Fatty Acid, Triacylglycerol, and Ketone Body Metabolism

Overview of Fatty Acids

Fatty acids are fundamental biomolecules that exist both as free molecules and as fatty acyl esters in the body. They play crucial roles in energy metabolism, membrane structure, and signaling.

• Free Fatty Acids (FFA): Present in all tissues; small amounts circulate in plasma.

• Energy Source: Fatty acids are oxidized by many tissues to provide energy and serve as substrates for ketone body synthesis.

• Structural Role: Fatty acids are components of membrane lipids and can be attached to proteins to enhance membrane association.

• Precursors: Fatty acids are precursors for hormone-like prostaglandins.

• Energy Reserve: Fatty acids are the major energy reserve of the body, stored as triacylglycerols.

• Clinical Relevance: Disorders of fatty acid metabolism are associated with obesity and metabolic diseases.

Fatty Acid Structure

Chemical Structure of Fatty Acids

Fatty acids consist of a hydrophobic hydrocarbon chain and a terminal carboxyl group, giving them amphipathic properties.

• Hydrophobic Chain: The long hydrocarbon tail is water-insoluble.

• Carboxyl Group: The terminal carboxyl group (–COOH) is hydrophilic and can ionize to –COO−.

• Amphipathic Nature: The combination of hydrophobic and hydrophilic regions allows fatty acids to interact with both lipids and water.

• Transport: Long-chain fatty acids (LCFA) are transported in the blood as fatty acyl esters in lipoproteins or bound to serum albumin.

Fatty Acid Saturation

Saturated vs. Unsaturated Fatty Acids

The degree of saturation refers to the presence or absence of double bonds in the hydrocarbon chain.

• Saturated Fatty Acids: No double bonds; straight chains allow tight packing.

• Unsaturated Fatty Acids: One or more double bonds, usually in the cis configuration, causing bends or kinks in the chain.

• Cis vs. Trans: Naturally occurring double bonds are almost always cis, not trans.

• Polyunsaturated Fatty Acids: Multiple double bonds, typically spaced at three-carbon intervals.

Fatty Acid Chain Length

Classification by Chain Length

Fatty acids are classified based on the number of carbon atoms in their chain, which affects their physical and metabolic properties.

• Even Numbered Chains: Most naturally occurring fatty acids have an even number of carbons.

• Very Long Chain Fatty Acids (VLCFA): >22 carbons, important for brain function.

• Numbering: Carbon atoms are numbered starting from the carboxyl carbon (carbon 1).

• Omega (ω) Carbon: The terminal methyl group is called the ω-carbon, regardless of chain length.

Fatty Acid Bond Positions

Omega and Delta Notation

The position of double bonds in fatty acids is described using omega (ω) and delta (Δ) notation.

• Omega (ω) Notation: Counts from the terminal methyl group.

• Delta (Δ) Notation: Counts from the carboxyl end.

• Essential Fatty Acids: Linoleic acid (18:2(9,12)) is the major ω-6 fatty acid; α-linolenic acid (18:3(9,12,15)) is the major ω-3 fatty acid.

• Example: In linoleic acid, the terminal double bond is six bonds from the ω end.

Summary Table: Fatty Acid Types and Properties

Key Equations

• General Formula for Saturated Fatty Acid:

• Numbering of Carbons: -carbon: first carbon after carboxyl group -carbon: terminal methyl group

Example: Linoleic Acid

• Structure: 18:2(9,12)

• Classification: Polyunsaturated, essential ω-6 fatty acid

• Function: Precursor for arachidonic acid and eicosanoids

Additional info:

• Essential fatty acids must be obtained from the diet because humans lack the enzymes to introduce double bonds beyond carbon 9 in the fatty acid chain.

• Fatty acid nomenclature is important for understanding metabolic pathways and clinical implications of fatty acid deficiencies.