Lipids and Fatty Acids: Structure, Classification, and Biological Roles

Les lipides / Lipids

Definition of Lipids

Lipids are a heterogeneous group of biological molecules that are insoluble in water but soluble in organic solvents (such as ether, chloroform, and benzene). They include fats, oils, waxes, certain vitamins, hormones, and most non-protein membrane components.

• Heterogeneous group: Lipids vary widely in structure and function.

• Insoluble in water: Due to their nonpolar nature.

• Soluble in organic solvents: Such as chloroform, ether, and benzene.

Examples: Butter, oils, margarine, steroids, and fat-soluble vitamins (A, D, E, K).

Biological Role of Lipids

• Source of Energy: 1 g of fat provides 9 kcal (energy reserve).

• Constituents of Biological Membranes: Essential for membrane structure and function.

• Precursors of Biological Molecules: Steroids and eicosanoids (e.g., prostaglandins).

Example: Cholesterol is a precursor for steroid hormones; prostaglandins are derived from fatty acids and play roles in inflammation and signaling.

Classification of Lipids

Lipids are classified based on their structure and function:

Main Class	Subclasses	Examples
True Lipids (Saponifiable, based on fatty acids)	Fatty acids, Glycerides, Waxes, Sterols	Triglycerides, cholesterol
Lipid Compounds (Non-saponifiable)	Eicosanoids, Isoprene lipids (terpenoids, quinones, steroids)	Prostaglandins, vitamins A/E/K, corticosteroids
Complex Lipids	Glycerophospholipids, Glycolipids	Phosphatidylcholine, cerebrosides

Additional info: Saponifiable lipids can be hydrolyzed to yield fatty acids, while non-saponifiable lipids cannot.

Fatty Acids (Acides Gras)

Definition

Fatty acids are unbranched linear chain monocarboxylic acids that may be saturated or unsaturated. They usually have an even number of carbon atoms (from 4 to 32) and are found in the free state in small quantities.

• General formula: $\mathrm{CH_3-(CH_2)_n-COOH}$

• Polar carboxylic group and a nonpolar hydrocarbon chain

Saturated Fatty Acids

• No double bonds in the hydrocarbon chain.

• General formula: $\mathrm{C_nH_{2n}O_2}$

• n is an even number (4–32).

• Examples:

  • Butyric acid (C4:0)

  • Palmitic acid (C16:0)

  • Stearic acid (C18:0)

  • Lignoceric acid (C24:0)

• Nomenclature: Based on chain length and saturation (e.g., n-hexadecanoic acid for palmitic acid).

Example: Palmitic acid: $\mathrm{CH_3-(CH_2)_{14}-COOH}$

Branched-Chain Saturated Fatty Acids

• Generally have only one branch.

• Example: Tuberculostearic acid (10-methyl-octadecanoic acid): 18 carbons, 1 methyl group at position 10.

Describing Saturated Fatty Acids

Descriptor	Example (Palmitic acid)
Common name	Palmitic acid
Symbol	C16:0
Systematic name	n-hexadecanoic acid
Molecular formula	C16H32O2
Structural formula	CH3-(CH2)14-COOH

Unsaturated Fatty Acids

• Contain at least one double bond in the aliphatic chain.

• Mono-unsaturated: One double bond (e.g., oleic acid).

• Polyunsaturated: Two or more double bonds (e.g., linoleic acid, linolenic acid).

Nomenclature:

• Chemical: $C_{18:1}^{\Delta 9}$ (18 carbons, 1 double bond at C9 from COOH end)

• Biochemical: $C_{18:1} \omega 9$ (numbering from terminal methyl, omega carbon)

Systematic Nomenclature for Unsaturated Fatty Acids

• Prefix for chain length

• Number of carbons carrying double bonds

• "Mono" or "di" for number of double bonds

• Suffix "enoic" for unsaturation

• Carboxylic function indicated by "acid"

Cis and Trans Configurations

• Cis (Z): Hydrogens on the same side of the double bond; causes a kink in the chain.

• Trans (E): Hydrogens on opposite sides; chain remains straight.

• Natural fatty acids are primarily in the cis configuration.

Example: Oleic acid is a cis-monounsaturated fatty acid.

Health Implications of Fatty Acid Configuration

• Trans fats (produced by hydrogenation) increase the risk of cardiovascular disease by raising LDL ("bad") cholesterol.

• Cis fats are generally considered healthier and are predominant in natural sources.

Examples of Unsaturated Fatty Acids

• Oleic acid (omega-9): 18C, one double bond at C9 ($C_{18:1}^{\Delta 9}$ or $C_{18:1} \omega 9$)

• Linoleic acid (omega-6): 18C, two double bonds at C9 and C12 ($C_{18:2}^{\Delta 9,12}$ or $C_{18:2} \omega 6$)

• Arachidonic acid (omega-6): 20C, four double bonds at C5, C8, C11, C14 ($C_{20:4}^{\Delta 5,8,11,14}$ or $C_{20:4} \omega 6$)

• Linolenic acid (omega-3): 18C, three double bonds at C9, C12, C15 ($C_{18:3}^{\Delta 9,12,15}$ or $C_{18:3} \omega 3$)

Special Positions of Double Bonds

• Malonic position: Double bonds separated by one CH2 group (common in PUFAs).

• Succinic position: Double bonds separated by two CH2 groups (rare).

• Conjugated position: Double bonds separated by one single bond (rare, e.g., conjugated linoleic acid, CLA).

Summary Table: Common Fatty Acids

Name	Carbons:Double Bonds	Systematic Name	Omega Family
Palmitic acid	16:0	n-hexadecanoic acid	—
Stearic acid	18:0	n-octadecanoic acid	—
Oleic acid	18:1	cis-Δ9-octadecenoic acid	ω9
Linoleic acid	18:2	cis, cis-Δ9,12-octadecadienoic acid	ω6
Arachidonic acid	20:4	cis, cis, cis, cis-Δ5,8,11,14-eicosatetraenoic acid	ω6
Linolenic acid	18:3	cis, cis, cis-Δ9,12,15-octadecatrienoic acid	ω3

Health and Dietary Implications

• Excess intake of saturated and trans fats is associated with increased risk of cardiovascular disease.

• Polyunsaturated fatty acids (PUFAs), especially omega-3 and omega-6, are essential for human health and must be obtained from the diet.

Example: A study showed that excess palm oil (rich in saturated fat) increases visceral fat, while sunflower oil (rich in unsaturated fat) increases lean mass.