Lipids: Structure, Classification and Function

Four Bioorganic Classes

• Lipids are a bioorganic class, along with carbohydrates, proteins, and nucleic acids
• Lipids/fats store chemical energy and carbon efficiently within the body
• Fats protect vital organs by surrounding and insulating them
• Phospholipids, glycolipids, and cholesterol are key components of cell membranes
• Some cholesterol derivatives act as chemical messengers (hormones)

Lipids: Structure and Classification

• Lipids lack a common structural feature
• Solubility determines their classification
• Lipids are organic compounds in living organisms and insoluble (or sparingly soluble) in water
• Lipids are soluble in nonpolar organic solvents

Diversity & Classification

• Lipids exhibit structural diversity of molecules
• Esters, amides, alcohols, acyclic, cyclic, and polycyclic structures can all be lipids
• Solubility is the defining characteristic, with all lipids being insoluble in water
• Lipids are classified into families using two common methods:
  • The biochemical function of the lipid
  • Or whether or not a lipid can be broken down into smaller units through basic hydrolysis
• Lipids are divided into five categories based on biochemical function:
  • Energy-storage lipids (triacylglycerols)
  • Membrane lipids (phospholipids, sphingoglycolipids, and cholesterol)
  • Emulsification lipids (bile acids)
  • Messenger lipids (steroid hormones and eicosanoids)
  • Protective-coating lipids (biological waxes)

Saponification

• Lipids are divided into two categories based on saponification in a basic aqueous solution:
  • Saponifiable lipids (triacylglycerols, phospholipids, sphingoglycolipids, and biological waxes)
  • Nonsaponifiable lipids (cholesterol, steroid hormones, bile acids, and eicosanoids)
• Saponifiable lipids convert into two or more smaller molecules through hydrolysis
• Nonsaponifiable lipids cannot be broken down this way, since they do not react with water

Carbohydrate & Lipid parallels

• Monosaccharides are the basic structural unit in carbohydrate chemistry
• Basic "building blocks" exist for lipid molecules
• A frequently encountered lipid building block is a fatty acid
• All energy-storage lipids contain fatty acid building blocks
• Most membrane lipids contain this building block

Fatty Acids

• A fatty acid is a naturally occurring monocarboxylic acid
• Biosynthesis results in an even number of carbon atoms and an unbranched carbon chain.
• Fatty acids are characterized as long-chain (C12 to C26), medium-chain (C8 and C10), or short-chain (C4 and C6) based on carbon chain length
• Fatty acids are rarely found free in nature and occur as parts of complex lipid molecules

SFAs, MUFAs, PUFAs

• The carbon chain of a fatty acid may or may not contain carbon-carbon double bonds
• Fatty acids are classified
  • As saturated fatty acids (SFAs)
  • Monounsaturated fatty acids (MUFAs)
  • Or polyunsaturated fatty acids (PUFAs).
• Over 500 different fatty acids have been isolated from microorganisms, plants, animals, & humans
• These fatty acids differ in:
  • Carbon chain length
  • Degree of unsaturation (number of double bonds)
  • Positions of double bonds in the chains.
• Naturally occurring lipids almost always have:
  • An unbranched carbon chain
  • An even number of carbon atoms
  • Double bonds, when present, in a cis configuration

Palmitic Acid

• A saturated fatty acid has a carbon chain with all carbon-carbon single bonds.
• The structural formula for the 16-carbon SFAs.
• The IUPAC name is hexadecanoic acid
• The Common name is palmitic acid
• Palmitic acid is usually written in a more condensed form

MUFAs

• A monounsaturated fatty acid has a carbon chain with one carbon-carbon double bond
• In biochemically important MUFAs, the configuration about the double bond is nearly always cis
• Different depiction exists

cis double bonds

• The presence of a cis double bond in the carbon chain puts a rigid 30° bend in the chain
• The bend affects the physical properties of a fatty acid

PUFAs

• A polyunsaturated fatty acid has a carbon chain in which two or more carbon-carbon double bonds are present.
• Up to six double bonds exist in biochemically important PUFAs
• Fatty acids and IUPAC names are nearly always referred to using their common names.

Unsaturated Fatty Acids and Double-Bond Position

• Numerical shorthand specifies structural parameters for fatty acids.
• Two numbers separated by a colon are used to specify carbon atoms and carbon-carbon double bonds
• 18:0 denotes a C18 fatty acid with no double bonds
• 18:2 signifies a C18 fatty acid in which two double bonds are present
• Double-bond positioning within the carbon chain is specified by adding the Greek capital letter delta (Δ) with superscript numbers
  • 18:3(Δ9,12,15) is a C18 PUFA with three double bonds at locations between carbons 9 and 10, 12 and 13, and 15 and 16.
• MUFAs are usually Δ9 acids
• PUFAs are generally at the Δ12 and Δ15 locations
• Exception: biochemically important arachidonic acid, a PUFA with structural parameters 20:4(Δ 5,8,11,14)

Families of Unsaturated Fatty acids

• Several Different exists
• These family relationships appear if double-bond position is specified relative to the noncarboxyl end of the fatty acid's carbon chain
• Double-bond positioning using the Greek lowercase letter omega (ω):
  • An omega-3 fatty acid has its endmost double bond three carbon atoms from its methyl end

Organisms & Fatty Acids

• Different organisms differ on fatty acid characteristic
  • Humans and animals have 5%-7% of fatty acids with 20 or 22 carbon atoms, while fish have 25%-30%
  • Humans and animals have less than 1% of their fatty acids with 5-6 double bonds, while plants have 5%-6%, and fish 15%-30%

Physical Properties & Solubility

• Physical properties are largely determined by carbon chain length and degree of unsaturation
• Water solubility is a function of carbon chain length
  • Solubility decreases as carbon chain length increases
  • Short-chain fatty acids have a slight solubility in water
  • Long-chain fatty acids are essentially insoluble in water
  • Short-chain is related to the polarity of the carboxyl group
  • Nonpolar nature of the hydrocarbon chain dominates in longer-chain fatty acids

Melting Points

• Strongly influenced by carbon chain length and degree of unsaturation
• Melting point increases as carbon chain length increases
• Saturated fatty acids have higher melting points than unsaturated fatty acids with the same # of carbon atoms
• The greater the unsaturation, the greater the reduction in melting points
• Long-chain saturated fatty acids tend to be solids at room temperature
• Long-chain unsaturated fatty acids tend to be liquids

Molecular Actions

• Decreased molecular attractions explain the decreasing melting points associated with increasing unsaturation
• Double bonds generally have the cis configuration
• Bends exist in the carbon chains and prevent fat molecule pack ability

Triacylglycerols

• Human cells store small amounts of energy-providing materials when energy demand is high
• Most common energy-storage material within cells is the carbohydrate glycogen, present in small amounts
• Lipids known as triacylglycerols function within the body for energy-storage
• They are primarily concentrated in adipocytes
• Triacylglycerols are more efficient at storing energy because large quantities pack in smal
• Energy-storage lipids are the most abundant type of lipid in the human body
• Triacylglycerols are triesters; with three ester functional groups
• Triacylglycerol formation is always glycerol (a three-carbon alcohol with three hydroxyl groups)

Glycerol

• Fatty acids forming triacylglycerol are the carboxylic acids
• The esterification reaction produces a single molecule of glycerol that reacts with three fatty acid molecules, each esterified

Representation

• Each of the fatty acids is attached to glycerol through an ester linkage
• A triacylglycerol is a lipid formed by esterification of three fatty acids to a glycerol molecule
• An acyl group is the portion of a carboxylic acid that remains after the -OH group is removed from the carboxyl carbon atom.
• Triacylglycerol molecules contain three fatty acid residues (three acyl groups) attached to a glycerol residue
• Older name that is still frequently used for a triacylglycerol is triglyceride

Types of Triaclyglycerols

• The triacylglycerol produced from glycerol and three molecules of stearic acid.
• A simple triacylglycerol is a triester formed from the esterification of glycerol with three identical fatty acid molecules
• If the reacting fatty acid molecules are not all identical, then the result is a mixed triacylglycerol
• A mixed triacylglycerol is a triester formed from the esterification of glycerol with more than one kind of fatty acid molecule

Fats and Oils

• Fats are naturally occurring mixtures of triacylglycerol molecules
• Oils are also naturally occurring mixtures of triacylglycerol molecules
• Physical state at room temperature distinguishes a fat from an oil
  • A fat is a triacylglycerol mixture that is a solid or a semi-solid at room temperature (25°C). Generally, fats are obtained from animal sources.
  • An oil is a triacylglycerol mixture that is a liquid at room temperature (25°C). Generally, oils are obtained from plant sources.
• Fats are composed of triacylglycerols, saturated fatty acids predominate
• Oils contain triacylglycerols with larger amounts of mono- and polyunsaturated fatty acids

Health Factors

• Nations whose citizens have high intakes of triacylglycerols (fats and oils) tend to have higher incidences of heart disease and cancers
• Mediterranean countries and the Inuit people of Greenland are exceptions, and diet impacts risk factors.

Current Diet Fact Recommendations

• Limit total fat intake to 30% of total calories
• Up to 15% from monounsaturated fat
• Up to 10% from polyunsaturated fat
• Less than 10% from saturated fats

Dietary Fat

• Ongoing studies show that the type and amount consumed determines responses to dietary fat
• High/small amount of extra-virgin olive oil helps reduce medication by 48%
• Substitution with sunflower reduces this to 4%