Download LIPIDS

LIPIDS
Low molecular weight (generally < 1000) substances of biological origin that are relatively
more soluble in non-polar than polar solvents. They are characterized by their low oxygen
content and are therefore energy dense. Neutral lipids (fats, oils) are generally found as
energy storage molecules whereas the more polar lipids (phospholipids, sphingolipids) are
key components of cellular and subcellular membranes.
The majority of lipids are derivatives of fatty acids. In these so called acyl lipids, the fatty
acids are present as esters, and in some minor lipid groups in amide forms. The acyl residue
influences strongly the hydrophobicity and the reactivity of the acyl lipids. Some lipids act as
building blocks in the formation of biological membranes, which surround cells and
subcellular particles. Primarily, triacylglycerols are deposited in some animal tissues and
organs of some plants. Lipid content in such storage tissues can rise to 15%–20% or higher
and so serve as a commercial source for isolation of triacylglycerols .
Lipids are broadly defined as any fat-soluble (lipophilic), naturally-occurring molecules, such
as fats, oils, waxes, cholesterol, steroids, fat-soluble vitamins (such as vitamins A, D, E and
K), monoglyceride, diglyceride, phospholipids, and others. Although the term lipid is
sometimes used as a synonym for fat, fats are in fact a subgroup of lipids called triglycerides
and should not be confused with the term fatty acid. The term is also used more specifically
to refer to fatty-acids and their derivatives (including tri-, di-, and monoglycerides and
phospholipids), as well as other fat-soluble sterol-containing metabolites such as
cholesterol.
FATTY ACIDS
Fatty acid is a carboxylic acid often with a long, unbranched aliphatic chain, which is either
saturated or unsaturated. Carboxylic acids as short as butyric acid (four carbon atoms) are
considered to be fatty acids, whereas fatty acids derived from natural fats and oils may be
assumed to have at least eight carbon atoms, e.g., caprylic acid (octanoic acid). Fatty acids
are aliphatic monocarboxylic acids derived from or contained in an esterified form in an
animal or vegetable fat, oil, or wax. Natural fatty acids commonly have a chain of 4–28
carbons (usually unbranched and even numbered), which may be saturated or unsaturated.
SATURATED FATTY ACIDS
Saturated fatty acids have no double bonds between the carbon atoms of the fatty acid
chain; hence, they are fully saturated with hydrogen atoms. There are several kinds of
naturally occurring saturated fatty acids, their only difference being the number of carbon
atoms - from 1 to 24. Saturated fat is fat that consists of triglycerides containing only
saturated fatty acids.
unsaturated fatty acids
An unsaturated fat is a fat or fatty acid in which there are one or more double bonds in the
fatty acid chain. A fat molecule is monounsaturated if it contains one double bond, and
polyunsaturated if it contains more than one double bond. Where double bonds are
formed, hydrogen atoms are eliminated. Thus, a saturated fat is "saturated" with hydrogen
atoms.
3 - Octenoic Acid
3, 6 - Octadienoic Acid
In biochemistry and nutrition, monounsaturated fats are fatty acids that have a single
double bond in the fatty acid chain and all the carbon atoms in the chain are single-bonded.
By contrast, polyunsaturated fatty acids have more than one double bond.
Fatty acids are long-chained molecules having a methyl group at one end and a carboxylic
acid group at the other end. Fatty acid fluidity increases with increasing number of double
bonds. Therefore, monounsaturated fatty acids have a higher melting temperature than
polyunsaturated fatty acids but lower than saturated fatty acids.
In nutrition, polyunsaturated fat is an abbreviation of polyunsaturated fatty acid. That is a
fatty acid in which more than one double bond exists within the representative molecule.
That is, the molecule has two or more points on its structure capable of supporting
hydrogen atoms not currently part of the structure. Polyunsaturated fatty acids can assume
a cis or trans conformation depending on the geometry of the double bond.
Cis 9 - Octadecenoic Acid (oleic)
Trans 9 - Octadecenoic Acid (elaidic acid)
Essential fatty acids
Essential fatty acids (EFAs) are 18- to 20-carbon unsaturated fatty acids having at least two
double bonds. The term ‘essential fatty acids’ means that they are essential for life and must
be provided in the diet to prevent death. These fatty acids are dietary essentials because
animal systems do not have enzymes that can insert a double bond distal to n-9 carbon in a
fatty acid.
the three polyunsaturated fatty acids usually thought of as EFAs – linoleate (18:2),
linolenate (18:3) and arachidonate (20:4) – the highest biopotency for growth is seen with
arachidonate. Arachidonic acid is found predominantly in animal tissues whereas linoleic
acid is distributed in plant oils. Arachidonic acid itself cannot be classified as an essential
fatty acid since it is derived from linoleic acid in metabolism.
The classical symptoms of EFA deficiency are growth depression, decreased reproduction,
decrease in skin integrity (more evaporative loss), tissue membrane degradation, and
changes in fatty acid concentrations in blood and tissue lipids. Humans given a fat-free diet
intravenously had evidence of an EFA deficiency by 14 days and in some cases by 2 days.
Ruminants seem to be unexpectedly resistant to similar treatments.
A number of hormones are derived from the essential fatty acids. Each EFA is a precursor for
a separate series of prostaglandins, thromboxanes and leukotrienes – hormones intimately
involved in cell and tissue metabolism.
LIPIDS
Compound Lipids
or
Heterolipids
Simple lipids or
homolipids
Fats
and
oils
Waxes
Phospho
lipids
Glyco
lipids
Derived Lipid
Steroids
Terpenes
Carotenoids
ACYLGLYCEROLS
A form of lipid made up of one glycerol molecule combined with three individual (not
necessarily identical) fatty acid molecules attached to the glycerol by ester bonds. Neutral
fats are mono-, di-, and tri-esters of glycerol with fatty acids, and are termed
monoacylglycerols, diacylglycerols, and triacylglycerols, respectively.
Monoglyceride (a - monostearin)
Diglyceride (a, a' - distearin)
Triglyceride (b - palmityl distearin)
OILS AND FATS
All oils and fats, with their high carbon and hydrogen content, can be traced back to organic
sources. Oils and fats are also produced by plants, animals, and other organisms through
organic processes. Oils are fats that are liquids at room temperature. Solid fats are fats that
are solids at room temperature. Solid fats come from many animal foods and can be made
from vegetable oils through a process called hydrogenation.
Waxes
Fatty acids and an alcohol. They are more hyrophobic than fats. And they are the natural
coatings for apples and pears.
Important in fruits:
1. Natural protective layer in fruits, vegetables, etc.
2. Added in some cases for appearance and protection.
PHOSPHOLIPIDS
are a class of lipids, and a major component of all biological membranes, along with
glycolipids, cholesterol and proteins.. They have two regions named as head and tail. Heads
are hydrophilic while tails are hydrophobic .
Phospholipids are diglycerides that are covalently bonded to a phosphate group by an ester
linkage. The diglyceride is composed of a glycerol backbone that has esterified to two fatty
acids the phosphate group is ionic and therefore hydrophilic The fatty acid side groups are
hydrophobic.
Glycerophospholipids
CH2OH
1. (phosphoglycerides), are common constituents of cellular membranes.
H
2. They have a glycerol backbone.
Formation of an ester:
R'OH + HO-C-R"
OH
CH2OH
3. Hydroxyls at C1 & C2 are esterified to fatty acids.
O
C
O
R'-O-C-R'' + H2O
An ester forms when a hydroxyl reacts with a carboxylic acid, with loss of H2O.
glycerol
Phosphatidate
In phosphatidate:
1. fatty acids are esterified to hydroxyls on C1 & C2
2. the C3 hydroxyl is esterified to Pi.
3. In most glycerophospholipids (phosphoglycerides),
Pi is in turn esterified to OH of a polar head group (X): e.g., serine, choline,
ethanolamine, glycerol, or inositol.
4. The 2 fatty acids tend to be non-identical. They may differ in length and/or the
presence/absence of double bonds.
O
O
R1
C
H2C
O
O
CH
H2C
C
R2
O
O
P
O
X
O
glycerophospholipid
GLYCOLIPIDS
These are the compounds of fatty acids with carbohydrates and contain nitrogen but no
phosphoric acid .
Steroids
A steroid is a terpenoid lipid characterized by a carbon skeleton with four fused rings,
generally arranged in a 6-6-6-5 fashion. belongs to this class.
1. Male & female sex hormones.
2. Bile acids.
3. Vitamin D.
4. Adrenal corticosteroids.
5. Cholesterol .
Cholesterol
1. Mother molecule for many important molecules in body, including sex hormones and
vitamin D, which is needed for calcium absorption.
2. The liver produces cholesterol, so none is needed in the diet.
3. “bad” cholesterol is associated with heart disease.
4. Dietary recommendation - <300 mg/d.
HO
Cholesterol
5. Sources – egg yolks, liver, shellfish, organ foods.
Lipoproteins
1. Clusters of lipids and proteins are used to transport fats in the blood stream.
2. Solve problem of transport through a watery fluid.
Types of lipoproteins
1. Chylomicrons
a. Largest.
b. Least dense.
c. Transport lipids derived from diet.
d. Cells remove lipids from them over about a 14 hour period, so they get smaller
as time passes.
e. When depleted only cholesterol, protein remnants and phosphlipids remain.
f. The depleted chylomicrons are removed from circulation by the liver.
g. The liver dismantles them and uses or recycles the pieces.
2. VLDL ( very low density lipoproteins)
a. Lipids made in the liver together with chylomicron remnants.
b. Released by liver into circulation.
c. Cells remove triglycerides.
d. VLDL shrink and become denser with a high ratio of cholesterol.
e. Eventually is transformed into LDL (low density lipoprotein.
3. LDL (low density lipoprotein)
a. Circulate in blood
b. All body cells remove cholesterol, phospholipids and remaining triglycerides to.
make membranes, hormones, etc.
c. Liver has receptors for LDL and removes what remains from circulation.
4. HDL (high density lipoprotein)
a. Made by liver.
b. Carries cholesterol released along with fatty acids, triglycerides and fatty acids
from fat cells back to liver.
c. Liver recycles or disposes cholesterol.
LDL
Bad cholesterol
Linked to higher risk of heart attack
HDL
Good cholesterol
Possibly protective of heart
Represents cholesterol returning to liver for
disposal