Lipid Classification: Fatty Acids

Classification of Lipids

• Lipids are divided into eight categories, including fatty acids, glycerolipids, glycerophospholipids, sterol lipids, prenol lipids, sphingolipids, saccharolipids, and polyketides.

Fatty Acids Composition

• Fatty acids consist of a hydrocarbon chain with a carboxylic acid group at the end.
• Fatty acids have two ends: a polar (hydrophilic) end, and a nonpolar (hydrophobic) one
• Length typically ranges between 4 and 24 carbons.
• They can be saturated or unsaturated.
• Fatty acids may have functional groups containing oxygen, halogens, nitrogen, and sulfur.

Cis and Trans Configuration

• Fatty acids which contain a double bond can be in a cis or trans configuration
• cis configuration: functional groups are on the same side of the carbon chain
• trans configuration: functional groups are on opposing sides of the carbon chain
• Most natural fatty acids have a cis configuration.
• trans fats and oils have higher melting points than cis fats because their packing is not affected as much.
• Naturally occurring unsaturated fats and oils tend to be in the cis form.
• Eicosanoids are biologically important fatty acids that mediate inflammation, tissue homeostasis, and blood clotting

Isomerism

• Two types of isomers can occur in an unsaturated fatty acid: geometric isomers.
• Geometric Isomers depend on the orientation of the radicals around the axis of the double bonds.
• 'cis' form: the H are on the same side of the bond
• 'trans' form: the H are on the opposite side
• 'Cis' form is comparatively unstable and more reactive.
• Oleic acid and elaidic acid have the same molecular formula: C17H33COOH.

Common Properties of Lipids

• Lipids float on water
• Lipids are insoluble in water
• Lipids are greasy to the touch and lubricating
• Lipids are not readily volatile
• Lipids burn without leaving residue
• Lipids have low density, melting points, and boiling points

Physical Properties of Fatty Acids

• The melting point of saturated fatty acids increases with carbon atoms in the chain.
• Butyric acid has a MP of 8°C, Palmitic acid's MP is 62°C, and Stearic acid shows 70°C.
• Higher saturation and increased chain length result in a higher melting point.
• Fatty acid solubility increases with each double bond.
• The solubility decreases with increased chain length because Methylene groups increase

Chemical Properties of Fatty Acids

• Fatty acids form salts with alkali and alkaline earth metals producing salts of Na, K, Ca and Mg, which are soaps
• Sodium and potassium salts are soluble; calcium and magnesium salts are insoluble.
• Reduction of COOH in fatty acids produces alkyl alcohols, sulfated into alkyl sulfates as detergents.
• Fatty acids combine with alcohol to form esters like mono-, di-, and tri-glycerides.

Eicosanoids

• Eicosanoid is a collective term for oxygenated derivatives of 20-carbon fatty acids
• Eicosapentaenoic acid (EPA) is an ω-3 fatty acid with 5 double bonds.
• Arachidonic acid (AA) is an ω-6 fatty acid with 4 double bonds.
• Dihomo-gamma-linolenic acid (DGLA) is an ω-6 fatty acid with 3 double bonds.
• Oxidation of 20-carbon fatty acids created signalling molecules

Eicosapentaenoic acid (EPA)

• Omega-3 fatty acid found in fish oils, benefiting heart health, eases blood flow, reduces triglycerides, pain, and swelling.

Arachidonic acid (AA)

• Polyunsaturated omega-6 fatty acid; if the diet contains linoleic acid, AA forms to create leukotrienes, prostaglandins, and thromboxanes

Dihomo-y-linolenic acid (DGLA)

• 20-carbon carboxylic ω-6 fatty acid, helps the immune system, prevents depressions, and develops nervous system in infants

Formation of Eicosanoids

• Eicosanoids involve prostaglandins and related compounds derived from arachidonic acid, a 20-carbon polyunsaturated fatty acid.
• Three classes: prostaglandins, thromboxanes, and leukotrienes.
• Prostaglandins: hormone-like compounds that affect Blood clotting, Inflammation, Menstruation, Labor, and Tissue damage.
• Thromboxanes: Lipids produced by platelets and endothelial cells that are involved with blood clotting, vasoconstriction, and platelet aggregation.
• Leukotrienes: inflammatory mediators produced by leukocytes, involved in asthma, allergic inflammation, and innate immunity

Biological processes Regulated by Eicosanoids

• Blood clotting is regulated by thromboxane A2 constricting blood vessels and platelet aggregation and prostacyclin dilating blood vessels and inhibiting platelet aggregation.
• Inflammatory response is mediated by prostaglandins
• Reproductive system is stimulated by smooth muscle by PGE2
• Gastrointestinal tract is aided by prostaglandins inhibiting gastric secretion, and Prostaglandins increasing secretion of protective mucus.
• Kidneys function by prostaglandins dilating renal blood vessels which results in increased water and electrolyte excretion
• Respiratory tract which leukotrienes promote the constriction of bronchi while prostaglandins promote bronchodilation

Chemical Properties of Lipids

• Hydrogenation: saturates double bonds in oils by adding hydrogen (hardening of oils).
• Halogenation: unsaturates double bonds in fatty acids (F, I, and Br halogens).
• Iodine Number: grams of iodine double bonds in 100 grams of fat

Lipid Oxidation

• Double bonds in unsaturated fatty acids are oxidizable.
• Spontaneous oxidation forms transparent coating on a surface, used in paint manufacturing.

Chemical Properties - Rancidity

• Rancidity occurs the oxygen in air affects lipid-rich foods by spoiling
• The unpleasant taste and smell is caused by oxidative cleavage of double bonds, thus volatility increases.

Improving Shelf Life

• Improves the shelf life of commercial vegetable oils used by employing partial hydrogenation to enhance stability.
• Converts cis double bonds into saturated single bonds, increasing oils melting point

Undesirable Hydrogenation

• Partial hydrogenation converts cis double bonds to trans double bonds.
• Dietary trans fatty acids raise triacylglycerols and LDL, lowering HDL, elevating coronary heart disease risk.

Essential Fatty Acids

• Polyunsaturated fatty acids that consist of linoleic acid, linolenic acid and arachidonic acid.
• They cannot be produced by the body and must be obtained through diet.
• EFA deficiency results in growth retardation and deficiency symptoms.

Docosahexaenoic Acid (DHA)

• Polyunsaturated fatty acid synthesized from α-linolenic acid or from fish oil.
• Concentrations in retina, cerebral cortex, testes, and sperm, needed for brain development via placenta and breast milk

Alcohols

• Glycerol, cholesterol, and higher alcohols (e.g., cetylalcohol)
• Glycerol is called glycerin, simplest trihydric alcohol with hydroxyl groups in molecule

Complex Lipids

• Complex lipids are divided into three subgroups: phospholipids, glycolipids, and lipoamino acids.
• Phospholipids: have a phosphorous base, found in the brain and include lecithin, cephalin and sphingomyelin.
• Glycolipids: consist of long-chain sugar derivatives with base of gylcerol in plants & bacteria or a ceramide in animals.
• Lipoamino acids: amino acids, linked into a long-chain alcohol and glycerolipid

Characteristics of Complex Lipids:

• They’re generally insoluble in water, but soluble in organic solvents.
• They present polar properties and include chemicals with fatty acids, alcohols, phosphates, and nitrogenous bases.

Fats vs Lipids

• Fats are triglycerides
• Lipids include fatty acids, phospholipids, cholesterols, sphingolipids, vitamin A, D, etc.

Fats vs Oil

• Fats are solid at room temperature, and are saturated.
• Oils are liquid at room temperature and are unsaturated.

Cholesterol Function

• Required for membranes construction & maintenance via hydroxyl group interaction with polar membrane phospholipids/sphingolipids and bulky hydrocarbon chain embedded in membrane like other lipids.
• Cholesterol increases membrane packing and reduces membrane fluidity via fatty-acid chains

Bile Acids

• Steroid acids dominant in mammals & other vertebrates bile.
• Bile Salts are bile acids with positive ions, usually sodium.
• Primary bile acids are synthesized by the liver
• Secondary bile acids come from bacterial activity in the colon.
• In humans, taurocholic & glycocholic acid (cholic acid derivatives) & taurochenodeoxycholic & glycochenodeoxycholic acid (chenodeoxycholic acid derivatives) make approx 80% of bile salts
• Conjugated salts also found in human bile include 7-alpha dehydroxylated derivatives, deoxycholic, lithocholic acid

Bile Self Study

• Bile contains 80% organic compounds (with phospholipids & cholesterol)
• Increased secretion increases bile flow
• Aids micelles formation for digestion, fat absorption, plus hormonal effects.
• Bile salts include five/eight-carbon side-chain steroid w/ four rings ending in carboxylic acid & hydroxyl groups.