How are lipids saponified



Lipids (from Greek λίπος lípos "Fat") is a collective term for completely or at least largely water-insoluble (hydrophobic) natural substances, which, on the other hand, dissolve very well in hydrophobic or lipophilic solvents such as hexane due to their low polarity. Their insolubility in water is mainly due to the long hydrocarbon residues that most lipids have.

In living organisms, lipids are mainly used as structural components in cell membranes, as energy stores or as signal molecules. Most biological lipids are amphiphilic, i.e. they have a lipophilic hydrocarbon residue and a polar hydrophilic head group, which is why they form micelles or membranes in polar solvents such as water. Often the term fat Used as a synonym for lipids, but fats represent only a subgroup of lipids (namely the group of triglycerides).

The lipids can be divided into seven groups: fatty acids, triacylglycerides (fats and oils), waxes, phospholipids, sphingolipids, lipopolysaccharides and isoprenoids (steroids, carotenoids, etc.)

Fatty acids, triacylglycerides (fats and fatty oils) and waxes

The triacylglycerols (triglycerides) make up the main part of the dietary lipids with> 90%. They are an important supplier of energy (1 g of fat contains 39 kJ of energy, 1 g of sugar only 17 kJ). In addition, triglycerides form the body's most important energy store (sugar, i.e. glucose, is stored in much smaller quantities in the liver than glycogen), they are good protection against the cold in the skin and also protect it from injuries. All important organs are protected by a layer of fat.

Fatty acids

Fatty acids are mostly unbranched monocarboxylic acids that consist of a chain of carbon atoms with a carboxyl group at one end (see picture).

A distinction is made between saturated fatty acids, in which there are no double bonds, and unsaturated fatty acids, which have one or more double bonds (in nature usually in the cis position and not in conjugation with one another). The simplest saturated fatty acid is butyric acid and contains only four carbon atoms. Important representatives of the unsaturated fatty acids are oleic acid (monounsaturated) and arachidonic acid (fourfold unsaturated). The more double bonds a fatty acid contains, the lower its melting point. The animal organism can only synthesize unsaturated fatty acids with restrictions. All those fatty acids that have to be consumed with food are therefore called 'essential fatty acids' (see below).

Triacylglycerides

As mentioned above, triacylglycerides (generally: fats and fatty oils) represent the largest group of dietary lipids. Both fats and oils are triple esters (esters are compounds according to scheme R1-CO-O-R2) of glycerol and are called Triacylglycerides designated. If triacylglycerides are split by saponification, glycerol and the corresponding fatty acid salts are formed.

There are two types of triacylglycerides, simple and mixed: With simple triacylglycerides the side chains (i.e. the fatty acid residues) are identical, with mixed ones they are different.

The reason why fat is solid and oil is liquid is the significantly higher proportion of unsaturated fatty acids in oils. The unsaturated fatty acids mostly possess cis-Double bonds, which makes crystal formation difficult and thus lowers the melting point.

If the side chains R1 and R3 different, a chiral molecule is present and an optical activity can be observed (i.e. the mirror image of the molecule is not congruent with the original and a solution of the molecule is able to rotate incident polarized light).

Waxes

Waxes are single esters of fatty acids and as such differ from the triple esters of fats and oils. Both the acid and alcohol parts of waxes have long saturated alkyl groups. In contrast to triglycerides, waxes are less “oily” and also harder and more porous.

Another definition (German Society for Fat Science) sees waxes as a class of substances that is defined exclusively by their mechanical-physical properties. According to this definition, waxes are kneadable at 20 ° C, solid to brittle and hard, they have a coarse to finely crystalline structure, in terms of color they are translucent to opaque (opaque), but not glass-like, above 40 ° C they melt without decomposition, a little above At their melting point they are slightly liquid (not very viscous), have a strongly temperature-dependent consistency and solubility and can be polished under slight pressure.

Membrane-forming lipids

Membrane-forming lipids are lipids that have a hydrophilic and a hydrophobic part - i.e. they are amphiphilic. This allows them to form either micelles (spherical aggregates of amphiphilic molecules that spontaneously assemble in a dispersion medium) or double lipid layers in polar solvents such as water - the hydrophilic part always interacting with the polar solvent. All biomembranes are built up from these double lipid layers, which demarcate the content of a cell from the environment and thus make membrane-forming lipids one of the basic requirements of all life.

Phospholipids

Phospholipids form the main component of biomembranes. A distinction is made between phosphoglycerides and sphingomyelins. The structure of the phosphoglycerides is derived from the phosphatidic acid, which is similar to the triglycerides, with the difference that there is a phosphoryl group on the C3 hydroxyl group instead of the acyl residue. Sphingomyelins, on the other hand, differ from glycerolipids in their sphingosine basic structure. The phosphoric acid diester group of all phospholipids is hydrophilic (i.e. interacts with water) and is "head" called. The acyl residues or the non-polar part of sphingosine are called "tail“And are hydrophobic. This opposing character leads to the formation of lipid bilayers in which the hydrophobic part of the membrane lipids point inwards and the hydrophilic part outwards. The most important phospholipids involved in the construction of biomembranes are the phosphoglycerides phosphatidylcholine (also lecithin), phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine and also sphingomyelins. The latter are both phospho- and sphingolipids. Phosphatidylethanolamine and phosphatidylserine are also known as cephalins. An important group of phosphoglycerides, especially in the intracellular transmission of extracellular signals (signal transduction), are the phosphatidylinositols, which occur in various stages of phosphorylation; as a head group they have a phosphoinositol.

Sphingolipids

Sphingolipids are also part of cell membranes. Their basic structure consists of a fatty acid and sphingosine. They are divided into the groups of ceramides, sphingomyelins and glycolipids. Sphingolipids are found in nerve tissue, they play an important role in signal transmission and the interaction of individual cells.

Glycolipids

Glycolipids are phosphate-free, sphingosine-containing lipids with a carbohydrate component glycosidically bound to the 1-hydroxyl group of the sphingosine. They often form the outside of biological membranes, with their carbohydrate content being presented on the cell membrane. It is believed that these play a role in the communication and interaction between individual cells. Glycolipids are divided into cerebrosides, gangliosides and sulfatides.

Isoprenoids

Isoprenoids (also terpenoids) are compounds that are based on isoprene units. Compounds that count among the lipids are the steroids and the carotenoids. Naturally occurring steroids belong to the triterpenoid derivatives (triterpenoid means it consists of 30 carbon atoms), since they are all biosynthesized from squalene. Carotenoids are counted among the tetraterpenoid derivatives (40 carbon atoms), they are derived from lycopene.

Steroids

All steroids have as a basic structure a system of four, usually trans-linked carbon rings, three hexagonal and one pentagonal. The best-known representative of steroids is cholesterol, which is one of the sterols. Among other things, it is also an essential component of all cell membranes with the exception of the inner membrane of the mitochondria, and can therefore also be counted among the membrane lipids in the broader sense.

Bile acids, which are involved in fat digestion, have a hydrophobic and a hydrophilic part, so they can coat fats and thus facilitate their absorption in the digestive tract.

Sex hormones are steroids produced in the ovaries and testes that control reproduction and the formation of secondary sexual characteristics. The female sex hormones are progesterone and estrogen, the male androgens (e.g. testosterone and androsterone).

Further examples are the sterols and sterol esters phytosterol, ergosterol, vitamin D and cardiac glycosides (e.g. digitalis and strophantine).

Carotenoids

Carotenoids are polymerization products of isoprene, which are produced exclusively in plants and function there as yellow to reddish coloring agents. They usually consist of unsaturated hydrocarbon chains and their oxidation products, and are made up of eight isoprene units. They are divided into carotenes and xanthophylls. The best known and most common carotenoid is β-carotene, also known as provitamin A. It is converted in the body into retinol (vitamin A), which plays an important role in the visual process.

Biological functions

The biological functions of lipids are as diverse as their chemical structure. They serve as

  • Fuel (β-oxidation of fatty acids)
  • Energy storage (triacylglycerols)
  • Membrane building blocks (phospholipids)
  • Signal molecules (diacylglycerol; IP3-Cascade)
  • Hormones (eicosanoids; prostaglandins etc.)
  • Fat-soluble vitamins (vitamins A, D, E, K)
  • Cofactors (Dolichol)
  • Pigments (carotenoids)

While some lipids can be produced by the human body in the fat metabolism itself, others have to be ingested with food. Therefore, these are called essential lipids designated.

Essential fatty acids

Double bonds in the hydrocarbon chain of a fatty acid that are more than nine carbon atoms away from the carboxyl group cannot be produced independently by the organism. However, these are important and must therefore be ingested through food, which is why they are called essential. The representatives of the essential omega-3 fatty acids include linolenic acid, eicosapentaenoic acid and docosahexaenoic acid. Essential omega-6 fatty acids are linoleic acid and arachidonic acid. Eicosanoids are synthesized from arachidonic acid; these are important tissue hormones and mediators in the body. Omega-9 fatty acids are not essential as they can be synthesized from omega-3 and omega-6 fatty acids. Possible sources of omega-3 and omega-6 fatty acids in food are fish, flaxseed, soybean oil, hemp oil, pumpkin seeds or walnuts.

Essential fatty acids play an important role in many metabolic processes, and there is evidence that deficiencies or imbalances in the absorption of essential fatty acids are the cause of numerous diseases.

Fat-soluble vitamins

The fat-soluble vitamins are:

  • Vitamin A, a terpene that plays an important role in the visual process on the one hand, and in the growth, function and structure of the skin and mucous membranes on the other,
  • Vitamin D, responsible for regulating the calcium and phosphorus concentrations in the blood and thus of crucial importance for bone stability
  • Vitamin E, a terpenoid with antioxidant effects and
  • Vitamin K, a terpenoid that helps blood clot.

literature

  • Georg Löffler, Petro E. Petrides: Biochemistry and pathobiochemistry. Springer, Berlin 2003, ISBN 3-540-42295-1
  • Florian Horn, Isabelle Moc, Nadine Schneider: Human biochemistry. Thieme, Stuttgart 2005, ISBN 3-13-130883-4
  • Charles E. Mortimer, Ulrich Müller: Chemistry. Thieme, Stuttgart 2003, ISBN 3-13-484308-0
  • Jeremy M. Berg, John L. Tymoczko, Lubert Stryer: "Biochemistry" 2002. Chapter 12 (available online)

Categories: Fabric Group | Lipid