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FIGURE 10-10 Three glycolipids of chloroplast membranes. In monogalactosyldiacylglycerols (MGDGs) and digalactosyldiacylglycerols (DGDGs), almost all the acyl groups are derived from linoleic acid (18:2(A9,12)) and the head groups are uncharged. In the sulfolipid 6-sulfo-6-deoxy-ff-d-glucopyranosyldiacylglycerol, the sulfonate carries a fixed negative charge.

6-Sulfo-6-deoxy-a-D-glucopyranosyldiacylglycerol (a sulfolipid)

6-Sulfo-6-deoxy-a-D-glucopyranosyldiacylglycerol (a sulfolipid)

FIGURE 10-10 Three glycolipids of chloroplast membranes. In monogalactosyldiacylglycerols (MGDGs) and digalactosyldiacylglycerols (DGDGs), almost all the acyl groups are derived from linoleic acid (18:2(A9,12)) and the head groups are uncharged. In the sulfolipid 6-sulfo-6-deoxy-ff-d-glucopyranosyldiacylglycerol, the sulfonate carries a fixed negative charge.

Archaebacteria Contain Unique Membrane Lipids

The archaebacteria, most of which live in ecological niches with extreme conditions—high temperatures (boiling water), low pH, high ionic strength, for example—have membrane lipids containing long-chain (32 carbons) branched hydrocarbons linked at each end to glycerol (Fig. 10-11). These linkages are through ether bonds, which are much more stable to hydrolysis at low pH and high temperature than are the ester bonds found in the lipids of eubacteria and eukaryotes. In their fully extended form, these archaebacterial lipids are twice the length of phospholipids and sphingolipids and span the width of the surface membrane. At each end of the extended molecule is a polar head consisting of glycerol linked to either phosphate or sugar residues. The general name for these compounds, glycerol dialkyl glycerol tetraethers (GDGTs), reflects their unique structure. The glycerol moiety of the archaebacterial lipids is not the same stereoisomer as that in the lipids of eubacteria and eukaryotes; the central carbon is in the R configuration in archaebacteria, in the S configuration in the other kingdoms (Fig. 10-7).

Sphingolipids Are Derivatives of Sphingosine

Sphingolipids, the fourth large class of membrane lipids, also have a polar head group and two nonpolar tails, but unlike glycerophospholipids and galactolipids they contain no glycerol. Sphingolipids are composed of one molecule of the long-chain amino alcohol sphingo-sine (also called 4-sphingenine) or one of its derivatives, one molecule of a long-chain fatty acid, and a polar head group that is joined by a glycosidic linkage in some cases and by a phosphodiester in others (Fig. 10-12).

Carbons C-1, C-2, and C-3 of the sphingosine molecule are structurally analogous to the three carbons of glycerol in glycerophospholipids. When a fatty acid is attached in amide linkage to the —NH2 on C-2, the resulting compound is a ceramide, which is structurally similar to a diacylglycerol. Ceramide is the structural parent of all sphingolipids.

There are three subclasses of sphingolipids, all derivatives of ceramide but differing in their head groups: sphingomyelins, neutral (uncharged) glycolipids, and gangliosides. Sphingomyelins contain phosphocholine or phosphoethanolamine as their polar head group and are therefore classified along with glycerophospholipids as phospholipids (Fig. 10-6). Indeed, sphingomyelins resemble phosphatidylcholines in their general properties and three-dimensional structure, and in having no net charge on their head groups (Fig. 10-13). Sphingo-myelins are present in the plasma membranes of animal cells and are especially prominent in myelin, a membranous sheath that surrounds and insulates the axons of some neurons—thus the name "sphingomyelins."

Glycosphingolipids, which occur largely in the outer face of plasma membranes, have head groups with one or more sugars connected directly to the —OH at C-1 of the ceramide moiety; they do not contain phosphate. Cerebrosides have a single sugar linked to ceramide; those with galactose are characteristically found in the plasma membranes of cells in neural tissue, and those with glucose in the plasma membranes of cells in nonneural tissues. Globosides are neutral (uncharged) glycosphingolipids with two or more sugars, usually D-glucose, D-galactose, or V-acetyl-D-galactosamine. Cere-brosides and globosides are sometimes called neutral glycolipids, as they have no charge at pH 7.

Gangliosides, the most complex sphingolipids, have oligosaccharides as their polar head groups and one or more residues of V-acetylneuraminic acid (Neu5Ac), a sialic acid (often simply called "sialic acid"), at the

Glycerol phosphate

Glycerol c h2c

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