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Lipids
Bioc. 201
ميحرلا نمحرلا هللا مسب
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Lipids
• Lipids, commonly referred to as fats, are substances of biological origin.
• Soluble in organic solvents such as chloroform and methanol.
• Fats, oils, certain vitamins and hormones, and
most non protein membrane components are
lipids.
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Functions of Lipids
Lipids play three major roles in human biochemistry:
1. Store energy within fat cells. Because they are composed of mostly carbon-hydrogen (C—H) bonds, they are a rich source of energy and an efficient way for the body to store excess calories.
2. Parts of membranes that separate compartments of aqueous solutions from each other. Because of their unique physical properties, lipids are also an integral part of cell membranes and, therefore, also play an important structural role in cells.
3. Serve as chemical messengers such as steroid
hormones.
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Lipids Classification
The major classes of lipids are:
1. Fatty acids.
2. Triglycerols.
3. Glycerphospholipids.
4. Sphingolipids.
5. Steroids.
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Lipids Classification
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Fatty Acids
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Fatty Acids
• Fatty acids are simply linear long-chains of C—H bonds that terminate with a carboxyl group (—
COOH).
• Composed of a carboxylic acid “head group” and a
long hydrocarbon “tail”.
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Fatty Acids
Fatty acids are variable in length and can be classified as:
• Short-chain (4-6 carbon atoms) fatty acids.
• Medium-chain (8-12 carbon atoms) fatty acids.
• Long-chain (more than 12 carbon atoms) fatty
acids.
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Fatty Acids
• Most fatty acids in our diet are of the long-chain variety and contain an even number of carbon atoms.
• In higher plant and animals, the predominant fatty acid residues are those of the C16 and C18 species palmitic, oleic, linoleic, and steric acids.
• Fatty acids with <14 or >20 carbon atoms are uncommon.
Structural formula of some C18 fatty acids. The double bonds all have the cis conformation.
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Fatty Acids
• Not all of the carbon atoms on fatty acids are fully saturated or bonded with hydrogen atoms: some of them may instead form carbon=carbon (C=C) double bond.
• Depending on the number of C=C double- bonds, fatty acids can be classified as:
Saturated (no double-bonds).
Unsaturated (contain double bonds).
Polyunsaturated (two or more double-bonds).
• Over half of the fatty acid residues of plant and animal lipids are unsaturated and polyunsaturated.
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Fatty Acids
CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 C OH 1O
4 2 6 5
8 7 3
Saturated Fatty Acids: Octanoic Acid
Unsaturated Fatty Acids:
CH
3CH
2CH
2CH
2CH
2CH
2CH
2C OH 1 O
4 2 6 5
8 7 3
CH
3CH
2CH
2CH
2CH
2CH
2CH
2C OH 1 O
4 2 6 5
8 7 3
3 - Octenoic Acid
3, 6 - Octadienoic Acid
Short hand: 8:1 (Δ3) 8:2 (Δ3,6)
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Fatty Acids
Cis 9 - Octadecenoic Acid (oleic)
Trans 9 - Octadecenoic Acid (elaidic acid)
O
CH
3(C H
2)
7C C (CH
2)
7C OH H H
9 10
O
CH
3(C H
2)
7C C (CH
2)
7C OH H
H
Cis And Trans Fatty Acids:
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Fatty Acids
• Linoleic acid:
Cis, cis, 9, 12 - Octadecadienoic acid
• Linolenic acid:
Cis, cis, cis 9, 12, 15 - Octadecatrienoic acid
• Arachidonic acid:
Cis, cis, cis, cis 5, 8, 11, 14 - Eicosatetraenoic acid
Linoleic Acid Linolenic Acid Arachidonic Acid
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Fatty Acids
• The first double bond of an unsaturated fatty acid commonly occurs between its C9 and C10 atoms counting from the carboxyl C atom (a Δ
9- or 9-double bond).
• Triple bonds rarely occur in fatty acids or any
other compound of biological origin.
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Fatty Acids
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Fatty Acids
Cis and trans configuration:
• The C=C double-bonds of unsaturated fatty acids are typically arranged in the cis form, with both hydrogen atoms on the same side of the C=C double-bonds, which causes a bend in their structure.
• Fatty acid C=C double-bonds can also occur in the
trans configuration, with both hydrogen atoms on
opposite side of the C=C double-bond.
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Fatty Acids
Cis and trans configuration:
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Fatty Acids
Cis and trans configuration:
• Fatty acid double bonds almost always have the cis configuration.
• The trans fatty acids are not commonly in
nature; however, they are present in our diet
because the chemical hydrogenation
treatment used in food processing for
converting polyunsaturated plant oils into
margarine introduces trans double bonds.
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Fatty Acids
• In plasma, only a relatively small amount of fatty acids exists in the free or unesterified form.
• The majority of plasma fatty acids are found as a constituent of triglycerides or phospholipids.
• Fatty acids are covalently attached to the glycerol
backbone of triglycerides and phospholipids by
an ester bond that forms between the carboxyl
group (CO) on the fatty acid and the hydroxyl
group (OH)on glycerol.
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Simple Lipids:
Triglycerides
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Triglycerides
• Triglycerides (also referred to as Triacylglycerols or neutral fats).
• contain three fatty acid molecules attached to one molecule of glycerol by ester bonds.
• Each fatty acid in the triglyceride molecules can potentially by different in structure, thus producing many possible structural forms of triglycerides.
• Fats and oil that occur in plants and animals consist largely of mixtures of Triglycerides.
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Triglycerides
• Triglycerides have no charged or polar hydrophilic groups, making them very hydrophobic and virtually water insoluble.
• The hydrophobic character of triglycerides is caused by the long hydrocarbon chains.
• Because it has no charge, triglyceride is classified as neutral lipid.
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Triglycerides
• Triglycerides are the most common lipid materials, although mono and diglycerides are not infrequent.
• In mono- and di- types, only one or two -OH groups of the glycerol are esterified by fatty acids.
1 Fatty Acid + Glycerol = monoacylglycerol
2 Fatty Acids + Glycerol = diacylglycerol
3 Fatty Acids + Glycerol = triacylglycerol
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Triglycerides
Triacylglycerols differ according to the identity and placement of their three fatty acid residues:
1. Simple triacylglycerols contain one type of fatty acid residue and are named accordingly. For example:
• Tristearoylglycerol or tristearin contains three stearic acid residues.
2. Mixed triacylglycerols contain two or three different types of fatty acid residues and are named according to their placement on the glycerol moiety.
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Triglycerides
Oils and Fats:
• Oils: most triglycerides from plant sources, such as corn and sunflower seeds, are rich in polyunsaturated fatty acids and are oils (liquid at room temperature).
• Fats: triglycerides from animal sources contain mostly saturated fatty acids and are usually solid at room temperature.
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Pure Fats
Pure fats and oils are colorless, odorless, and tasteless.
• This statement may seem surprising because
we all know the tastes and colors of such fats
and oils as butter, and olive oil. The tastes,
odors, and colors are caused by small amounts
of other substances dissolved in the fat or oil.
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Triglycerides are efficient energy reserves
Fats are highly efficient form in which to store metabolic energy because:
• Fats are less oxidized than are carbohydrates or proteins and hence yield significantly more energy on oxidation.
• Fats, being non polar substances, are stored in
anhydrous form, whereas glycogen bind about twice
its weight of water. Fats therefore provide about six
times the metabolic energy of an equal weight of
hydrates glycogen.
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Adipocytes (fat cells)
Adipocytes (fat cells) are specialized for the synthesis and storage of triacylglycerols in animals.
Adipose tissue is most
abundant in a
subcutaneous layer and
in the abdominal cavity.
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Adipocytes (fat cells)
Functions:
1. Fat content of normal human (21% for men, 26% for women) enables them to survive starvation for 2 to 3 months. In contrast, the body’s glycogen supply, which functions are a short-term energy store, can provide for the body’s metabolic need for less than a day.
2. The subcutaneous fat layer also provides thermal insulation.
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Saponification (Soap making)
• Saponification is a process by which triglycerides are reacted with sodium or potassium hydroxide to produce glycerol and a fatty acid salt, called 'soap'.
• Lipids that contain fatty acid ester linkages can undergo hydrolysis. This reaction is catalyzed by a strong acid or base. Saponification is the alkaline hydrolysis of the fatty acid esters.
