Biosynthesis of Lipids VII

Suaib Luqman

epgp books

 

 

1.    Objectives

 

v  To understand the biosynthesis of Glycerol

v   What are the significance of their synthesis

 

 

2.  Concept Map

 

  1. Description

 

Glycerol also known as glycerin or glycerine, is a sugar alcohol (simple polyol) compound. It is viscous, odorless and colorless liquid, extensively used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids known as triglycerides. Glycerol is sweet-tasting and is non-toxic.

 

Production

 

 

Glycerol can be either synthetic, or derived from plants (usually soybeans or palm) or animals (usually tallow). It can also be a blend of both animal and vegetable oils. Approximately 950,000 tons per year are produced in the United States and Europe; 350,000 tons of glycerol were produced per year in the United States alone from 2000 to 2004.[5] Production will increase as the EU directive 2003/30/EC is implemented, which requires the replacement of 5.75% of petroleum fuels with biofuel across all Member States by 2010, as glycerol is a byproduct in the production of biodiesel. It is projected that by the year 2020, production will be six times more than demand.[6]From fats and oils Triglycerides found in fats and oils are by definition esters of glycerol with long-chain carboxylic acids; the hydrolysis (saponification) or transesterification of these triglycerides produces stoichiometric quantities of glycerol. In this scheme, glycerol is produced as a co-product in the production of long-chain carboxylate salts used as soaps (see soap-making):

Glycerol from triglycerides is produced on a large scale, but the crude product is of variable quality, with a low selling price of as low as 1–8 U.S. cents per pound in 2011. It can be purified, but the process is expensive. As a result, a good fraction of crude glycerol is disposed of as waste. Some glycerol is burned for energy, but the heat value is low.[7]

 

Crude glycerol from the hydrolysis of triglycerides can be purified by treatment with activated carbon to remove organic impurities, alkali to remove unreacted glycerol esters, and ion exchange to remove salts. High purity glycerol (> 99.5%) is obtained by multi-step distillation; vacuum is helpful due to the high boiling point of glycerol (290 °C).[6]

Synthetic glycerol

 

Synthetic glycerol refers to material obtained from non-triglyceride sources. Glycerol may also be produced by various routes from propylene. The epichlorohydrin process is the most important; it involves the chlorination of propylene to give allyl chloride, which is oxidized with hypochlorite to dichlorohydrins, which reacts with a strong base to give epichlorohydrin. Epichlorohydrin is then hydrolyzed to give glycerol. Chlorine-free processes from propylene include the synthesis of glycerol from acrolein and propylene oxide.[6] Because of the emphasis on biodiesel, where glycerol is a waste product, the market for glycerol is depressed, and these old processes are no longer economical on a large scale. Due to the glycerol glut, efforts are being made to convert glycerol to its precursors, such as acrolein and epichlorohydrin.[8] (See the Chemical intermediate section of this article.)

 

In food and beverages, glycerol serves as a humectant, solvent, and sweetener, and may help preserve foods. It is also used as filler in commercially prepared low-fat foods (e.g., cookies), and as a thickening agent in liqueurs. Glycerol and water are used to preserve certain types of plant leaves.[9] As a sugar substitute, it has approximately 27 kilocalories per teaspoon (sugar has 20) and is 60% as sweet as sucrose. It does not feed the bacteria that form plaques and cause dental cavities. As a food additive, glycerol is labeled as E number E422. It is added to icing (frosting) to prevent it from setting too hard.

 

As used in foods, glycerol is categorized by the Academy of Nutrition and Dietetics as a carbohydrate. The U.S. Food and Drug Administration (FDA) carbohydrate designation includes all caloric macronutrients excluding protein and fat. Glycerol has a caloric density similar to table sugar, but a lower glycemic index and differentmetabolic pathway within the body, so some dietary advocates    accept glycerol as a sweetener compatiblewith low carbohydrate diets.

 

 

It is also recommended as an additive when using polyol sweeteners such as erythritol and xylitol which have a cooling effect, due to its heating effect in the mouth, if the cooling effect is not wanted.[10]

 

Pharmaceutical and personal care applications

 

 

Glycerol is used in medical and pharmaceutical and personal care preparations, mainly as a means of improving smoothness, providing lubrication and as a humectant. It is found in allergen immunotherapies, cough syrups, elixirs and expectorants, toothpaste, mouthwashes, skin care products, shaving cream, hair care products, soaps and water-based personal lubricants. In solid dosage forms like tablets, glycerol is used as a tablet holding agent.

 

For human consumption, glycerol is classified by the U.S. FDA among the sugar alcohols as a caloric macronutrient.

 

Glycerol is a component of glycerin soap. Essential oils are added for fragrance. This kind of soap is used by people with sensitive, easily irritated skin because it prevents skin dryness with its moisturizing properties. It draws moisture up through skin layers and slows or prevents excessive drying and evaporation.

 

Glycerol can be used as a laxative when introduced into the rectum in suppository or small-volume (2–10 ml) (enema) form; it irritates the anal mucosa and induces a hyperosmotic effect.[11]

 

Taken orally (often mixed with fruit juice to reduce its sweet taste), glycerol can cause a rapid, temporary decrease in the internal pressure of the eye. This can be useful for the initial emergency treatment of severely elevated eye pressure.

 

Botanical extracts

 

When utilized in ‘tincture’ method extractions, specifically as a 10% solution, glycerol prevents tannins from precipitating in ethanol extracts of plants (tinctures). It is also used as an ‘alcohol-free’ alternative to ethanol as a solvent in preparing herbal extractions. It is less extractive when utilized in a standard tincture methodology. Alcohol-based tinctures can also have the alcohol removed and replaced with glycerol for its preserving properties. Such products are not ‘alcohol-free’ in a scientific sense, as glycerol contains three hydroxyl groups. Fluid extract manufacturers often extract herbs in hot water before adding glycerol to make glycerites.[13][14]

 

When used as a primary ‘true’ alcohol-free (e.g. no ethanol ever being used) botanical extraction solvent in innovative non-tincture based ‘dynamic’[clarification needed] methodologies, glycerol has been shown to possess a high degree of extractive versatility for botanicals including removal of numerous constituents and complex compounds, with an extractive power that can rival that of alcohol and water/alcohol solutions.[citation needed] That glycerol possesses such high extractive power assumes it is utilized with dynamic methodologies as opposed to standard passive ‘tincturing’ methodologies that are better suited to alcohol. Glycerol possesses the intrinsic property of not denaturing or rendering a botanical’s constituents inert (as alcohols – i.e. ethyl (grain) alcohol, methyl (wood) alcohol, etc., do). Glycerol is a stable preserving agent for botanical extracts that, when utilized in proper concentrations in an extraction solvent base, does not allow inverting or reduction-oxidation of a finished extract’s constituents, even over several years.[citation needed] Both glycerol and ethanol are viable preserving agents. Glycerol is bacteriostatic in its action, and ethanol is bactericidal in its action.[15][16][17]

Electronic cigarette liquid

 

Vegetable glycerin is a common component of e-liquid, a solution used with electronic cigarettes that is heated with an atomizer to produce vapor in order to deliver flavors and optionally nicotine.

 

Antifreeze

 

 

Main article: antifreeze

 

 

Like ethylene glycol and propylene glycol, glycerol is a non-ionic kosmotrope that forms strong hydrogen bonds with water molecules, competing with water-water hydrogen bonds. This disrupts the crystal lattice formation of ice unless the temperature is significantly lowered. The minimum freezing point temperature is at about −36 °F / −37.8 °C corresponding to 70% glycerol in water.Glycerol was historically used as an anti-freeze for automotive applications before being replaced by ethylene glycol, which has a lower freezing point. While the minimum freezing point of a glycerol-water mixture is higher than an ethylene glycol-water mixture, glycerol is not toxic and is being re-examined for use in automotive applications.[18][19]

 

In the laboratory, glycerol is a common component of solvents for enzymatic reagents stored at temperatures below 0 °C due to the depression of the freezing temperature. It is also used as a cryoprotectant where the glycerol is dissolved in water to reduce damage by ice crystals to laboratory organisms that are stored in frozen solutions, such as bacteria, nematodes, and mammalian embryos.

 

Internal Combustion Fuel

 

Used to power diesel generators supplying electricity for the FIA Formula E series of electric race cars.[20]

 

 

Chemical intermediate

 

 

Glycerol is used to produce nitroglycerin, which is an essential ingredient of various explosives such as dynamite, gelignite, and propellants like cordite. Reliance on soap-making to supply co-product glycerol made it difficult to increase production to meet wartime demand. Hence, synthetic glycerol processes were national defense priorities in the days leading up to World War II. Nitroglycerin, also known as glyceryl trinitrate (GTN) is commonly used to relieve angina pectoris, taken in the form of sub-lingual tablets, or as an aerosol spray. Allyl iodide, a chemical building block for polymers, preservatives, organometallic catalysts, and pharmaceuticals, can be synthesized by using elemental phosphorus and iodine on glycerol.[21] A great deal of research is being conducted to try to make value-added products from crude glycerol (typically containing 20% water and residual esterification catalyst) obtained from biodiesel production.[22] The use of crude glycerol as an additive to biomass for a renewable energy source when burned or gasified is also being explored.

 

·      Hydrogen gas production unit[23]

·      Glycerine acetate (as a potential fuel additive)[24]

·      Conversion to propylene glycol[25]

·      Conversion to acrolein[26][27]

·      Conversion to ethanol[28]

·      Conversion to epichlorohydrin,[29] a raw material for epoxy resins

 

 

Film Industry

 

 

Glycerol is used by the film industry when filming scenes involving water in order to stop areas drying out too

 

quickly.[30]

 

 

Metabolism

 

 

Glycerol is a precursor for synthesis of triacylglycerols and of phospholipids in the liver and adipose tissue.

When the body uses stored fat as a source of energy, glycerol and fatty acids are released into the bloodstream.

 

Circulating glycerol does not glycate proteins as do glucose or fructose, and does not lead to the formation of advanced glycation endproducts (AGEs). In some organisms, the glycerol component can enter the glycolysis pathway directly and, thus, provide energy for cellular metabolism (or, potentially, be converted to glucose (gluconeogenesis)).

 

Before glycerol can enter the pathway of glycolysis or gluconeogenesis (depending on physiological conditions), it must be converted to their intermediate glyceraldehyde 3-phosphate in the following steps:

 

The enzyme glycerol kinase is present mainly in the liver and kidneys, but also in other body tissues, including muscle and brain.[31][32][33] In adipose tissue, glycerol 3-phosphate is obtained from dihydroxyacetone phosphate (DHAP) with the enzyme glycerol-3-phosphate dehydrogenase.

 

Glycerol has very low toxicity when ingested; its LD50  oral dose for rats is 12600 mg/kg and 8700 mg/kg for mice.

 

 

Historical cases of contamination with diethylene

 

On 4 May 2007, the US Food and Drug Administration advised all US makers of medicines to test all batches of glycerol for the toxic diethylene glycol.[34] This followed an occurrence of hundreds of fatal poisonings in Panama resulting from a falsified import customs declaration by Panamanian ex/im firm Aduanas Javier de Gracia Express, S. A.. The cheaper diethylene glycol was relabeled as the more expensive glycerol.[35][36]

 

  1. Summary

 

In this lecture we learnt about:

 

  • The Biosynthesis of TAGs
  • The role of Lipins
  • The Biosynthesis of Phospholipids
you can view video on Biosynthesis of Lipids VII

 

Weblinks

 

 

 

Books

 

  1. Lehninger Principles of Biochemistry by David L. Nelson, Albert‎ L. Lehninger, Michael‎ M. Cox. 2008. https://books.google.co.in/books?isbn=071677108X
  2. The Role of Glycerol in the Biosynthesis of Pyridine Ring … by James Robert Fleeker. 1965. https://books.google.co.in/books?id=V0eJtgAACAAJ
  3. Glycerol by Anthony Armin Newman, L. V. Cocks. 1968. https://books.google.co.in/books?id=S7xTAAAAMAAJ
  4. Future of Glycerol by Mario Pagliaro, Michele‎ Rossi. 2010. https://books.google.co.in/books?isbn=1849730466
  5. Glycerol: The Platform Biochemical of the Chemical … by Mario Pagliaro. 2013. https://books.google.co.in/books?isbn=886369964X