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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
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Description of Module
Paper : 03 Structure and function of biomolecules II Module : 17 Lipids: An introduction
Principal Investigator Dr. Sunil Kumar Khare,
Professor Dept. of Chemistry,I.I.T. Delhi
Content Reviewer:
Paper Co-ordinator Dr. M.N.Gupta, Emeritus
Professor Dept. of Biochemical Engg. and Biotechnology, I.I.T. DelhiDr. Sunil Kumar Khare,
Professor Dept. of Chemistry,I.I.T. Delhi
Content Writer Dr. M.N.Gupta, Emeritus
Professor Dept. of Biochemical Engg. and Biotechnology, I.I.T. DelhiDr. Prashant Mishra,
Professor Dept. of Biochemical Engg. and Biotechnology, I.I.T. Delhi2
Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Subject Name Biochemistry
Paper Name 03 Structure and Function of Biomolecules II Module Name/Title 17 Lipids: An introduction
Dr. Vijaya Khader Dr. MC Varadaraj
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
1. Objectives
To understand the general importance of lipids in biology and Biotechnology
2. Concept Map
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
3. Description
For quite sometime lipids were considered not as exciting a class of biological molecules as proteins and nucleic acids! Nucleic acids were genetic material and so they attracted the attention of
mainstream biochemists even prior to rDNA era.
Enzymes were proteins and as catalysis of all the reactions in living organisms is carried out by enzymes, they were the focus of microbiologists, biochemists and many others.
Lipids were of interest to nutritionists. Now ofcourse the general population is also interested in fats/oils and ὠ-fatty acids in view of massive advertisement by oil industry in the media.
The role of lipids goes beyond nutritional aspects. Let us look at a brief overview and we will cover their various biological roles in this set of modules exclusively devoted to lipids.
3.1 Definition
All the classes of biological molecules discussed so far had a structural basis. Carbohydrates are sugars or their dimmers/oligomers or polymers. Proteins and nucleic acids are made up of amino acids and nucleotides as monomers respectively.
Lipids differ in their structures widely. As we will see, structure of a fatty acid has little resemblance to a cholesterol molecule. Both nevertheless are classified as lipids.
To start with, both show higher solubility in organic solvents than in water. So, it is a physical property which was responsible for their being classified together as lipids.
Interesting enough, later work showed that fortuitously, this turned out to be a judicious choice.
Cholesterol and lipoproteins have a close relationship in metabolism. Look at the clinical reports on the blood of the people. You would find triglyceride content, cholesterol, LDL and HDL all mentioned together. So, it turns out that biologically, it is not a bad idea to classify these different kinds of molecules as lipids.
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Figure 1
Historically, the class of lipids emerged as any compound which was part of a fraction which was extracted from a biological source (e.g.: animal or plant) by a so called “fat solvent” or non-polar solvent (e.g.: benzene, ether, chloroform or petroleum ether etc.)
So, the basis for compounds being classified together is operational rather than structural.
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Figure 2
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Table 1.
Albumins Soluble in water and
Salt Solution
Globulins Barely soluble in water but freely soluble in salt solutions
Prolamins Soluble in 70-80%
Ethanol
Glutelins Soluble in acid or alkali Sceleroproteins Insoluble in aqueous
solvents
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
The simplest sub-classes of lipids are fatty acids and lipids which are derivatives of glycerol.
I. Fatty Acids
Figure 3A
II. Lipids which are derivatives of glycerol
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Figure 3B
Glycerol is a trihydric alcohol. It is used as moisturizer/solvent and commonly called glycerin.
Glycerol is freely soluble in water, esterifying its alcoholic groups with long chain fatty acids turns this into fats/oils.
During soap making, a process which has been called saponification for a long time, glycerol is produced as a by product.
Currently, most of the industries which were major suppliers of glycerol in the world are shut down, it is mostly available as a by product during biodiesel production!
(A) Neutral Fats
1. Mono-, Di-, Triglycerides 2. Glyceryl Ethers
3. Glycosyl Glycerides
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Figure 3C: Mono-, Di-, triglycerides
Neutral fats are actually what we generally call fats or oils. Structurally, both fats and oils are identical.
If triglycerides are present as a free flowing liquid, these are called oils. If present as solid, we call these fats.
While something like lard will be solid at wide range of temperatures which will be prevalent as room temperature, in a country like India where temperature vary widely in different seasons, this may be confusing.
In Delhi winter, “coconut oil” is generally solid! In other seasons, it may be semi solid. As we will learn in a later module, there is a process in oleochemical industry which is called “winterization”.
Nevertheless, we should not get confused.
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Figure 3D: Glycerol Ethers
While trimesters of glycerol, fats and oils are well known compounds, glycerol also forms ethers. Here are examples of one of its terminal C-atom as a part of ether linkage.
As the other two –OH groups are free in these compounds, these compounds are called alcohols.
The long chain R of the other component of the ether glycerol –OR ensures poor solubility of these compounds in water and solubility in organic solvents. Hence glycerol ethers are lipids.
(B) Phospholipids
1.
Phosphatides or glycerophospholipids12
Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
The nitrogeneous compounds choline, ethanolamine, L-serine etc. containing the alcohol group forms ester bonds with the free acidic group of the phosphoric acid of the L-α-phosphatidic acid.
These are now generally called glycerophospholipids.
Figure 4
The isomer is called L-form because of its stereochemical relationship with L-glyceryl phosphate One way to look at phosphatide structure is as monophosphate ester of 1,2 diglycerides.
When the phosphate groups form another ester bond with these nitrogeneous bases containing alcohol, many important phospholipids are formed. As we will see in a later module, glycophospholipids are the phospholipids which are generally present in biological membranes.
2.
PhosphoglyceridesThese are derivatives of glycerol phosphate and do not contain nitrogen. All these are, as we just learnt, also called phosphatidic acids.
The common phosphoglycerides present in animals and plants are diphosphatidyl glycerols and mono phosphatidyl glycerols respectively.
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
So, It is necessary to realise that phosphatidic acid is not a single component but a class of
phosphoglycerides, different members have different fatty acids esterifying the –OH on the glycerol chain.
Figure 5: Phosphatidyl Gycerols
3.
PhosphoinositidesThese are also derivatives of α-phosphatidic acid without nitrogen but contains inositol.
Inositol is a sugar alcohol.
Myo-inositol is the best known member of this group as it is widely distributed among micro-organisms, plants and animals, often as a phosphorylated form. In animals, myo-inositol also occurs in free form in tissues like muscle, heart, lung and liver.
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Figure 6: Myo-inositol
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Figure 7: Phosphoinositides
Just to clarify again, currently phosphotides, phosphoglycerides and phosphoinositides are all considered members of a bigger class glycerophospholipids.
Glycerophospholipids are also excellent surfactants. Dipalmitoyl phosphatidyl choline (DPCC), as we will see is an important lung surfactant required for good pulmonary performances.
III. Sphingolipids
A. Lipids containing sphingosine or dihydrosphingosine are called sphingolipids
Figure 8A: Sphingosine
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Figure 8B: Dihydrophingosine Sphingolipids are also part of membranes.
In most sphingolipids, sphingosine, the C18 amino alcohol has trans configuration around its single double bond.
In dihydrosphingosine, ofcourse, this double bond is abolished.
Sphingosine has two –OH groups and one –NH2 group as main functional groups.
In fact the biosynthesis of sphingosine is as follows:
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Figure 9: Biosynthesis of sphingosine
Apart from these most common C18 sphingosines, less common C16, C17, C19 and C20 sphingosines also occur as a part of some naturally occurring sphingolipids
B. Ceramides
These are sphingolipids in which free amino group of sphingosine is aylated by a C16, C18, C20 or C22
fatty acid.
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Figure 10: Ceramide
Hence ceramides are essentially N-acylsphingosine. Ceramides differ from each other in this constituent fatty acid.
Ceramides are the basic structural units of sphingolipids Ceramides are part of both sphingomyelin and gangliosides
C. Sphingomyelins
Sphingomyelins are important constituents of brain lipids and are also found in blood.
Plasma membrane of the animal cells contain sphingomyelins.
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Sphingomyelins are phosphodiesters of a ceramide and choline.
Cerebrosides, on the other hand, are ceramides which can be called glycosphingolipids. The common cerebrosides are galactocerebroside and glucocerebroside. Here, the galactose or glucose is linked to the terminal –OH group of sphingosine unit of ceramides.
Please note that cerebrosides do not contain phosphate group and hence are neither phospholipids nor ionic in nature!
So, sphingomyelins are phospholipids. Ceramides and cerebrosides are not!
Figure 11: Structure of a sphingomyelin
Some important cerabrosides are:
Kerasin: Contains saturated C24 lignoceric acid in the ceramide part
Phrenosin: Contains cerabronic acid (2-hydroxy derivative of lignoceric acid) instead
Sulfatides: These are group of S-containing cerabrosides. Often, S is present as sulfoesters in the sugar part.
Ceramide oligosaccharide: Contains more than one sugar residue and hence are called cerabroside disaccharide etc. Some examples are Ceramide lactoside and cytolipin K (has a tetrasaccharide moiety).
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Figure 13: Lactosyl ceramide
Gangliosides
As mentioned earlier, gangliosides are also formed from ceramides.
Figure 12: Synthesis of gangliosides
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Please note that just like cerebrosides and gangliosides do not contain phosphorous. So, these are not phospholipids but glycolipids.
Most complex sphingolipids are also ceramide oligosaccharides but deserve to constitute a class of its own. The oligosaccharide chain in gangliosides contains an acidic sugar called sialic acid or N-acetyl neuraminate.
Myelin sheath are membranes wrapped around each nerve axon. About 10-15 layers thick, it has high lipid content and hence an electrical insulator.
Myelin sheath is rich in sphingomyelins. The name sphingomyelin, in fact, owes its origin to this fact.
Pyranose Form Open form
Figure 14: N-acetyl neuraminic acid The so called grey matter in the brain is rich in gangliosides.
Tay Sach disease, generally fatal for infants is an inherited disorder associated with the deficient hydrolase which breaks down ganglioside GH2
Waxes
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Waxes are esters of long chain fatty acids with long chain alcohols
So, while fats/oils are esters of glycerol, waxes contain long chain alcohols such as cetyl alcohol.
Figure 15: Cetyl alcohol
Other important classes of lipids are terpenes and steroids.
Many essential oils are terpenoids. Vitamin A, the visual pigment and many plant pigments are terpenoids.
Steroids are compounds having Perhydrocyclopentanophenanthrene fused ring system
Figure 16: Perhydrocyclopentanophenanthrene fused ring system
Cholesterol, bile acids and sex hormones are important examples of steroids.
Summary
All compounds soluble in organic solvents and mostly insoluble in aqueous buffers are called lipids
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Biochemistry Structure and Function of Biomolecules II Lipids: An Introduction
Structurally, this is a very diverse class
This, as we shall see, results in lipids having diverse roles and functions in biological systems
Broadly we have gained preliminary knowledge about:
Fats/oils
Phospholipids
Sphingolipids
Waxes, terpenes and steroids
Lipids also combine with other classes of biological compounds to form lipoproteins and lipopolysaccharides.
