Biological Membranes - A. N. Ogurtsov 2012

Structure and Functions of Biomembranes
Membrane Lipids
Classification of Lipids

Although organic chemistry classifies Fatty acids and their derivatives as Lipids, Biochemistry and Other biological fields also include hydrophobic or amphiphilic substances of different chemical nature in this category.

Above all, lipids include products of the interaction of fatty acids with alcohols (Simple Lipids), amino alcohols, and Other Compounds (Complex Lipids), as well as Prostaglandins and isoprenoid lipids (e.g., carotenoids, chlorophyll, Vitamins E and K).

Triacylglycerols are simple lipids that are esters of the trihydric alcohol glycerol and three fatty acids (Figure 15(a)). They vary in type depending on The Nature of the three fatty acid residues attached to the hydroxyl groups of glycerol.

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Figure 15 - Schemes of biological tri- and diglycerides: a - triacylglycerol; b - diacylglycerol

A typical membrane consists of phosphoglycerides, Sphingolipids, and Steroids. These three classes of lipid molecules are amphiphilic, featuring a polar (hydrophilic) HEAD and a hydrophobic (hydrocarbon) tail.

The hydrophobic effect and Structure/103.html">Van der Waals forces drive the self-assembly of Cell/29.html">The Lipid Bilayer in such a way that the hydrocarbon tails are sequestered within the interior of the membrane, while the hydrophilic heads are immersed in the surrounding aqueous environment (Figure 5).

However, while all Membrane Lipids share a similar amphiphilic nature, different classes vary in their chemical structure, Abundance, and Membrane Functions.

Phosphoglycerides. Phosphoglycerides are the most abundant class of lipids in most membranes; they are derivatives of glycerol-3-phosphate (Figure 16).

Figure 16 - Structure of phosphoglycerides

A typical phosphoglyceride molecule consists of two fatty acid chains attached to the hydroxyl groups of glycerol-3-phosphate, and a polar head attached to the phosphate group.

The two fatty acid tails can differ in the number of units in the hydrocarbon chain and in the number of saturated and unsaturated C-C and C=C bonds (Table 1).

Phosphoglycerides are classified according to The properties of their polar heads. In phosphatidylcholines (designated as PC in Figure 16), which are the most common Phospholipids in cell Plasma Membranes, the polar head consists of Choline—a positively charged alcohol attached to a negatively charged phosphate.

In other phosphoglycerides, molecules containing either hydroxyl groups or positive charges (like choline) are attached to the phosphate.

Ethanolamine, Serine, and Inositol are most commonly used for this purpose (Figure 16). The corresponding phospholipids are designated as:

✵ phosphatidylethanolamine (PE);

✵ phosphatidylserine (PS);

✵ phosphatidylinositol (PI).

The polar heads of phosphoglycerides, featuring a negatively charged phosphate group and positively charged or hydroxyl groups, interact extensively with polar Water molecules.

Plasmalogens are a group of phospholipids in which one of the fatty acid chains is attached to glycerol via an ester bond (just as in regular phosphoglycerides), while the other hydrocarbon chain is attached via an ether bond (C-O-C) (Figure 17(a)).

Plasmalogens account for about 20% of phosphoglycerides in The Human Body, being particularly abundant in Brain and Heart Cells. It is possible that the enhanced chemical Stability of the ether bond in plasmalogens, or minor differences in their Spatial Structure compared to other phosphoglycerides, holds a specific physiological significance that has not yet been established.

Sphingolipids. The second class of membrane lipids consists of sphingolipids. All these compounds are derivatives of sphingosine (Figure 17(b))—an amino alcohol with a long fatty acid tail—and contain a second fatty acid tail attached to the amino group of sphingosine (Figure 18). N-acyl fatty acid derivatives of sphingosine are called ceramides (Figure 17(b)).

Figure 17 - Lipid structure: a - plasmalogen; b - sphingosine; c - ceramide; R - fatty acid tails; X - polar head

In sphingomyelin (sphingomyelin, SM) — the most abundant sphingolipid — phosphocholine is attached to the hydroxyl group of sphingosine (Figure 18). Therefore, The structure of sphingomyelin is analogous to that of phosphatidylcholine, and sphingomyelin can also be classified among phospholipids.

Another type of sphingolipids is represented by Glycolipids, in which The Role of the polar head is played by either a monosaccharide or a branched oligosaccharide. For example, glucosylcerebroside (glucosyl-cerebroside, GlcCer), the simplest glycosphingolipid, contains a glucose monosaccharide attached to sphingosine. In complex glycosphingolipids, called gangliosides, one or two branched Oligosaccharides containing N-acetylneuraminic (sialic) acid residues are attached to sphingosine.

Glycolipids account for 2–10% of the total lipids in The Plasma Membrane. Their highest concentration is found in Nervous Tissue.

Figure 18 - Structure of sphingolipids

Steroids. Cholesterol and its derivatives form the third class of membrane lipids: steroids. The core of steroids is a hydrocarbon structure consisting of four rings. Cholesterol, the major sterol component of animal cells, has a hydroxyl group at the end of one of the rings (Figure 19).

Cholesterol is particularly abundant in the plasma membranes of mammalian cells, whereas it is entirely absent in most Prokaryotic Cells. In plants, up to 30–50% of lipids are specific plant-cell steroids.

At neutral pH, phosphoglycerides (such as phosphatidylcholine and phosphatidylethanolamine) are uncharged, whereas others (such as phosphatidylinositol and phosphatidylserine) are negatively charged. Despite this, the polar heads of all phospholipids orient themselves to form a lipid bilayer.

Figure 19 - Cholesterol: a - structural formula; b - van der Waals molecular model

Sphingomyelins are structurally similar to phosphoglycerides and assemble into a mixed bilayer. Cholesterol and other steroids are too hydrophobic to form a bilayer structure on their own, but when mixed with phospholipids, they intercalate into bilayer membranes.