Chapter 19: Lipids - Detailed Notes

19.1 Structure and Classification of Lipids

  • A lipid is an organic compound found in living organisms.
    • Insoluble (or sparingly soluble) in water.
    • Soluble in non-polar organic solvents.
  • Lipids lack a common structural feature, unlike other biomolecules.
  • Classification is based on solubility, not functional groups.
    • Insoluble or sparingly soluble in water.
    • Soluble in non-polar organic solvents.

Five Categories of Lipids (based on function)

  • Energy-storage lipids: triacylglycerols
  • Membrane lipids: phospholipids, sphingoglycolipids, and cholesterol
  • Emulsification lipids: bile acids
  • Chemical messenger lipids: steroid hormones and eicosanoids
  • Protective-coating lipids: biological waxes

Structural Formulas of Lipids

  • Lipids exhibit structural diversity.
  • Some are esters, some are amides, some are alcohols (acyclic and cyclic), and some are polycyclic.

19.2 Types of Fatty Acids

  • Fatty acids are naturally occurring monocarboxylic acids with linear (unbranched) carbon chains.
  • They typically have an even number of carbon atoms.
    • Long-chain fatty acids: C<em>12−C</em>26C<em>{12} - C</em>{26}
    • Medium-chain fatty acids: C<em>6−C</em>11C<em>6 - C</em>{11}
    • Short-chain fatty acids: C<em>4−C</em>5C<em>4 - C</em>5
  • Two Types:
    • Saturated: all C-C bonds are single bonds.
    • Unsaturated:
      • Monounsaturated: one C=C bond.
      • Polyunsaturated: two or more C=C bonds (up to six double bonds).

Saturated Fatty Acids

  • Numbering starts from the end of the -COOH group.
  • Structural notation indicates the number of C atoms.
    • Example: Lauric acid has 12 C atoms and no double bonds, so it is (12:0).

Unsaturated Fatty Acids

  • A monounsaturated fatty acid has one carbon–carbon double bond in the carbon chain.
  • Different ways of depicting the structure are used.

Polyunsaturated Fatty Acids (PUFAs)

  • A polyunsaturated fatty acid has two or more carbon–carbon double bonds.
  • Up to six double bonds are found in biochemically important PUFAs.
  • Two types:
    • Omega (ω)-3 fatty acids: The endmost double bond is three carbon atoms away from the methyl end.
    • Omega (ω)-6 fatty acids: The endmost double bond is six carbon atoms away from the methyl end.

Selected Unsaturated Fatty Acids of Biological Importance

  • Numbering starts from the end opposite the -COOH group.
  • Structural notation indicates the number of C atoms and double bonds.
    • E.g., 18:2 means 18 carbons and 2 double bonds.

Omega Acids

  • Essential Fatty Acids: These must be part of the diet because they cannot be synthesized by the body.
  • Nutritionally important Omega-3 and Omega-6 fatty acids:
    • Linolenic acid: Omega-3
    • Linoleic acid: Omega-6
  • Linoleic Acid Deficiency:
    • Skin redness and irritation
    • Infections and dehydration
    • Liver abnormalities
    • Children are most susceptible.
    • Human milk has a higher concentration than cow’s milk.

American Diet

  • Sufficient in omega-6 fatty acids.
  • Deficient in omega-3 fatty acids.
    • Fish is a good source of omega-3 fatty acids.
  • A high rate of heart disease may be due to an imbalance in omega-3 and 6 fatty acids.
    • Ideal ratio: Omega 6 : Omega 3 (4 - 10 g: 1g).

19.3 Physical Properties of Fatty Acids

  • Water solubility:
    • Short-chain fatty acids have some solubility due to the polar carboxylic acid group.
    • Long-chain fatty acids are insoluble.
  • Physical properties like melting point depend on the number of C atoms and the degree of unsaturation.

The Melting Point

  • Depends upon:
    • Length of the carbon chain
    • Degree of unsaturation (number of double bonds).

Space-Filling Molecules

  • The number of bends in a fatty acid chain increases as the number of double bonds increases.
  • Less packing occurs.
  • Melting point is lower.
  • They tend to be liquids at room temperature.

19.4 Energy-Storage Lipids: Triacylglycerols

  • With the exception of nerve cells, human cells store small amounts of energy-providing materials.
    • The most widespread energy storage material is carbohydrate glycogen, but it is present in small amounts.
  • The primary storage material is triacylglycerols.
    • Triacylglycerols are concentrated primarily in special cells (adipocytes), nearly filling them.

Two Types of Triacylglycerols

  • Simple Triacylglycerols: Three identical fatty acids are esterified to glycerol.
    • Naturally occurring simple triacylglycerols are rare.
  • Mixed Triacylglycerols: A triester formed from the esterification of glycerol with more than one kind of fatty acid.
    • In nature, mostly mixed triacylglycerols are found, even from the same source, depending on the feed (e.g., corn, peanut, and wheat-fed cows have different triacylglycerols).

Difference Between Fats and Oils

  • Physical State:
    • Fats:
      • Predominantly saturated.
      • Solids or semi-solids at room temperature.
    • Oils:
      • Predominantly unsaturated.
      • Liquids at room temperature.
  • Source:
    • Fats: Animal source and tasteless.
    • Oils: Plants and fish oil.
    • Pure oils and fats are colorless and odorless.

19.5 Dietary Considerations and Triacylglycerols

  • Studies indicate that nations with high dietary intakes of fats and oils tend to have higher incidences of heart disease and certain types of cancers.
  • The typical American diet contains too much fat, so Americans are advised to reduce their total dietary fat intake.
  • Other studies show that the risk factor is more than just the total amount of triacylglycerols consumed.

“Good Fats” Versus “Bad Fats”

  • Studies indicate that the type and amount of dietary fat are important for a balanced diet.
    • Current recommended amounts:
      • Monounsaturated fat: 15% of total fat intake in calories
      • Polyunsaturated fat: 10%
      • Saturated fats: <10%
    • Saturated fats are considered “bad fats.”
    • Monounsaturated fats are considered “good fats.”
    • Trans-monounsaturated fats are considered “bad fats.”
    • Polyunsaturated fats can be both “good” and “bad.”
      • Omega-3 and 6 are important “good fats.”

Essential Fatty Acids

  • Fatty acids that must be obtained from dietary sources because they are not synthesized within the body.
  • Two most important essential fatty acids:
    • Linoleic acid (18:2) - omega-6
    • Linolenic acid (18:3) - omega-3
  • Both are needed for:
    • Proper membrane structure
    • Serving as starting materials for the production of several nutritionally important longer-chain omega-6 and omega-3 fatty acids.
  • Deficiencies of these acids may result in skin redness, infections, dehydration, and liver abnormalities.

Fat and Fatty Acid Composition of Nuts

  • Numerous studies indicate that eating nuts can have a strong protective effect against coronary heart disease.
    • Nuts have low amounts of saturated fatty acids.
    • Nuts also contain valuable antioxidant vitamins, minerals, and plant fiber protein.

19.6 Chemical Reactions of Triacylglycerols

  • Chemical Properties are due to two functional groups: esters and alkenes
    • Hydrolysis: Partial hydrolysis of triacylglycerols breaks 1-2 ester bonds to give rise to mono- or diacylglycerol and fatty acid(s).
    • Enzymes produced by the pancreas carry out the hydrolysis.

Saponification

  • Hydrolysis in basic solution produces the salt of a fatty acid and glycerol.
  • RCOOR′+NaOH→RCOONa (soap)+R′OHRCOOR' + NaOH \rightarrow RCOONa \text{ (soap)} + R'OH

Hydrogenation

  • Addition of hydrogen across a double (=) bond increases the degree of saturation.
  • Many food products are produced by partial hydrogenation of oils and fats.
    • Peanut oil + H2H_2 → Peanut Butter
    • Vegetable oil + H2H_2 → Margarine

Oxidation

  • Double bonds in triacylglycerols are subject to oxidation with oxygen in the air (an oxidizing agent), leading to C=C breakage.
  • Oxidation of alkenes may result in two short-chain molecules: an aldehyde or a carboxylic acid.
    • The aldehydes and/or carboxylic acids produced often have objectionable odors; fats and oils are said to be rancid.
    • Antioxidants are added as preservatives to avoid this unwanted oxidation process (e.g., Vitamin C and vitamin E).

19.7 Membrane Lipids: Phospholipids

  • All cells are surrounded by a membrane that confines their contents.
  • Up to 80% of the mass of a cell membrane can be lipid materials, primarily phospholipids.
  • A phospholipid contains one or more fatty acids, a phosphate group, a platform molecule (glycerol or sphingosine) to which the fatty acid(s) and the phosphate group are attached, and an alcohol attached to the phosphate group.

Glycerophospholipids

  • A glycerophospholipid is a lipid that contains two fatty acids and a phosphate group esterified to a glycerol molecule and an alcohol esterified to the phosphate group.
  • All attachments (bonds) between groups in a glycerophospholipid are ester linkages.
  • Glycerophospholipids have four ester linkages, compared to three in triacylglycerols.
  • Glycerophospholipids undergo hydrolysis and saponification reactions similar to triacylglycerols.
  • The alcohol attached to the phosphate group is usually one of three amino alcohols: choline, ethanolamine, or serine, leading to phosphatidylcholines, phosphatidylethanolamines, and phosphatidylserines.

Glycerophospholipids - Structure and Function

  • Structurally, glycerophospholipids are similar to triacylglycerols but have different biochemical functions.
    • Triacylglycerols serve as energy storage molecules.
    • Glycerophospholipids function as components of cell membranes.
  • A major structural difference is their "polarity."
    • Triacylglycerols are non-polar.
    • Glycerophospholipids are polar.

Sphingophospholipids

  • Structures are based on the 18-carbon monounsaturated aminodialcohol sphingosine.
  • They contain one fatty acid and one phosphate group attached to a sphingosine molecule, and an alcohol attached to the phosphate group.
  • Saponifiable lipids.
  • Sphingophospholipids in which the alcohol esterified to the phosphate group is choline are called sphingomyelins.
  • Sphingomyelins are found in all cell membranes and are important structural components of the myelin sheath of neurons.

19.8 Membrane Lipids: Sphingoglycolipids

  • Sphingoglycolipids contain both a fatty acid and a carbohydrate.
  • Simple sphingoglycolipids are called cerebrosides and contain a single monosaccharide unit (glucose or galactose).
    • They occur primarily in the brain (7% of dry mass).

Gangliosides

  • Complex sphingoglycolipids are called Gangliosides; they contain a branched chain of up to seven monosaccharide residues.
  • They occur in the gray matter of the brain as well as in the myelin sheath.

19.9 Membrane Lipids: Cholesterol

  • Lipids featuring fused rings.
  • Cholesterol: C27C_{27} steroid molecule.
  • A steroid is a lipid whose structure is based on a fused ring system of three 6-carbon rings and one 5-carbon ring.
  • Important in human cell membranes, nerve tissue, and brain tissue.
  • Important in chemical synthesis: hormones and vitamins essential for life.
  • Function: Third major type of membrane lipid

Cholesterol in Food

  • The liver synthesizes cholesterol: ~ 1g everyday, so dietary intake is not necessary.
  • Cholesterol synthesis decreases if it is ingested, but the reduction is not sufficient, leading to cardiovascular disease.
  • Animal food has a lot of cholesterol.
  • Plant food has no cholesterol.

19.10 Cell Membranes

  • Cells are surrounded by plasma membranes that separate the aqueous interior of a cell from the aqueous environment surrounding the cell.
  • Up to 80% of the plasma membrane is lipid material.
  • The membranes are lipid bilayers made up of phospholipids.
  • Bilayer structure: Nonpolar tails of phospholipids in the middle, and polar heads are on the surface.
  • 6 - 9 billionths of a meter thick or 6-9 nanometers thick.
  • The membrane is a liquid-like structure due to unsaturation in lipid tails.

Cholesterol in Cell Membranes

  • Cholesterol molecules are also components of plasma membranes.
  • Cholesterol helps regulate membrane fluidity.
    • The fused ring system does not allow rotation of fatty acid tails in the vicinity.
  • Fits between fatty acid chains of the lipid bilayer, making it rigid.
  • Cholesterol thus acts as a membrane plasticizer.

Proteins in Cell Membranes

  • The membranes also contain proteins that are responsible for moving substances such as nutrients and electrolytes across the membrane.
  • Receptors for hormones and neurotransmitters.
  • The membrane proteins and some lipids are further reacted with carbohydrate molecules.
    • Act as markers for cell recognition.

Passive Transport

  • Transport Across Cell Membranes: To maintain cellular processes, various molecules are transported across cell membranes.
  • Three types of transport:
    • Passive transport
    • Facilitated transport
    • Active transport
  • Passive transport: a substance moves across a cell membrane by diffusion from a region of higher concentration to a region of lower concentration.
    • Only a few types of molecules, including O<em>2O<em>2, N</em>2N</em>2, H2OH_2O, urea, and ethanol, can cross membranes by passive transport.

Facilitated Transport

  • Facilitated transport: a substance moves across a cell membrane with the aid of a membrane protein from a region of higher concentration to a region of lower concentration.
  • Specific protein carriers or transporters are involved in the process.

Active Transport

  • Active transport: a substance moves across a cell membrane, with the aid of membrane proteins, against a concentration gradient with the expenditure of cellular energy.
  • Proteins involved in active transport are called “pumps.” The needed energy is supplied by molecules such as ATP.

19.11 Emulsification Lipids: Bile Acids

  • An emulsifier is a substance that can disperse and stabilize water-insoluble substances as colloidal particles in an aqueous solution.
  • Bile Acids: Cholesterol derivatives that function as emulsifying agents that make dietary lipids soluble in the aqueous environment of the digestive tract.
    • Approximately one-third of cholesterol produced by the liver is converted to bile acids.
    • Their action is similar to soap in washing.

Bile Acids Structure

  • Bile acids are tri- or dihydroxy cholesterol derivatives.
  • The carbon 17 side chain of cholesterol has been oxidized to a carboxylic acid.
  • The oxidized acid side chain is bonded to an amino acid (either glycine or taurine) through an amide linkage.
  • Bile is a fluid containing emulsifying agents (bile acids) secreted by the liver, stored in the gallbladder, and released into the small intestine during digestion.

19.12 Messenger Lipids: Steroid Hormones

  • A hormone is a biochemical substance produced by a ductless gland that has a messenger function.
  • Hormones serve as a means of communication between various tissues.
    • Some hormones are lipids.
  • Lipids that play the role of “chemical messengers” include:
    • Steroid hormones: derivatives of cholesterol
    • Eicosanoids: derivatives of arachidonic acid
  • Two major classes of steroid hormones:
    • Sex hormones: control reproduction and secondary sex characteristics
    • Adrenocorticoid hormones: control numerous biochemical processes in the body

Sex Hormones

  • Classified into three major groups:
    • Estrogens: the female sex hormones
    • Androgens: the male sex hormones
    • Progestins: the pregnancy hormones

Adrenocorticoid Hormones

  • Produced by the adrenal glands, small organs located on top of each kidney.
  • 28 different hormones have been isolated from the adrenal cortex.
  • Two types of adrenocorticoid hormones:
    • Mineralocorticoids: control the balance of Na and K ions in cells
    • Glucocorticoids: control glucose metabolism and counteract inflammation

19.13 Messenger Lipids: Eicosanoids

  • Eicosanoids: Arachidonic acid (20:4) derivatives.
    • Have profound physiological effects at extremely low concentrations.
    • Eicosanoids are hormone-like molecules.
    • Exert their effects in the tissues where they are synthesized.
    • Eicosanoids usually have a very short “life.”
  • Physiological effects of eicosanoids:
    • Inflammatory response
    • Production of pain and fever
    • Regulation of blood pressure
    • Induction of blood clotting
    • Control of reproductive functions, such as induction of labor
    • Regulation of the sleep/wake cycle

Three Principle Types of Eicosanoids

  • Prostaglandins: C20C_{20}-fatty-acid derivative containing a cyclopentane ring and oxygen-containing functional groups.
    • Involved in raising body temperature,
    • Inhibiting the secretion of gastric juices,
    • Increasing the secretion of a protective mucus layer into the stomach,
    • Relaxing and contracting smooth muscle, directing water and electrolyte balance, intensifying pain, and enhancing inflammation responses.

Three Principle Types of Eicosanoids

  • Thromboxanes: C20C_{20}-fatty-acid derivative containing a cyclic ether ring and oxygen-containing functional groups.
    • Promote platelet aggregation.
  • Leukotrienes: C20C_{20}-fatty-acid derivative containing three conjugated double bonds and hydroxy groups.
    • Promote inflammatory and hypersensitivity (allergy) responses.

19.14 Protective-Coating Lipids: Biological Waxes

  • A biological wax: a monoester of a long-chain fatty acid and a long-chain alcohol.
  • The fatty acids found in biological waxes:
    • Generally are saturated fatty acids
    • Contain 14 to 36 carbon atoms.
  • The alcohols found in biological waxes:
    • May be saturated or unsaturated
    • May contain 16 to 30 carbon atoms.

Properties and Function of Biological Waxes

  • Properties: Water-insoluble and water-repellent because of long nonpolar hydrocarbon chains.
    • Humans and animals secrete biological waxes from skin glands.
  • Function of biological waxes:
    • Protect hair and skin and keep it pliable and lubricated.
    • Impart water repellency to animal fur.
    • Birds keep their feathers water repellent and help minimize loss of body heat.
    • Plants coat their leaves with a thin layer of biological waxes to prevent excessive evaporation of water and to protect against parasite attack.