Structural Lipids in Membranes:- 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 sphingosine (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 ONH₂ 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 proper ties and three-dimensional structure, and in having no net charge on their head groups (Fig. 10–13). Sphingomyelins 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 N-acetyl-D-galactosamine. Cerebrosides 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 N-acetylneuraminic acid (Neu5Ac), a sialic acid (often simply called “sialic acid”), at the termini. Sialic acid gives gangliosides the negative charge at pH=7 that distinguishes them from globosides. Gangliosides with one sialic acid residue are in the GM (M for mono-) series, those with two are in the GD (D for di-) series, and so on (GT, three sialic acid residues; GQ, four).

FIGURE 10–11 A typical membrane lipid of archaebacteria. In this diphytanyl tetraether lipid, the diphytanyl moieties (yellow) are long hydrocarbons composed of eight five-carbon isoprene groups condensed end-to-end (on the condensation of isoprene units, see Fig. 21–36; also, compare the diphytanyl groups with the 20-carbon phytol side chain of chlorophylls in Fig. 19–40a). In this extended form, the diphytanyl groups are about twice the length of a 16-carbon fatty acid typically found in the membrane lipids of eubacteria and eukaryotes. The glycerol moieties in the archaebacterial lipids are in the R configuration, in contrast to those of eubacteria and eukaryotes, which have the S configuration. Archaebacterial lipids differ in the substituents on the glycerols. In the molecule shown here, one glycerol is linked to the disaccharide -glucopyranosyl-(1→2)-β-galactofuranose; the other glycerol is linked to a glycerol phosphate head group.

FIGURE 10–12 Sphingolipids. The first three carbons at the polar end of sphingosine are analogous to the three carbons of glycerol in glycerophospholipids. The amino group at C-2 bears a fatty acid in amide linkage. The fatty acid is usually saturated or monounsaturated, with 16, 18, 22, or 24 carbon atoms. Ceramide is the parent compound Gal GalNAc for this group. Other sphingolipids differ in the polar head group (X) attached at C-1. Gangliosides have very complex oligosaccharide head groups. Standard abbreviations for sugars are used in this figure: Glc, D-glucose; Gal, D-galactose; GalNAc, N-acetyl-D-galactosamine; Neu5Ac, N-acetylneuraminic acid (sialic acid).

اخر الاخبار

اخبار العتبة العباسية المقدسة

جامعة العميد تعلن توفر وظائف شاغرة

سماحة السيد الصافي يؤكد دعم العتبة العباسية المقدسة لأي نشاط يرتقي بمستوى الرعاية الصحية المقدمة للمواطنين والزائرين

قسم بين الحرمين الشريفين ينفّذ حملةً لغسل مناهل مياه الشرب

المجمع العلمي يقيم دورة قرآنية خاصة بأحكام التلاوة في صلاح الدين

المزيد

الآخبار الصحية

ما هي أفضل العصائر لصحة القلب وتقوية المناعة والنوم العميق؟

الأطعمة الغنية بالبروتينات النباتية تقلل خطر الأمراض المزمنة

وجع الأوتار قد يخفي خطرا صحيا مرتبطا بالقلب

أزمة "السفينة الموبوءة"..الأرجنتين تحقق في مصدر تفشي "هانتا"

المزيد

الآخبار التكنلوجية

باحثون يحددون تأثير القهوة على الميكروبيوم والمزاج

روسيا.. ابتكار تقنية لمعالجة سبائك التيتانيوم لحماية الطائرات والسيارات

بكين وواشنطن تدرسان إطلاق محادثات رسمية حول الذكاء الاصطناعي

لأول مرة في الولايات المتحدة.. تشغيل مفاعل مصغر نووي يغذي الذكاء الاصطناعي بالطاقة

المزيد

مواضيع ذات صلة

Lipids as Signals, Cofactors, and Pigments: -Plants Use Phosphatidylinositols, Steroids, and Eicosanoidlike Compounds in Signaling

Lipids as Signals, Cofactors, and Pigments:- Steroid Hormones Carry Messages between Tissues

Lipids as Signals, Cofactors, and Pigments:- Phosphatidylinositols and Sphingosine Derivatives Act as Intracellular Signals

Structural Lipids in Membranes:- Sterols Have Four Fused Carbon Rings

Structural Lipids in Membranes:- Sphingolipids at Cell Surfaces Are Sites of Biological Recognition

Structural Lipids in Membranes:- Chloroplasts Contain Galactolipids and Sulfolipids

Structural Lipids in Membranes:- Some Phospholipids Have Ether-Linked Fatty Acids

Structural Lipids in Membranes:- Glycerophospholipids Are Derivatives of Phosphatidic Acid

Storage Lipids:- Many Foods Contain Triacylglycerols

Storage Lipids:- Triacylglycerols Provide Stored Energy and Insulation

Storage Lipids:- Triacylglycerols Are Fatty Acid Esters of Glycerol

Storage Lipids:- Triacylglycerols Are Fatty Acid Esters of Glycerol