Suaib Luqman

epgp books

 

  1. Objectives
  • v To understand what is lipid
  • v Why they are important
  • v How they occur in nature
  1. Concept Map

 

  1. Description

 

3.1 Prologue to Lipids

 

In 1943, the term lipid was first used by BLOOR, a German biochemist.

 

Lipids are heterogeneous group of compounds present in plants and animal tissues related either actually or potentially to the fatty acids. They are amphipathic molecules, hydrophobic in nature originated utterly or in part by thioesters (carbanion-based condensations of fatty acids and/or polyketides etc) or by isoprene units (carbocation-based condensations of prenols, sterols, etc). Lipids have the universal property of being:

 

  1. Quite insoluble in water (polar solvent)
  2. Soluble in benzene, chloroform, ether (non-polar solvent)

 

Thus, lipids include oils, fats, waxes, steroids, vitamins (A, D, E and K) and related compounds, such as phospholipids, triglycerides, diglycerides, monoglycerides and others, which are allied more by their physical properties than by their chemical assests.

 

They are vital constituents related to diet because of high energy value, essential fatty acids and fat soluble vitamins present in the content of fat foods. Lipids serve as adept source of energy when stored in adipose tissue. Fat also dole out as Thermal Insulators in the subcutaneous tissues and in the region of some organs and non-polar lipids acts as Electrical Insulators permitting quick propagation of depolarization waves by the side of myelinated nerves. Lipoproteins (combinations of lipid and protein) transport lipids in the blood and the biochemical acquaintance of lipids is obligatory in understanding atherosclerosis, diabetes mellitus, obesity etc. The function of diverse polyunsaturated fatty acids (PUFAs) in nutrition and health can also be best understood by studying its biochemical profile.

 

Chemically, lipids are defined as esters of glycerol and fatty acids or else refer as the triglycerides of fatty acids. They are the substances of natural origin, soluble in non-polar solvents and hence may be extorted by using organic solvents such as methanol. Lipids could be fractionated by either using adsorption chromatography (thin layer chromatography) or reverse-phase chromatography.

 

Fatty acid

 

It may be defined as an organic acid that occurs in a natural triglyceride and is a mono carboxylic acid ranging in chain length from C4-C24 carbon atoms. Fatty acids are obtained from the hydrolysis of fats. They are naturally occurring straight chain derivatives containing carbon atoms with even numbers (4-28) as they assemble from two carbon units. Typically derived from triglycerides or phospholipids, fatty acids are vital sources of fuel yielding huge quantities of ATP. Fatty acids that contain C=C are recognized as unsaturated fattyacid (UFAs) and those lacking double bonds are recognized as saturated fatty acid (SFAs). They are named after corresponding hydrocarbons and vary in length. UFAs end with suffix-enoic and SFAs ends with suffix-anoic. Based on length as short to very long, they may be categorized as under:

 

  • SCFA: Short Chain Fatty Acids with less than six carbons (e.g. butyric acid).
  • MCFA: Medium Chain Fatty Acids with 6-12 carbons.
  • LCFA: Long Chain Fatty Acids with 13-21 carbons.
  • VLCFA: Very Long Chain Fatty Acids with more than 22 carbons.

 

The condensation of Acetyl Co-A, a coenzyme, results in the biosynthesis of fatty acids as it carries two carbon units. That is why all FAs have even numbers of carbon atoms.

 

Unsaturated Fatty Acid (UFA)

 

UFAs contain carbon units linked by double bonds, saturated with hydrogen atoms that convert double bonds to single bonds (one or more double bonds between carbon units). The carbon atoms occur either in a cis or a trans configuration. When two hydrogen atoms nearby to the double bond fasten on the chain (same side), it is cis configuration of fatty acid (Oleic acid, Linoleic acid etc). When the neighboring two hydrogen atoms lie on the chain (opposite side), it is trans configuration of fatty acid (Elaidic acid, Vaccenic acid etc). Unsaturated fatty acid may be of following types.

 

  • Monounsaturated: Presence of one double bond
  • Polyunsaturated: Presence of two or more double bond
  • Eicosanoid: are signaling molecules made by the oxidation of 20-carbon fatty acids
  • ü Prostanoid: It includes (a) Prostaglandins e.g. PGE1, (b) Prostacyclin e.g. PCI2, (c) Thromboxanes e.g. TXA2
  • ü Leukotriene: Containing three double bonds sequentially e.g. LTB4, LTE4

Table 1. Selected examples of Unsaturated Fatty Acids

 

 

Saturated Fatty Acid (SFA)

 

They are long chain carboxylic acids without double bonds but with 12-24 carbon units. As its name indicates they are saturated with hydrogen atoms having solitary bonds with each carbon units inside the chain has 2 hydrogen atoms (except 3 hydrogens at the end for omega carbon). Examples of the SFAs are Capric acid, Palmitic acid, Stearic acid etc.

 

Table 2. List and Examples of Saturated Fatty Acids

 

Essential Fatty Acid (EFA)

 

They are indispensable for the human body not produced in adequate amount from substrates, and consequently ought to be obtained from diet (food). The idiom ‘EFA’ refers to fatty acids obligatory for biological processes and excludes those fats that merely act as fuel. Two vital series of EFAs have been reported: (i) Three carbon units with double bond removed from the methyl end and (ii) Six carbon units with double bond removed from the methyl end. Omega 3-fatty acid (α-linolenic acid = ALA) and Omega 6-fatty acid (Linoleic acid = LA) are the two EFAs widely distributed in plant oils. Humans lack the ability to synthesize these two EFAs due to absence of desaturase enzymes required for their production. In 1923, these two EFAs were preferred as Vitamin F but later (1929), studies on mice revealed that these two EFAs should be categorized under fats rather vitamins. Omega-3 fatty acid (Docosahexaenoic acid) and Omega 6-fatty acid (γ- Linoleic acid) are occasionally referred to as ‘Conditionally Essential’ as they happen to indispensable under disease or some developmental circumstances. In the human body, EFAs dole out numerous functions such as:

 

  • Customized to make

 

o Eicosanoids: Distressing several cellular functions including inflammation
o Endocannabinoids: Upsetting mood, behavior and inflammation
o Lipoxins: A faction of eicosanoid derivatives produced from ω-6 EFAs via the
lipoxygenase pathway and resolvins from ω-3 (down regulating
inflammation in the presence of acetylsalicylic acid)

 

  • o Epoxyeicosatrienoic acids (EETs), Hepoxilins, Isofurans, Isoprostanes, Neurofurans and Neuroprotectin D
  • EFAs affect cellular signaling by forming lipid rafts.
  • They either activate or inhibit transcription factors such as NF-κB and act on DNA.

 

 

Examples of the food sources with EFAs are canola (rapeseed) oil, chia seeds, fish and shellfish, flaxseed (linseed), hemp seed, leafy vegetables, pumpkin seeds, soya oil, sunflower seeds and walnuts. Nearly, all the PUFAs in the human diet are EFAs that play a vital part in the existence and loss of cardiac cells. The deficiency of EFAs results in depression, dermatitis and osteoporosis.

 

Free Fatty Acid

 

When fatty acids do not affixed to supplementary molecules, they are acknowledged as ‘Free Fatty Acids’ (FFAs) or ‘Uncombined Fatty Acids’ (UCFAs). The FFAs or UCFAs ensued from the triglyceride breakdown, insoluble in water, circulated, solubilized and transported through albumin (a plasma protein). However, their blood echelon is restricted by the accessibility of binding sites of albumin.

 

Table 3. Composition of dietary fats

 

Triglycerides (TG)

 

Are most abundant form of lipids and constitute about 98% of total dietary lipids. TGs are esters of glycerol with three fatty acid molecules. Glycerol (Alcohols) contains a hydroxyl (OH) moiety and Fatty acid (Organic acids) encloses a carboxyl (-COOH) group. Both join together to form esters. During each esterification one molecule of water is released. In TGs, the OH group of the glycerol connects the COOH group of the fatty acid to form ester bonds. They may be fats and oils and are also known as Triacylglycerol (TAG) or Triacylglyceride. TGs contain three moles of fatty acids which may be similar or dissimilar. Similar kind of FAs in all the three positions are called simple TGs e.g. Tripalmitin, triolein etc. Most of the TGs contains different kinds of fatty acids in position 1, 2 or 3 and are called mixed TGs e.g. Oleodipalmitin etc. There are a lot of triglycerides obtainable from the oil source, a number of them are highly unsaturated and a few are unsaturated. Saturated are those having single bonds between the carbon atoms (C-C) where hydrogen atoms bonds with carbon atoms while unsaturated compounds bears double bonds between carbon units (C=C), plummeting the integer of places wherever hydrogen atoms bonds with carbon atoms. Furthermore, at room temperature saturated have an elevated melting point and are solid while unsaturated have a lower melting point and are liquid.

 

TGs are the vital constituents of animal fats (saturated) including human skin oils and vegetable oil (unsaturated). In naturally occurring TGs, the chain lengths of the FAs include even number (16, 18, 20) of carbon units. However, in bacteria and ruminants fat odd number (15) carbon atoms are present. Majority of natural fats include an intricate blend of individual TGs and due to this, they deliquesce over a wide array of temperatures. In TGs form, lipids cannot be engrossed by the duodenum unless broke dowm into fatty acids, monoglycerides and a few diglycerides. In the intestine, TGs ripped into FFAs and monoacylglycerol following the secretion of bile and lipases in a process called lipolysis. TGs advances to the intestine through enterocyte cells and reinstate it from their wreckage, wrap up with proteins and cholesterol to guise chylomicrons. An array  of tissues incarcerates the chylomicrons and releases the TGs to be worn as an energy source. TGs can pass through cell membranes freely via the fatty acid transporter (FAT) only after its split into fatty acid and glycerol by lipoprotein lipases. TGs being the foremost components of chylomicrons and VLDL (very low density lipoprotein) perform an imperative role (energy source) in metabolism and transporters of dietary fat (38 kJ/g or 9 kcal/g compared to carbohydrates: 17 kJ/g or 4kcal/g). A high level of TGs in human body has been related to atherosclerosis, stroke risk and heart disease.

 

In oil paints and coating, di and triunsaturated fatty acid components present in linseed and related oil are used which apt to congeal in the presence of oxygen. Using trans-esterification phenomenon, TGs are also ripped into their components in the biodiesel manufacturing. The ensuing esters of FA be able to be worn as a fuel in diesel engines. The glycerin is used in the production of pharmaceuticals and food. Lysochromes (Fat soluble dye, Oil Red O, Sudan Black B, Sudan IV) has been employed for staining fatty acids, triglycerides, lipoproteins, and other lipids.

 

Lipids contain hydrocarbons which are the base for the structure and function of living cells. The biological functions of the lipids are as distinct as their chemistry. Oils and fats are the primary stored arrangement of energy in numerous organisms. Sterols and phospholipids are key structural essentials of biological membranes. Other lipids, though present in reasonably small quantities, perform crucial roles as electron carriers, anchors for proteins (hydrophobic), enzyme cofactors, light absorbing pigments, as intracellular messengers, as chaperones in protein folding of membranes and as an emulsifying agents in the digestive tract.

 

Being largely hydrocarbon, lipids yield huge amounts of energy on oxidation and represent exceedingly reduced forms of carbon. Based on biochemical subunits, lipids may be alienated into the following class: saccharolipids,sphingolipids, sterol lipids, polyketides (consequential of ketoacyl subunits condensation), prenol lipids (resultant of isoprene subunits condensation), glycerophospholipids, glycerolipids and fatty acids.

 

The major functions of lipids include energy storage, integral part of cell membrane components and signaling. Oils and fats are the main source of energy in innumerable organisms. Sterols and phospholipids are key structural elements of membranes. Other present in comparatively diminutive quantities, perform essential roles as electron carriers, enzyme cofactors, light absorbing pigments, hormones, as intracellular messenger, as an anchor for hydrophobic proteins, as chaperones in membrane proteins folding, as an emulsifying agent in the digestive tract. Lipids have a burly relevance in nanotechnology as well as in food and cosmetic industries.

 

 

In a nutshell, lipids:

 

  • Acts as a fuel in the body
  • Yields 9.0 kcal of energy per gram
  • Exerts an insulating effect in the body
  • Provide padding and protect the internal organs like kidney
  • Supply EFAs for normal health, development and growth
  • Vital for fat soluble vitamins
  • Fundamental constituent of cell wall, cell membrane and cell organelle like mitochondria

 

The present prologue introduces lipids and their representative of every type with a prominence on their physical properties and chemical structure.

 

  1. Summary

 

In this lecture we learnt about:

 

  • The Definition of Lipids
  • Their Solubility
  • Chemical and Biological nature
you can view video on Prologue to Lipids

 

 

Weblinks:

 

 

 

Books:

 

  1. Lipids by Markus R. Wenk, ‎Gilbert Di Paolo. 2012. https://books.google.co.in/books?isbn=0123864879
  2. Biochemistry of Lipids, Lipoproteins and Membranes by J.E. Vance, ‎Dennis E. Vance. 2008. https://books.google.co.in/books?isbn=0080559883
  3. Lipids: Current perspectives – Issue 12259 by D John Betteridge. 1996. https://books.google.co.in/books?isbn=1853172316
  4. The Lipid Handbook with CD-ROM, Third Edition by Frank D. Gunstone, ‎John L. Harwood, ‎Albert J. Dijkstra. 2007. https://books.google.co.in/books?isbn=1420009672
  5. Bhagavan NV (2002). Medical Biochemistry. San Diego: Harcourt/Academic Press. ISBN 978-0-12-095440-7.
  6. Devlin TM (1997). Textbook of Biochemistry: With Clinical Correlations (4th ed.). Chichester: John Wiley & Sons. ISBN 978-0-471-17053-2.
  7. Stryer L, Berg JM, Tymoczko JL (2007). Biochemistry (6th ed.). San Francisco: W.H. Freeman. ISBN 978-0-7167-8724-2.
  8. van Holde KE, Mathews CK (1996). Biochemistry (2nd ed.). Menlo Park, California: Benjamin/Cummings Pub. Co. ISBN 978-0-8053-3931-4.