Lipids and Fatty Acids Overview

Lipids

• Lipids are diverse but generally hydrophobic
• All biomolecules, even lipids, are amphipathic
• Lipids are classified into those containing fatty acids (complex lipids) and those that do not.

Fatty Acids

• Fatty acids are carboxylic acids with hydrocarbon chains containing 4 to 36 carbons.
• Most natural fatty acids have an even number of carbons and are unbranched.
• Saturated fatty acids lack double bonds between carbons in the chain.
• Unsaturated fatty acids contain one or more C=C double bonds in the alkyl chain. Monounsaturated fatty acids have one double bond, and polyunsaturated fatty acids have multiple double bonds.
• Cis double bonds introduce kinks (approximately 30 degrees) into the alkyl chain.

Fatty Acid Nomenclature

• Systematic names (e.g., cis-9-octadecanoic acid), common names (e.g., oleic acid), delta and omega numbering systems.

Double Bonds in Polyunsaturated Fatty Acids (PUFAs)

• Double bonds in PUFAs are not usually conjugated; they are spaced by 3 carbon atoms.
• Examples include linoleic acid (18:2, Δ9,12 or ω-6).

Trans Fatty Acids

• Trans fatty acids form in the presence of metal catalysts used for partial hydrogenation.
• Trans fatty acids increase shelf life, stability at cooking temperatures, and result in solid consistency.
• A trans double bond allows a conformation that mimics the Van der Waals interactions found in saturated fats.
• Trans fatty acids are strongly linked with heart disease.

PUFAs and Lipid Peroxidation

• PUFAs are susceptible to lipid peroxidation due to hydrogen abstraction by free radicals, initiating a chain reaction.
• The peroxidation products include malondialdehyde, 4-hydroxynonenal, and others.

Triacylglycerols

• Triacylglycerols (TAGs) are storage lipids consisting of glycerol esterified to three fatty acids.
• Can be a mixed triacylglycerol.

Glycerophospholipids

• Glycerophospholipids are major membrane lipids.
• Two fatty acids are esterified to the first and second hydroxyl groups of glycerol-3-phosphate.
• The phosphoryl group is charged at physiological pH.
• The phosphate group can be further substituted with different head groups. Examples include phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, and phosphatidylglycerol.
• Unsaturated fatty acids commonly connect to C2 of glycerol-3-phosphate.

Sphingolipids

• Sphingolipids are membrane lipids with sphingosine as their backbone.
• A fatty acid is linked to sphingosine by an amide linkage.
• A polar head group is attached to sphingosine.
• Glycosphingolipids contain sugars and are located in the outer leaflet of plasma membranes.
• Sphingomyelin is structurally similar to phosphatidylcholine.

Lipids and Lipases

• Lipids can be hydrolyzed by lipases.
• Phospholipase A1, A2, C, and D, are types of lipases.

Cholesterol

• Sterols have a steroid nucleus with four fused rings, a hydroxyl group, and various nonpolar side chains.
• Examples include Cholesterol.
• Cholesterol acts as a fluidity buffer in cell membranes. Below the transition temperature cholesterol disrupts close packing, increasing fluidity. Above the transition temperature, cholesterol reduces acyl chain mobility and decreases fluidity. Cholesterol also broadens/abolishes phase transitions.

Lipid Assemblies

• Lipid assemblies include micelles, bilayers, and liposomes.

Bilayer Fluidity

• Bilayer fluidity is the opposite of viscosity and is inversely related to lipid packing.
• It's influenced by the strength of van der Waals interactions (increased by saturated fatty acids) and increased unsaturation, size of head groups, and temperature.

Hydrocarbon Conformations

• Hydrocarbon conformations include eclipsed and staggered forms, and anti and gauche conformations.
• Staggered conformations are typically the most stable and common (especially the anti form).
• Anti conformations maximize packing and are favored at lower temperatures.
• Gauche conformations are more common at higher temperatures.

Two States of Bilayer Lipids

• Bilayer lipids exist in gel and liquid-crystalline phases.
• The transition temperature (T_m) depends on lipid composition (number of carbon atoms, degree of unsaturation, size of head groups).
• T_m for biological membranes is typically between 10-40 °C.

This quiz covers the essentials of lipids and fatty acids, including their structure, classification, and nomenclature. Explore the differences between saturated and unsaturated fatty acids, as well as the significance of double bonds in polyunsaturated fatty acids. Test your understanding of these important biomolecules!