Study Notes: Lipid Structures and Nomenclature

Lipid Structures and Nomenclature

I. Fatty Acids: Structures and Naming

Fatty acids are carboxylic acids with long hydrocarbon chains. They are fundamental building blocks of many lipids and are classified as saturated or unsaturated based on the presence of double bonds.

• Saturated Fatty Acids: No double bonds in the hydrocarbon chain. Examples include:

  • Lauric acid: Formula C12:0, acyl group name lauryl

  • Myristic acid: Formula C14:0, acyl group name myristyl

  • Palmitic acid: Formula C16:0, acyl group name palmitoyl

  • Stearic acid: Formula C18:0, acyl group name stearoyl

• Unsaturated Fatty Acids: Contain one or more double bonds. Examples include:

  • Palmitoleic acid: Formula C16:1(Δ9) or C16:1ω7, acyl group name palmitoleoyl

  • Linoleic acid: Formula C18:2(Δ9,12) or C18:2ω6,9, acyl group name linoleoyl

  • Linolenic acid: Formula C18:3(Δ9,12,15) or C18:3ω3,6,9, acyl group name linolenyl

  • Arachidonic acid: Formula C20:4(Δ5,8,11,14) or C20:4ω5,8,11,14, acyl group name arachidonyl

Key Points:

• Fatty acids are named using both delta (Δ) and omega (ω) nomenclature, indicating the position of double bonds.

• Acyl group names are derived from the parent fatty acid name.

• You should be able to name any lipid given its CN: format, Δ, or ω nomenclature.

Example:

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

• Linoleic acid: $\mathrm{CH_3(CH_2)_4CH=CHCH_2CH=CH(CH_2)_7COOH}$

II. Triacylglycerols (TAGs): Structure and Naming

Triacylglycerols are esters formed from glycerol and three fatty acids. They are the main storage form of energy in animals.

• Structure: Glycerol backbone esterified with three fatty acids.

• Naming: Named by specifying the acyl groups at each position (e.g., 1,2-dipalmitoyl-3-oleoyl glycerol).

Example:

• 1,2-dipalmitoyl-3-oleoyl glycerol

• Tristearin: All three positions occupied by stearic acid.

Formula:

• $\mathrm{TAG = Glycerol + 3 Fatty Acids}$

III. Glycerophospholipids: Recognition and Structure

Glycerophospholipids are major components of cell membranes, consisting of a glycerol backbone, two fatty acids, and a phosphate group with a head group.

• Common Types:

  • Phosphatidylcholine (lecithin)

  • Phosphatidylethanolamine

  • Phosphatidylserine

  • Phosphatidylinositol

  • Phosphatidylglycerol

  • Phosphatidic acid

• Structure: Fatty acids at sn-1 and sn-2 positions, phosphate at sn-3 position.

• Head groups: Choline, ethanolamine, serine, inositol, glycerol.

Example:

• 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine

IV. Ether Glycerophospholipids and Plasmalogens

Ether glycerophospholipids have an ether linkage at the sn-1 position of glycerol. Plasmalogens are a subclass with a vinyl ether linkage.

• Key Features:

  • Ether linkage at sn-1 position (alkyl or alkenyl group)

  • Fatty acid at sn-2 position

  • Phosphocholine or other head group at sn-3 position

• Plasmalogens: Contain a vinyl ether at sn-1 and are abundant in heart and brain tissue.

Example:

• Ether phospholipid with a 16-carbon ether tail and acetyl group at C2

• Plasmalogen with a 14-carbon ether tail and myristoyl group at C2

V. Sphingolipids: Structure and Types

Sphingolipids are based on a sphingosine backbone and play key roles in cell recognition and signaling.

• Types:

  • Sphingomyelin: Sphingosine + fatty acid + phosphocholine head group

  • Glycosphingolipids: Sphingosine + fatty acid + carbohydrate head group

  • Cerebroside: Sphingosine + fatty acid + single sugar (e.g., glucose or galactose)

  • Globoside: Sphingosine + fatty acid + multiple sugars

  • Ganglioside: Sphingosine + fatty acid + oligosaccharide with sialic acid

• Key Point: Gangliosides contain sialic acid; globosides do not.

Example:

• Sphingomyelin: Contains a phosphocholine head group

• Globoside: Contains multiple sugars, no sialic acid

• Ganglioside: Contains oligosaccharide with sialic acid

VI. Biochemical Waxes

Biochemical waxes are esters of long-chain fatty acids and long-chain alcohols. They serve as protective coatings in plants and animals.

• Structure: R-COO-R', where R and R' are long hydrocarbon chains.

• Example: Biological wax made from palmitic acid.

Formula:

• $\mathrm{CH_3(CH_2)_{14}COO(CH_2)_{29}CH_3}$

VII. Hydrophobic and Hydrophilic Regions in Lipids

Lipids have distinct hydrophobic (nonpolar) and hydrophilic (polar) regions, which are crucial for membrane formation.

• Hydrophobic region: Long hydrocarbon chains

• Hydrophilic region: Polar head groups (e.g., phosphate, choline, sugar)

VIII. Deducing Lipid Structure from Hydrolysis Products

Lipid hydrolysis yields characteristic products that can be used to deduce the original lipid's structure.

• Example: Hydrolysis yielding glycerol, phosphate, choline, and two equivalents of oleic acid indicates lecithin (phosphatidylcholine).

Table: Fatty Acid Nomenclature and Acyl Group Names

Name	Formula	Delta Nomenclature	Omega Nomenclature	Acyl Group Name
Lauric acid	C12:0	C12:0	C12:0	lauryl
Myristic acid	C14:0	C14:0	C14:0	myristyl
Palmitic acid	C16:0	C16:0	C16:0	palmitoyl
Stearic acid	C18:0	C18:0	C18:0	stearoyl
Palmitoleic acid	C16:1	C16:1Δ9	C16:1ω7	palmitoleoyl
Linoleic acid	C18:2	C18:2Δ9,12	C18:2ω6,9	linoleoyl
Linolenic acid	C18:3	C18:3Δ9,12,15	C18:3ω3,6,9	linolenyl
Arachidonic acid	C20:4	C20:4Δ5,8,11,14	C20:4ω5,8,11,14	arachidonyl

Additional info:

• These notes are most relevant for biochemistry or organic chemistry courses, but the molecular structure and physical properties of lipids are also important in biophysics and physical chemistry.

• Understanding lipid structure is essential for topics such as membrane biophysics, lipid bilayer formation, and energy storage in biological systems.