Lipids
Chapter 19
Structure and classification of lipids
• Lipids are organic
compounds that are
found in living organisms
that are soluble in nonpolar organic solvents.
• Unlike the other types of
compounds we’ve seen
so far, there are no
characteristic functional
groups in lipids that
indicate their structure.
Structure and classification of lipids
• Lipids can be divided into five categories, on
the basis of lipid function
1.
2.
3.
4.
5.
Energy-storage lipids (triacylglycerols)
Membrane lipids (e.g. phospholipids)
Emulsification lipids (bile acids)
Messenger lipids (e.g. steroid hormones)
Protective-coating lipids (biological waxes)
Fatty acids and types of fatty acids
• Fatty acids are structural components of all the lipids we’ll
study except cholesterol, bile acids and steroid hormones.
• They are naturally occurring monocarboxylic acids that tend
to have even numbers of carbon atoms, and may be classified
as:
– long-chain (C12 to C26)
– medium-chain (C8 and C10)
– short-chain (C4 and C6)
Fatty acids are not like most lipids in that they may be recognized distinctly by the presence of a
COOH group on a carbon chain.
Fatty acids and types of fatty acids
Saturated and unsaturated fatty acids
• Fatty acids may also be categorized with regards to the
presence (and number) of unsaturated units (double bonds).
– Saturated fatty acids (SFAs) contain no double bonds – all
C-C single bonds in the carbon chain component.
Fatty acids and types of fatty acids
Saturated and unsaturated fatty acids
• Monounsaturated fats (MUFAs) possess one C-C double bond
in a monocarboxylic acid structure, and nearly all naturally
occurring MUFAs have cis- stereochemistry.
Fatty acids and types of fatty acids
Saturated and unsaturated fatty acids
• Polyunsaturated fatty acids (PUFAs) possess more than one
double bond in the carbon chain component of the fatty acid.
Up to six double bonds may be found in biochemically
important PUFAs.
Fatty acids typically have the following characteristics:
an unbranched carbon chain
an even number of carbon atoms in the chain
when double bonds are present, they have cis-stereochemistry
Fatty acids and types of fatty acids
Unsaturated fatty acids and double-bond position
• A system exists for describing unsaturated fatty acids in terms
of the number of carbon atoms in the acid and in terms of
double bond position(s):
16:0
Fatty acid with 16 C-atoms
and no double bonds
18:1 D9
Fatty acid with 18 C-atoms
and one double bond at C-9
18:3 D9,12,15
Fatty acid with 18 C-atoms
and three double bonds at
C-9, C-12, and C-15
Fatty acids and types of fatty acids
Unsaturated fatty acids and double-bond position
• Several families of unsaturated fats may be recognized by the
number of saturated carbon atoms that follow the last double
bond (the placement of the methyl end of the chain with
respect to the double bond).
1
3
w-3 (omega-3 fatty acid)
2
2
1
4
3
6
5
w-6 (omega-6 fatty acid)
Physical properties of fatty acids
Water-solubility of fatty acids
• The length of the carbon chain in a fatty acid is important in
determining things like water-solubility and melting/boiling
points.
• Long carbon chains are non-polar, and things with long carbon
chains on them do not dissolve in water.
• Short chain fatty acids are slightly water-soluble, because the
carboxyl group (-COOH) is polar.
Physical properties of fatty acids
Melting points of fatty acids
• Melting points also increase with increasing molar mass
(London forces), so the longer the carbon chain, the higher
the melting point of the fatty acid.
• The presence of double bonds (all cis-stereochemistry) lowers
the melting point (makes it easier for the fatty acid to melt)
because these double bonds cause the molecule to become
bent (less attractions between chains).
Energy storage lipids
Fats and oils
• Carbohydrates store energy in the form of complex
carbohydrates (glycosides).
• Fats store energy in the form of triacylglycerols:
Older term for triacylglycerols: triglycerides
Triacylglycerols are lipids formed by the esterification of three fatty acids to a glycerol molecule.
Energy storage lipids
Fats and oils
• Simple triacylglycerols have three identical fatty acid
molecules.
• Mixed triacylglycerols have different fatty acid molecules
incorporated in them.
Energy storage lipids
Fats and oils
• Both fats and oils are complex mixtures in which many
different kinds of triacylglycerols are present
– fats are solids or semi-solids at room temperature
– oils are liquids at room temperature
• Generally, fats are obtained from animal sources, while oils
are obtained from plants.
• Fats involve triacylglycerols that contain mainly saturated fatty
acid components, while oils have more unsaturated fatty acids
components
Energy storage lipids
Fats and oils
Chemical reactions of triacylglycerols
Reactions of triacylglycerols
• Triacylglycerols have ester groups and (sometimes) double
bonds, so the reactions that can happen with triacylglycerols
involve those functional groups (hydrolysis, addition
reactions)
• They can also undergo oxidation (enzyme-mediated) at the
double bond to produce two carbonyl groups (aldehyde and
then carboxylic acid).
Chemical reactions of triacylglycerols
Reactions of triacylglycerols
• Hydrolysis: since triacylglycerols are tri-esters, the
ester groups can undergo hydrolysis to yield
carboxylic acids and glycerol (1,2,3-Propanetriol).
• This kind of reaction is important in digestion.
Chemical reactions of triacylglycerols
Reactions of triacylglycerols
• Enzymes catalyze the hydrolysis of triacylglycerols in a
stepwise fashion. The outside ester groups are hydrolyzed
first before the middle group.
• In certain cases, triacylglycerols are only partially hydrolyzed
(partial hydrolysis) – this happens when only one or two of
the fatty acid groups are removed from the triacylglycerol by
hydrolysis.
Chemical reactions of triacylglycerols
Reactions of triacylglycerols
• Saponification: as before, the hydrolysis of an ester carried
out under basic conditions is called saponification. The base
causes de-protonation of the carboxylic acid to form a
carboxylic acid salt (called fatty acid salts).
Chemical reactions of triacylglycerols
Reactions of triacylglycerols
• Saponification of a triacylglycerol yields fatty acid salts. The
anions (negative ions) in these salts have long carbon chains
and they have both polar and non-polar components.
• In the body (and other aqueous environments), these anions
form ball-shaped structures called micelles.
• Micelles are able to interact with both polar and non-polar
molecules, and serve as soaps/detergents.
Chemical reactions of triacylglycerols
Reactions of triacylglycerols
• Hydrogenation of a triacylglycerol can happen if the
triacylglycerol has unsaturated fatty acid chains.
• Hydrogenation of the double bonds (adding H2
across the C-C double bond) converts C-C double
bonds to C-C single bonds.
Chemical reactions of triacylglycerols
Reactions of triacylglycerols
• When hydrogenation occurs, the
physical properties of a
triacylglycerol will change. For
example, more saturated chains
will lower the melting point of
the triacylglycerol, perhaps
causing it to become a
semisolid/solid at room
temperature.
• This reaction is used in the food
industry to make margarine,
peanut butter, and other
partially hydrogenated products;
however, partial hydrogenation
also results in the conversion of
cis-double bonds to transdouble bonds (trans-fats).
Chemical reactions of triacylglycerols
Reactions of triacylglycerols
• Oxidation of the C-C double bonds can also occur.
• In this process, both bonds in the double bond are broken and
two C=O groups are made:
unsaturated fatty
acid chain
oxidation
oxidation
O2 from air
O2 from air
short-chain
aldehydes
short-chain
carboxylic acids
• This kind of reaction occurs when fats become rancid
(decompose in air), producing compounds that have bad
smells (short-chain aldehydes and carboxylic acids).