Lipids

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Lipids

• Chapter 19

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Structure and classification of lipids

• Lipids are organic compounds that are found in
  living organisms that are soluble in non-polar
  organic solvents.
• Unlike the other types of compounds weve seen so
  far, there are no characteristic functional
  groups in lipids that indicate their structure.

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Structure and classification of lipids

• Lipids can be divided into five categories, on
  the basis of lipid function
• Energy-storage lipids (triacylglycerols)
• Membrane lipids (e.g. phospholipids)
• Emulsification lipids (bile acids)
• Messenger lipids (e.g. steroid hormones)
• Protective-coating lipids (biological waxes)

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Fatty acids and types of fatty acids

• Fatty acids are structural components of all the
  lipids well 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.
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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.

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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.

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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
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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)

160 Fatty acid with 16 C-atoms and no double
bonds
181 D9 Fatty acid with 18 C-atoms and one double
bond at C-9
183 D9,12,15 Fatty acid with 18 C-atoms and
three double bonds at C-9, C-12, and C-15
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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).

w-3 (omega-3 fatty acid)
w-6 (omega-6 fatty acid)
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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.

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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).

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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.
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Energy storage lipids
Fats and oils

• Simple triacylglycerols have three identical
  fatty acid molecules.
• Mixed triacylglycerols have different fatty acid
  molecules incorporated in them.

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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

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Energy storage lipids
Fats and oils
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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).

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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.

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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.

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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).

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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.

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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.

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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 trans-double bonds
  (trans-fats).

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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 CO groups are made
• This kind of reaction occurs when fats become
  rancid (decompose in air), producing compounds
  that have bad smells (short-chain aldehydes and
  carboxylic acids).