BIOCHEMISTRY - Textbook - L. I. Ostapchenko - 2012

Chapter 13. STRUCTURE AND PROPERTIES OF LIPIDS

13.3. Phospholipids

Phospholipids represent a major Class of Membrane Lipids. In the Cells and Tissues of Bacteria, plants, and animals, they act alongside other lipids and Proteins to form Introduction/36.html">Biological Membranes and support their proper functioning.

Phospholipids, or phosphoacylglycerols, are glycerol derivatives containing two hydrophobic higher fatty acid residues and a hydrophilic component—residues of phosphoric acid, amino alcohols, Amino Acids, or glycerides, with the general formula:

where R and R' are hydrophobic radicals, and X is a hydrophilic substituent.

In phospholipid molecules, the hydroxyl groups of glycerol at positions 1 and 2 are esterified with Fatty acids, while the hydroxyl at position 3 is esterified with phosphoric acid. Diacylglycerol-3-phosphate, or phosphatidic acid (phosphatidate), serves as a key metabolic intermediate in the Synthesis of Other phosphoacylglycerols; thus, phospholipids are derivatives of phosphatidic acid formed through the Esterification of phosphatidate with the hydroxyls of amino alcohols, amino acids, glycerol, or Inositol (Table 13.2).

The presence of polar groups within the molecular Structure imparts amphipathic properties to phospholipids, specifically their ability to interact with electrolyte solutions.

Table 13.2

Water/126.html">Diversity of the Chemical Structure of Phospholipids

The Characteristic Properties of phosphoacylglycerols manifest upon their interaction with water. At the water-air interface, they form a monomolecular layer in which their polar "heads" are immersed in water, while their hydrophobic "tails" point toward the air. As the concentration of phospholipids In aqueous solutions increases, they form spherical structures known as micelles, which can further aggregate into cylindrical structures (Fig. 13.2, after R.P. Yevstigneyeva, 1976).

In certain properties, phosphoacylglycerols are identical to neutral lipids: they also exhibit several polymorphic forms, the transitions between which under physiological conditions are driven by Temperature changes. Upon heating, phosphoacylglycerols melt, passing through a liquid-crystalline phase. The temperature of phospholipid phase transitions depends on The Nature and degree of unsaturation of the hydrophobic radicals in their molecules: more unsaturated phospholipids have lower phase transition temperatures.

Fig. 13.2. Behavior of phospholipids in aqueous solutions

The Classification of phospholipids is based on the Structural Features of their hydrophilic substituent X.

Phosphatidylcholines are the most abundant among phosphoacylglycerols, accounting for up to 45–50% of total cellular lipid content in various tissues. These complex esters of the amino alcohol Choline and phosphatidic acid share the general formula:

и

where R and R' represent saturated and unsaturated hydrophobic radicals of palmitic, stearic, oleic, linoleic, or arachidonic acids, respectively.

Phosphatidylethanolamines, which can also be viewed as esters of ethanolamine and phosphatidate, constitute 30–40% of all phosphoacylglycerols in the cells of various organisms:

Phosphatidylserines consistently account for approximately 5% of the phosphoacylglycerols found in PLANT AND ANIMAL

tissues. These are acidic phospholipids due to the overall negative charge of the molecule:

Polyglycerophosphatides are phosphoacylglycerols whose molecules contain multiple glycerol residues. These include phosphatidylglycerols and cardiolipins. The latter are characteristic of mitochondrial membranes, where they are present in relatively large amounts:

Plasmalogen (aldelehydogenic) phospholipids are quite widespread in nature, with their content in animal tissues ranging from 25 to 50% of the total phospholipids. They are mainly represented by phosphatidalcholines and phosphatidylethanolamines, whose molecules contain an alkenyl ether group (-OCH=CH-):

Phosphoinositides are derivatives of phosphatidic acid and the cyclic alcohol inositol (myo-inositol), which can be phosphorylated. Therefore, depending on the number of phosphoric acid residues in the molecule, they are classified into mono- and polyphosphoinositides.

Polyphosphoinositides are found in the Tissues of the Central Nervous system in animals.