Lipid Chemistry Overview

Lipid Chemistry

• Lipids are a diverse group of naturally occurring organic compounds.
• Key types of lipids include fats, oils, and waxes.
• Fatty acids are the building blocks of many lipids.
• Fatty acids have the general formula R-COOH.
• R represents a hydrocarbon chain.
• Saturated fatty acids contain no double bonds in their hydrocarbon chains.
• Unsaturated fatty acids contain one or more double bonds in their hydrocarbon chains. Double bonds often have a cis conformation.
• Unsaturated fatty acids usually have lower melting points than saturated fatty acids.

Fatty Acids

• Fatty acids—when not attached to other molecules—are termed "free" fatty acids.
• Fatty acids are amphipathic molecules, possessing both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.
• Even-numbered saturated fatty acids are common in plants and animals.
• Odd-numbered fatty acids are less common, but occur in bacteria and ruminant animals.
• Free fatty acids are either saturated or unsaturated.
• Fatty acid chain lengths vary, but most have 16, 18, or 20 carbon atoms.

Chemical Properties

• Hydrogenation: Unsaturated fatty acids can react with hydrogen gas in the presence of a catalyst (such as platinum or nickel) to form saturated fatty acids; this is a key reaction in food processing (e.g., converting vegetable oil to solid margarine).
• Partial hydrogenation can produce trans fats. Trans fats have a different structure than cis fats, affecting their physical properties.
• Halogenation: Halogens (like bromine or iodine) can be added to unsaturated fatty acids at room temperature. This reaction is used to determine the degree of unsaturation (iodine number).
• Oxidation: Lipids can react with oxygen, often leading to rancidity (unpleasant odors/flavors). The amount of unsaturation in the oil greatly impacts susceptibility to oxidation.

Functions of Lipids

• Lipids, especially triacylglycerols, are a crucial form of energy storage.
• Lipids function as insulation and shock absorbers for the body.
• Lipids contribute to the structure of cell membranes (phospholipids, sphingolipids).
• They are linked to carbohydrates (glycolipids), to amino groups (sphingomyelins).
• Lipids are necessary for transporting fat-soluble vitamins.
• Some lipids can have roles in hormone synthesis, or other important biological functions.

Classification of Lipids

• Simple lipids (e.g., fats, oils, waxes)—are derived from alcohol and carboxylic acids (esterification).
• Compound lipids (e.g., phospholipids, glycolipids)—combine simple lipids with other components like phosphate or carbohydrates.
• Derived lipids (e.g., steroids, eicosanoids)—originate from simple or compound lipids (some function as hormones/chemical messengers).

Simple Lipids: Triacylglycerols (Triglycerides)

• Triacylglycerols are the most abundant lipids in the diet and a major component of fat storage depots.
• Triacylglycerols are synthesized in the body and found in plants and animals (adipocytes).
• Triacylglycerols are formed by esterification.
• They are insoluble in water.
• They have high energy density compared to carbohydrates and proteins.

Waxes

• Waxes are esters of long-chain fatty acids and long-chain alcohols.
• They are important for waterproofing in some organisms.

Compound Lipids: Phospholipids

• Phospholipids are the most abundant type of membrane lipids.
• They consist of a polar head and two nonpolar tails (amphipathic).
• This structure allows for the formation of lipid bilayers that comprise cell membranes.
• Phospholipids are crucial for transport and utilization of other lipids. Some examples are Lecithin.
• Phospholipids can assist in food processing by aiding in mixing of vegetable fats.

Compound Lipids: Sphingolipids

• Sphingolipids contain a backbone of sphingosine.
• These lipids are essential components of nerve tissue/myelin sheath, which help in the proper signaling, transmission through nerve tissues.
• Examples include sphingomyelins and glycolipids.

Compound Lipids: Glycolipids

• Glycolipids are comprised of lipids linked to carbohydrates.
• They are essential components of cell membranes, particularly within the nervous system and on the cell surfaces, participating in signaling processes.

Derived Lipids: Steroids (Cholesterol)

• Cholesterol is a crucial component of cell membranes in animals and is a precursor to steroid hormones.
• Cholesterol is a critical building block for hormones.
• High blood cholesterol levels are often associated with cardiovascular diseases (atherosclerosis).
• Bile salts aid in lipid absorption.

Lipoproteins

• Lipoproteins are complexes that transport cholesterol and other lipids in the blood.
• Different types of lipoproteins (HDL, LDL, VLDL, chylomicrons) carry various amounts of cholesterol and other lipids around the body.
• HDL typically removes excess cholesterol from tissues and delivers it to the liver.
• LDL carries cholesterol from the liver to tissues and may contribute to plaque buildup in arteries if excessive amounts are present.

Terpenoids

• Terpenoids are a large diverse class of compounds that come from plants.
• Many terpenoids are hydrocarbons, also known as terpenes.
• Other terpenoids contain oxygen.
• They are frequently used in fragrances and/or medicines, particularly in plant extracts.

Eicosanoids

• Eicosanoids are a class of lipid-based signaling molecules (chemical messengers) important for various bodily functions.
• They are produced within the body "at the site" of function.
• Examples include prostaglandins, thromboxanes, and leukotrienes.
• Eicosanoids affect processes such as inflammation, immunity, blood clotting, and regulating fluid and blood flow.