Ch. 17: Lipids

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Lipids

Introduction to Lipids

Lipids are a diverse class of naturally-occurring organic compounds grouped together based on their common solubility properties rather than a shared structure. They are generally insoluble in water but soluble in nonpolar solvents.

Key types of lipids:
- Fatty acids, triglycerides, sphingolipids, phospholipids, and glycolipids
- Lipid-soluble vitamins
- Prostaglandins, leukotrienes, and thromboxanes
- Cholesterol, steroid hormones, and bile acids

Classification of Lipids

Simple and Complex Lipids

Lipids can be classified as simple or complex based on their structure and composition.

Simple lipids: Esters of fatty acids with various alcohols (e.g., fats and oils, waxes)
Complex lipids: Contain additional functional groups (e.g., phospholipids, glycolipids)

Diagram: Simple lipids include fats and waxes; complex lipids include phospholipids and glycolipids. (See textbook for structural diagrams.)

Fatty Acids

Structure and Properties

Fatty acids are the building blocks of many lipids. They are primarily unbranched carboxylic acids with long hydrocarbon chains.

Typically contain 10 to 20 carbon atoms
Usually have an even number of carbon atoms
Except for the carboxyl group, they have no additional functional groups
In unsaturated fatty acids, the double bond is usually in the cis configuration
Two main nomenclature systems: common names and the "two-number" system (e.g., 18:1 for oleic acid)

Types of Fatty Acids

Saturated fatty acids: No carbon-carbon double bonds
Monounsaturated fatty acids: One carbon-carbon double bond (usually cis)
Polyunsaturated fatty acids: Multiple cis carbon-carbon double bonds

Most Common Fatty Acids

The four most common fatty acids found in nature are:

Palmitic acid (palmitate): 16:0
Stearic acid (stearate): 18:0
Oleic acid (oleate): 18:1
Linoleic acid (linoleate): 18:2

For unsaturated fatty acids, the double bonds are typically in the cis configuration.

Fatty Acid Isomerism: Cis vs. Trans

Cis-bond: Causes a kink (bend) in the hydrocarbon tail, affecting packing and melting point
Trans-bond: Almost never produced naturally; found in artificially hydrogenated oils; results in a straighter chain

Melting Points of Saturated vs. Unsaturated Fatty Acids

Saturated fatty acids: Pack tightly, have higher melting points due to strong London dispersion forces
Unsaturated fatty acids: Kinks from cis double bonds prevent tight packing, resulting in lower melting points

Effects of Trans-Fatty Acids

Trans-fats are associated with several negative health effects:

Increased levels of triglycerides
Contribute to hardening of arteries (atherosclerosis)
May increase inflammation
Damage to blood vessel linings, promoting heart disease

Omega Fatty Acids

Classification and Importance

Omega (ω) fatty acids are classified by the position of the first double bond from the methyl (omega) end of the fatty acid chain.

Omega-3 fatty acids: First double bond at the third carbon from the omega end
Omega-6 fatty acids: First double bond at the sixth carbon from the omega end
Most omega-3 fatty acids are polyunsaturated and have all double bonds in the cis configuration

Table: Examples of Omega Fatty Acids

Type	Example	Double Bond Position
Omega-3	α-Linolenic acid	3rd carbon from omega end
Omega-6	Linoleic acid	6th carbon from omega end
Omega-9	Oleic acid	9th carbon from omega end

Omega-3 Fatty Acids and Nutrition

Omega-3 fatty acids are essential for normal growth and health. They play roles in cardiovascular health, anti-inflammatory activity, and may have anti-cancer benefits.

Essential for normal growth (1–2% of total caloric intake)
Improve cardiovascular health
Anti-inflammatory and anti-anxiety effects
May reduce risk of certain cancers
Improve immune function and maturation

Balance of Omega-3, -6, and -9 Fatty Acids

A proper balance is necessary for good health
Omega-3 fatty acids may inhibit cyclooxygenase (COX) enzymes, reducing inflammation
Prostaglandins are important signaling molecules synthesized from fatty acids

Prostaglandins

Structure and Function

Prostaglandins are a family of compounds with a 20-carbon skeleton derived from prostanoic acid. They are powerful agents in the body, regulating smooth muscle activity, glandular secretions, blood clotting, and inflammation.

Regulate activity of smooth muscles (e.g., blood pressure)
Induce or prevent glandular secretions
Affect blood clotting and flow
Responsible for fevers and inflammation

Synthesis

Not stored in tissues; synthesized from membrane-bound polyunsaturated fatty acids (e.g., arachidonic acid) in response to physiological triggers

COX Enzymes

The cyclooxygenase (COX) enzyme catalyzes the production of prostaglandins. There are two forms:

COX-1 (constitutive): Normal physiological production
COX-2 (inducible): Production in response to inflammation or tissue injury

Aspirin and other NSAIDs inhibit COX enzymes, reducing inflammation.

Thromboxanes

Thromboxanes are derived from arachidonic acid via COX enzymes. They induce platelet aggregation and vasoconstriction, playing a key role in blood clotting.

Aspirin and NSAIDs inhibit thromboxane synthesis, which is why "aspirin therapy" is used for heart conditions

Summary Table: Key Lipid Types and Functions

Lipid Type	Main Function	Example
Fatty acids	Energy storage, membrane structure	Palmitic acid
Triglycerides	Long-term energy storage	Animal fats, vegetable oils
Phospholipids	Membrane structure	Lecithin
Steroids	Hormones, membrane fluidity	Cholesterol
Prostaglandins	Signaling, inflammation	PGE2
Thromboxanes	Blood clotting	Thromboxane A2

Key Equations and Structures

General structure of a fatty acid:

Example: Structure of arachidonic acid (precursor for prostaglandins and thromboxanes):

COX-catalyzed reaction (simplified):

Additional info: For more detailed structures and mechanisms, refer to your textbook or lecture slides.