BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 13. STRUCTURE AND PROPERTIES OF LIPIDS

13.4. Sphingolipids

Sphingolipids represent the second major Class of Lipids that serve as structural components in both animal and human Cell membranes.

Chemically, sphingolipids are complex esters of the aliphatic unsaturated amino alcohol sphingosine or its saturated analog, dihydrosphingosine:

Sphingosine belongs to a group of compounds known as sphingoid bases, which are characterized by the presence of an amino group, two hydroxyl groups, and a trans-configured double bond (at C₄).

The structural backbone of sphingolipids is formed by ceramides—N-acyl derivatives of sphingosine, in which acyl radicals are linked to the amino alcohol via an amide bond:

The Nature of the substituent on the primary alcohol group determines the division of sphingolipids into two main groups: phosphorus-containing sphingolipids and glycosphingolipids.

The first group includes sphingomyelins, which are ceramide derivatives representing complex esters of sphingosine with phosphorylcholine or phosphorylethanolamine:

Sphingomyelins are relatively simple and most abundant sphingolipids in animal tissues. They are particularly abundant and diverse in the myelin sheaths of Nervous Tissue.

Due to the presence of phosphorus, a molecular charge, and other specific features such as amphiphilic properties, sphingomyelins can be classified among Phospholipids.

Glycosphingolipids comprise more complex molecules that are also ceramide derivatives; they lack phosphoryl residues and net charge, but contain polar sugar residues.

This complex group of lipids is subdivided into cerebrosides and gangliosides, which are found in the Plasma Membranes of various Cells.

Cerebrosides belong to Glycolipids present in nervous tissue, particularly the Brain, from which they were first isolated. Cerebroside molecules consist of sphingosine, a long-chain fatty acid, and a carbohydrate moiety that can be a monosaccharide (galactose, less frequently glucose) or an oligosaccharide containing residues of galactose, glucose, and acetylated galactosamines and glucosamines. Characteristic specific acyl components of cerebroside molecules include lignoceric, nervonic, and cerebronic acids, all containing 24 carbon atoms.

Cerebrosides of brain tissue notably include cerasin:

The BIOLOGICAL Functions OF cerebrosides are determined by The properties of the carbohydrate moiety of their molecules, which participates in Molecular recognition Processes and cell-cell interactions.

Some cerebrosides are sulfated and are therefore referred to as sulfatides or acidic cerebrosides. The sulfuric acid residues in these molecules are typically linked to hexose residues via the hydroxyl group at carbon-3:

Chemically, gangliosides share structural similarities with cerebrosides. They represent a large group of compounds comprising various sphingosine bases, typically stearic acid residues, and a large polar carbohydrate moiety composed of several hexose residues, amino sugars, and sialic acids.

Gangliosides are predominantly found in the Gray matter of the brain, within the plasma membranes of neural and glial cells (localized mainly on the outer membrane surface), and are also present in the Mitochondria and nuclei of these cells.

Based on the number of sialic acid residues per molecule, gangliosides (G) are classified into mono- (G_M), di- (G_D), tri- (G_T), quadra- (G_Q), penta- (G_P), and other sialagangliosides, for example:

monosialoganglioside — G_M1 — has the Structure

where N-АНК is an N-acetylneuraminic (sialic) acid residue;

disialoganglioside — Gd₁ —

trisialoganglioside — Gt₁ —