of organic roles: they serve as fuel molecules, very focused energy stores, signal molecules, and components of membranes. The initially 3 functions of lipids will certainly be debated in later on chapters. Here, our emphasis is on lipids as membrane constituents. The three major kinds of membrane lipids are phospho-lipids, glycolipids, and cholesterol. We start through lipids discovered in eukaryotes and also bacteria. The lipids in archaea are distinct, although they have many kind of features concerned their membrane-creating attribute in widespread via lipids of various other organisms.
12.3.1. Phospholipids Are the Major Class of Membrane Lipids
Phospholipids are abundant in all organic membranes. A phospholipid molecule is built from four components: fatty acids, a platform to which the fatty acids are attached, a phosphate, and also an alcohol attached to the phosphate (Figure 12.3). The fatty acid components administer a hydrophobic obstacle, whereas the remainder of the molecule has hydrophilic properties to allow interactivity with the atmosphere.
The platcreate on which phospholipids are developed might be glycerol, a 3- carbon alcohol, or sphingosine, a much more complex alcohol. Phospholipids obtained from glycerol are dubbed phosphoglycerides. A phosphoglyceride is composed of a glycerol backbone to which two fatty acid chains (whose attributes were described in Section 12.2.2) and also a phosphorylated alcohol are attached.
In phosphoglycerides, the hydroxyl groups at C-1 and also C-2 of glycerol are esterified to the carboxyl groups of the two fatty acid chains. The C-3 hydroxyl team of the glycerol backbone is esterified to phosphoric acid. When no even more enhancements are made, the resulting compound is phosphati-date (diacylglycerol 3-phosphate), the most basic phosphoglyceride. Only little quantities of phosphatiday are present in membranes. However, the molecule is an essential intermediate in the biosynthesis of the other phosphoglycerides (Section 26.1). The absolute configuration of the glycerol 3-phosphate moiety of membrane lipids is shown in Figure 12.4.
Structure of Phosphatiday (Diacylglycerol 3-Phosphate). The absolute configuration of the center carbon (C-2) is shown.
The major phosphoglycerides are obtained from phosphatiday by the development of an ester bond between the phosphate group of phosphatiday and also the hydroxyl team of among a number of alcohols. The prevalent alcohol moieties of phosphoglycerides are the amino acid serine, ethanolamine, choline, glycerol, and also the inositol.
The structural formulas of phosphatidyl choline and the other primary phosphoglycerides—namely, phosphatidyl ethanolamine, phosphatidyl serine, phosphatidyl inositol, and diphosphatidyl glycerol—are given in Figure 12.5.
Sphingomyelin is a phospholipid found in membranes that is not acquired from glycerol. Instead, the backbone in sphingomyelin is sphingosine, an amino alcohol that consists of a lengthy, unsaturated hydrocarbon chain (Figure 12.6). In sphingomyelin, the amino team of the sphingosine backbone is attached to a fatty acid by an amide bond. In addition, the primary hydroxyl group of sphingosine is esterified to phosphoryl choline.
Structures of Sphingosine and Sphingomyelin. The sphingosine moiety of sphingomyelin is highlighted in blue.
12.3.2. Archaeal Membranes Are Built from Ether Lipids through Branched Chains
The membranes of archaea differ in composition from those of eukaryotes or bacteria in three essential ways. Two of these distinctions plainly relate to the hostile living conditions of many archaea (Figure 12.7). First, the nonpolar chains are joined to a glycerol backbone by ether quite than ester linkperiods. The ether linkage is more resistant to hydrolysis. Second, the alkyl chains are branched rather than straight. They are built up from repeats of a completely saturated five-carbon fragment. These branched, saturated hydrocarbons are even more resistant to oxidation. The capacity of archaeal lipids to stand up to hydrolysis and oxidation may aid these organisms to withstand also the too much conditions, such as high temperature, low pH, or high salt concentration, under which some of these archaea grow. Finally, the stereochemisattempt of the central glycerol is inverted compared via that displayed in Figure 12.4.
An Archaeon and also Its Environment. Archaea can thrive in habitats as harsh as a volcanic vent. Here, the archaea form an oarray mat surrounded by yellow sulfurous deposits.
12.3.3. Membrane Lipids Can Also Include Carbohydprice Moieties
Glycolipids, as their name implies, are sugar-containing lipids. Like sphingomyelin, the glycolipids in animal cells are derived from sphingosine. The amino group of the sphingosine backbone is acylated by a fatty acid, as in sphingomyelin. Glycolipids differ from sphingomyelin in the identification of the unit that is linked to the major hydroxyl group of the sphingosine backbone. In glycolipids, one or more sugars (quite than phosphoryl choline) are attached to this team. The easiest glycolipid, dubbed a cerebroside, contains a single sugar residue, either glucose or galactose.
More complex glycolipids, such as gangliosides, contain a branched chain of as many type of as seven sugar residues. Glycolipids are oriented in a completely asymmetric fashion via the sugar residues always on the extracellular side of the membrane.
12.3.4. Cholesterol Is a Lipid Based on a Steroid Nucleus
Cholesterol is a lipid through a framework quite different from that of phospholipids. It is a steroid, constructed from four linked hydrocarbon rings.
A hydrocarbon tail is connected to the steroid at one finish, and a hydroxyl team is attached at the other end. In membranes, the molecule is oriented parallel to the fatty acid chains of the phospholipids, and the hydroxyl group interacts through the adjacent phospholipid head groups. Cholesterol is absent from prokaryotes however is discovered to varying degrees in basically all animal membranes. It constitutes nearly 25% of the membrane lipids in certain nerve cells yet is fundamentally lacking from some intracellular membranes.
12.3.5. A Membrane Lipid Is an Amphipathic Molecule Containing a Hydrophilic and a Hydrophobic Moiety
The arsenal of membrane lipids is extensive, perhaps also bewildering, at first sight. However before, they possess an essential widespread structural theme: membrane lipids are amphipathic molecules (amphiphilic molecules). A membrane lipid consists of both a hydrophilic and also a hydrophobic moiety.
Let us look at a model of a phosphoglyceride, such as phosphatidyl choline. Its in its entirety shape is about rectangular (Figure 12.8A). The 2 hydrophobic fatty acid chains are approximately parallel to each other, whereas the hydrophilic phosphoryl choline moiety points in the opposite direction. Sphingomyelin has actually a comparable conformation, as does the archaeal lipid illustrated. Therefore, the following shorthand also has been embraced to represent these membrane lipids: the hydrophilic unit, also called the polar head team, is represented by a circle, whereas the hydrocarbon tails are portrayed by straight or wavy lines (Figure 12.8B).
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Representations of Membrane Lipids. (A) Space-filling models of a phosphoglyceride, sphingomyelin, and an archaeal lipid present their shapes and distribution of hydrophilic and also hydrophobic moieties. (B) A shorthand also depiction of a membrane lipid.
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