glyc•er•ol, ˈglɪs əˌrɔl
A sugar alcohol made up of two polyols derived from saponification of fats and oils, and functions chiefly as a metabolic intermediate and a structural component of the major classes of biological lipids, triglycerides and phosphatidyl phospholipids
Glycerol is one of the sugar alcohols. Sugar alcohols belong to a class of polyols characterized by being white, water-soluble, organic compounds with a general chemical formula of (CHOH)nH2. Sugar alcohols may be produced by the hydrogenation of sugars.
History and terminology
Glycerol was discovered in 1779 by the Swedish chemist Carl Wilhelm Sheele 1742-1786. He obtained glycerol when the glycerol was washed out of a heated mixture of lead oxide and olive oil. In olive oil, glycerol is the predominant triglyceride.1 Its name glycerol was coined by the French chemist Michel Eugéne Chevreul 1786–1889. Etymologically, glycerol came from the Greek glycos, meaning “sweet”. In 1836, the French chemist Théophile-Jules Pelouze 1807-1867 determined its chemical formula (C3H8O3). In 1872, it was first synthesized inside a laboratory by the French chemist Charles Friedel 1832–1899.2 Today, glycerol is artificially synthesized for its various uses in food, medicine, and other industries. As a food additive, glycerol has been approved as Generally Recognized As Safe (GRAS) by US Food and Drug Administration (FDA).3 Glycerol is also called glycerine (or glycerin). Nevertheless, the term “glycerol” is often used to indicate the presence of the compound as an ingredient of a product whereas “glycerine” (or glycerin) often pertains to the product name. For instance, the glycerin syrup is 99.7% glycerol. 3
Glycerol is a colorless, odorless, viscous, sweet-tasting polyol with a chemical formula of C3H8O3. It is a trihydric alcohol since it is comprised of three carbon atoms; each of the two end carbon atoms is bound to two hydrogen atoms and a hydroxyl group; the central carbon atom is bound to a hydrogen atom and a hydroxyl group. 1 This structure makes glycerol highly hygroscopic (readily attracts moisture) and soluble in water and in alcohol. Its melting point is 18°C. Its boiling point is 290 °C. It is less sweet than sucrose, i.e. 75% sweetness relative to sucrose.
Glycerol occurs naturally. One of the ways to biosynthesize glycerol is by removing the phosphate group from glycerol phosphate through the catalytic action of the enzyme phosphatase. Glycerol may also be derived from hydrolyzing fats.
Lipogenesis is the process of producing lipid or fat. It is carried out by esterification, which is a chemical reaction involving an alcohol and an acid that form an ester (the reaction product). Glycerides (also called acylglycerols) are esters formed from glycerol and fatty acid. Glycerol with its three hydroxyl groups can be esterified with up to three fatty acids. Depending on the number of fatty acids bound to the glycerol, a glyceride may be a monoglyceride (also called monoacylglycerol), a diglyceride (also called diacylglycerol), or a triglyceride (also called triacylglycerol). A monoglyceride forms from the condensation of a glycerol and one fatty acid joined via an ester bond. A diglyceride forms from the condensation of two fatty acids and a glycerol whereas a triglyceride, from three fatty acids and a glycerol. In triglyceride synthesis, three fatty acids are esterified to a glycerol in the endoplasmic reticulum. The cells that carry out lipogenesis are mostly adipocytes and hepatocytes.
Glycerol may be used in the biosynthesis of glycerol-3-phosphate (Gro3P). Gro3P is a phosphoric ester of glycerol. It forms by phosphorylating glycerol via the enzyme glycerol kinase. The enzyme catalyzes the transfer of a phosphate from ATP to glycerol to form Gro3P. Gro3P may then enter triglyceride (triacylglyceride) biosynthesis, phospholipid biosynthesis, glycolysis, and gluconeogenesis.
In triacylglyceride biosynthesis and phospholipid biosynthesis, glycerol acts as the structural backbone from where the fatty acids are bound to. In triacylglyceride synthesis, in particular, the carboxyl end of each of the three fatty acids reacts with each of the hydroxyl group of the glycerol. Their binding liberates a molecule of water per fatty acid (thus, releasing a total of three water molecules in the process).
Phospholipids serve as a major structural component of many biological membranes. Some of them function as second messengers in signal transduction. They are amphipathic compounds, meaning they have a hydrophilic head and a hydrophobic tail. In essence, the head is comprised of a phosphate group whereas the tail is comprised of two fatty acids. A glycerol joins the head and the tail of the phospholipid. In particular, the phosphate group is attached to one of the three carbons of the glycerol backbone whereas the remaining two carbons are bound to two fatty acid chains (mostly a saturated fatty acid on C-1 and an unsaturated fatty acid on C-2). The phosphate may further be bound to: hydrogen, choline, serine, ethanolamine, inositol, etc. The hydrophilic component determines the type of phospholipid: phosphatidic acid, phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, phosphatidylinositol, etc., respectively. Each of these phospholipids goes through a particular biosynthetic pathway. The phosphatidic acid is the most fundamental of these phospholipids as it serves as the precursor to many phospholipids. The biosynthesis of phospholipids usually starts at Gro3P.
In the glycolytic pathway, the glycerol may enter the pathway directly or indirectly, depending on the organism involved. In humans, glycerol has to be converted first prior to entering glycolysis. In particular, glycerol is converted into Gro3P by glycerol kinase. Gro3P is converted into dihydroxyacetone phosphate (DHAP) via the enzymatic activity of glycerol-3-phosphate dehydrogenase and the reduction of NAD+ to NADH. DHAP is converted into glyceraldehyde-3-phosphate via the action of the enzyme triosephosphate isomerase. The conversion of glycerol into glyceraldehyde 3-phosphate also occurs before entering gluconeogenesis. Gluconeogenesis is a metabolic pathway where glucose is formed from non-carbohydrate precursors, e.g. glycerol, lactate, pyruvate. It is one of the ways by which the body maintains blood glucose levels.
Nutritive glycerol is generally present in plant oils and animal fats. It may also be produced artificially for use as a food humectant, thickener, solvent, sweetener (e.g. fondant, jams, processed food products, energy bars, etc.). 3 Although glycerol is a type of sugar alcohol, it does not cause laxative effects as others do (e.g. sorbitol, mannitol, isomalt). That is because glycerol is fully absorbed in the small intestine. Digestion of a fatty food involves the action of lipases and bile. Lipids are hydrolyzed and broken down into fragments, such as monoglycerides, diglycerides, glycerol, and free fatty acids. These fragments are absorbed by (diffuse into) the intestinal cells (enterocytes) where they will be reverted into triglycerides to form chylomicrons. Chylomicrons are special particles formed in the endoplasmic reticulum of the enterocytes. They contain triglycerides (as the main component), cholesterol, and fat-soluble vitamins. At the basolateral surface of the enterocytes, chylomicrons are released by exocytosis. Because of the large size of chylomicrons, they are transported via the intestinal lymphatic capillaries called lacteals instead of through the small capillaries. Chylomicrons are collected in the lymphatic system, and then get mixed with the blood when they reach the large vessels near the heart that drain into the general circulation. Lipids are, therefore, transported from enterocytes to the bloodstream by way of these chylomicrons.4 Triglycerides, though, do not readily pass through the cell membranes of the cell. Thus, special enzymes in the walls of the blood vessels called lipoprotein lipases hydrolyze triglycerides (and other lipoproteins) in the chylomicrons into free fatty acids and glycerol. The free fatty acids and glycerol can then be absorbed by the cells (e.g. of adipose tissues, skeletal and cardiac muscle tissues) by way of the so-called fatty acid transproter. Remnants of the chylomicrons are taken up by the liver. Hepatocytes and adipocytes store triglycerides as energy fuel by lipogenesis.
The hormone glucagon triggers the liver to break down triglycerides to release fatty acids by way of the lipases. The glycerol component that is released from the process may serve as an alternative source of glucose especially when glucose level is low. It can be converted into glucose by way of gluconeogenesis and then it may also enter the glycolytic pathway. This is particularly important when there is not enough glucose for the brain to use aside from the fact that the brain cannot directly use fatty acid as an energy source.
Lipolysis is the process of breaking down lipids by hydrolyzing triglycerides into glycerol and free fatty acids. This occurs mainly in the adipose tissue and often as a response during intense exercise and fasting. Lipases become phosphorylated and activated. In particular, adipose triglyceride lipase catalyzes the hydrolysis of triacylglycerol to diacyglycerol. The conversion of diacylglycerol to monoacylglycerol is through the catalytic action of the hormone-sensitive lipase. Monoacylglycerol lipase, in turn, catalyzes the hydrolysis of monoacylglycerol to glycerol. The liberated fatty acids are released into the bloodstream. Hormone-sensitive lipases are regulated by the hormones insulin, glucagon, epinephrine, and norepinephrine.
Glycerol is an essential sugar alcohol for many living things. For one, it is a component of lipids, such as glycerides and phospholipids. Along with the fatty acids, glycerol forms glycerides that could serve as an energy fuel. Triglycerides, for instance, is a major component of animal fats and vegetable oils. Glycerol also serves as one of the substrates for the synthesis of glycerol-3-phosphate that could enter triglyceride biosynthesis, phospholipid biosynthesis, glycolysis, and gluconeogenesis. Phospholipids are one of the main structural components of biological membranes. They may also act as second messengers in signal transduction. There are various types of phospholipids, phosphatidic acid, phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, phosphatidylinositol, etc., each involved in various metabolic activities. Glycerol may be converted into glucose, the major metabolite of glycolysis, which is a metabolic pathway through which energy (ATP) is synthesized. This energy drives the various metabolic activities of a cell. When there is not enough glucose, glycerol is a major glucose precursor in gluconeogenesis. Unlike fatty acids, glycerol is more readily absorbed particularly by the brain cells. The brain cells can use glycerol-turned-glucose for glycolysis when glucose is insufficient.
Glycerol may be synthesized naturally or may be derived by consuming glycerol-containing fatty foods. It is also produced chemically by saponification or by the action of superheated steam for use as food sweetener, humectant, thickener, and emulsifier. Among the sugar alcohols, glycerol is classified as a caloric macronutrient by the FDA. Glycerol provides 4.3 kilocalories per gram.
- 1,2,3-trihydroxy-propane or propan-1,2,3-triol
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