Lec. 1: Biochemistry of Lipids

Introduction to Lipids

• Lipids are hydrophobic, nonpolar molecules that are soluble in nonpolar solvents and insoluble in polar solvents, such as water.
• Major types of lipids include fatty acids, triglycerides, steroids, and eicosanoids.

Functions of Lipids

• Lipids are a source of high energy value, with 1 gram of fat providing 9 calories.
• They are constituents of membrane structure and regulate membrane permeability.
• Lipids serve as a source of fat-soluble vitamins (A, D, K, and E).
• They are important as cellular metabolic regulators (steroid hormones and prostaglandins).
• Phospholipids participate in the electron transport chain as components of the inner mitochondrial membrane.

Fatty Acids

• Fatty acids contain a carboxylic acid group, which makes them polar, but they also have a long hydrocarbon tail that makes them nonpolar overall.
• Fatty acids typically contain between 12 and 20 carbons, usually an even number.
• Saturated fatty acids have no carbon-carbon double bonds, while unsaturated fatty acids have double bonds and polyunsaturated fatty acids (PUFAs) have multiple sites of unsaturation.
• Melting points for saturated fatty acids increase with the number of carbons.
• Unsaturated fatty acids can be monounsaturated or polyunsaturated, and have lower melting points than saturated fatty acids.
• The double bonds in fatty acids are usually cis, which lowers the melting points.

Essential Fatty Acids

• Essential fatty acids (EFAs) cannot be synthesized by the body and must be supplied in the diet.
• EFAs are unsaturated fatty acids, including linoleic acid (18:2;9,12), linolenic acid (18:3;9,12,15), and arachidonic acid (20:4;5,8,11,14).

Shorthand Representation of Fatty Acids

• The general rule for shorthand representation of fatty acids is to write the total number of carbon atoms, followed by the number of double bonds, and finally the position of double bonds, starting from the carboxyl end.
• Examples include palmitic acid (16:0), oleic acid (18:1;9), and arachidonic acid (20:4;5,8,11,14).

Delta System

• The site of unsaturation in a fatty acid is indicated by the symbol Δ and the number of the first carbon of the double bond relative to the carboxylic acid group (–COOH).
• The delta system is used to designate the position of double bonds, with carbon 1 being the carboxylic acid group.