Lipids Chapter 19 Structure and classification of lipids

Lipids Chapter 19

Structure and classification of lipids • Lipids are organic compounds that are found in living organisms that are soluble in nonpolar organic solvents. • Unlike the other types of compounds we’ve seen so far, there are no characteristic functional groups in lipids that indicate their structure.

Structure and classification of lipids • Lipids can be divided into five categories, on the basis of lipid function 1. 2. 3. 4. 5. Energy-storage lipids (triacylglycerols) Membrane lipids (e. g. phospholipids) Emulsification lipids (bile acids) Messenger lipids (e. g. steroid hormones) Protective-coating lipids (biological waxes)

Fatty acids and types of fatty acids • Fatty acids are structural components of all the lipids we’ll study except cholesterol, bile acids and steroid hormones. • They are naturally occurring monocarboxylic acids that tend to have even numbers of carbon atoms, and may be classified as: – long-chain (C 12 to C 26) – medium-chain (C 8 and C 10) – short-chain (C 4 and C 6) Fatty acids are not like most lipids in that they may be recognized distinctly by the presence of a COOH group on a carbon chain.

Fatty acids and types of fatty acids Saturated and unsaturated fatty acids • Fatty acids may also be categorized with regards to the presence (and number) of unsaturated units (double bonds). – Saturated fatty acids (SFAs) contain no double bonds – all C-C single bonds in the carbon chain component.

Fatty acids and types of fatty acids Saturated and unsaturated fatty acids • Monounsaturated fats (MUFAs) possess one C-C double bond in a monocarboxylic acid structure, and nearly all naturally occurring MUFAs have cis- stereochemistry.

Fatty acids and types of fatty acids Saturated and unsaturated fatty acids • Polyunsaturated fatty acids (PUFAs) possess more than one double bond in the carbon chain component of the fatty acid. Up to six double bonds may be found in biochemically important PUFAs. Fatty acids typically have the following characteristics: an unbranched carbon chain an even number of carbon atoms in the chain when double bonds are present, they have cis-stereochemistry

Fatty acids and types of fatty acids Unsaturated fatty acids and double-bond position • A system exists for describing unsaturated fatty acids in terms of the number of carbon atoms in the acid and in terms of double bond position(s): 16: 0 Fatty acid with 16 C-atoms and no double bonds 18: 1 D 9 Fatty acid with 18 C-atoms and one double bond at C-9 18: 3 D 9, 12, 15 Fatty acid with 18 C-atoms and three double bonds at C-9, C-12, and C-15

Fatty acids and types of fatty acids Unsaturated fatty acids and double-bond position • Several families of unsaturated fats may be recognized by the number of saturated carbon atoms that follow the last double bond (the placement of the methyl end of the chain with respect to the double bond). w-3 (omega-3 fatty acid) w-6 (omega-6 fatty acid)

Physical properties of fatty acids Water-solubility of fatty acids • The length of the carbon chain in a fatty acid is important in determining things like water-solubility and melting/boiling points. • Long carbon chains are non-polar, and things with long carbon chains on them do not dissolve in water. • Short chain fatty acids are slightly water-soluble, because the carboxyl group (-COOH) is polar.

Physical properties of fatty acids Melting points of fatty acids • Melting points also increase with increasing molar mass (London forces), so the longer the carbon chain, the higher the melting point of the fatty acid. • The presence of double bonds (all cis-stereochemistry) lowers the melting point (makes it easier for the fatty acid to melt) because these double bonds cause the molecule to become bent (less attractions between chains).

Energy storage lipids Fats and oils • Carbohydrates store energy in the form of complex carbohydrates (glycosides). • Fats store energy in the form of triacylglycerols: Older term for triacylglycerols: triglycerides Triacylglycerols are lipids formed by the esterification of three fatty acids to a glycerol molecule.

Energy storage lipids Fats and oils • Simple triacylglycerols have three identical fatty acid molecules. • Mixed triacylglycerols have different fatty acid molecules incorporated in them.

Energy storage lipids Fats and oils • Both fats and oils are complex mixtures in which many different kinds of triacylglycerols are present – fats are solids or semi-solids at room temperature – oils are liquids at room temperature • Generally, fats are obtained from animal sources, while oils are obtained from plants. • Fats involve triacylglycerols that contain mainly saturated fatty acid components, while oils have more unsaturated fatty acids components

Energy storage lipids Fats and oils

Chemical reactions of triacylglycerols Reactions of triacylglycerols • Triacylglycerols have ester groups and (sometimes) double bonds, so the reactions that can happen with triacylglycerols involve those functional groups (hydrolysis, addition reactions) • They can also undergo oxidation (enzyme-mediated) at the double bond to produce two carbonyl groups (aldehyde and then carboxylic acid).

Chemical reactions of triacylglycerols Reactions of triacylglycerols • Hydrolysis: since triacylglycerols are tri-esters, the ester groups can undergo hydrolysis to yield carboxylic acids and glycerol (1, 2, 3 -Propanetriol). • This kind of reaction is important in digestion.

Chemical reactions of triacylglycerols Reactions of triacylglycerols • Enzymes catalyze the hydrolysis of triacylglycerols in a stepwise fashion. The outside ester groups are hydrolyzed first before the middle group. • In certain cases, triacylglycerols are only partially hydrolyzed (partial hydrolysis) – this happens when only one or two of the fatty acid groups are removed from the triacylglycerol by hydrolysis.

Chemical reactions of triacylglycerols Reactions of triacylglycerols • Saponification: as before, the hydrolysis of an ester carried out under basic conditions is called saponification. The base causes de-protonation of the carboxylic acid to form a carboxylic acid salt (called fatty acid salts).

Chemical reactions of triacylglycerols Reactions of triacylglycerols • Saponification of a triacylglycerol yields fatty acid salts. The anions (negative ions) in these salts have long carbon chains and they have both polar and non-polar components. • In the body (and other aqueous environments), these anions form ball-shaped structures called micelles. • Micelles are able to interact with both polar and non-polar molecules, and serve as soaps/detergents.

Chemical reactions of triacylglycerols Reactions of triacylglycerols • Hydrogenation of a triacylglycerol can happen if the triacylglycerol has unsaturated fatty acid chains. • Hydrogenation of the double bonds (adding H 2 across the C-C double bond) converts C-C double bonds to C -C single bonds.

Chemical reactions of triacylglycerols Reactions of triacylglycerols • When hydrogenation occurs, the physical properties of a triacylglycerol will change. For example, more saturated chains will lower the melting point of the triacylglycerol, perhaps causing it to become a semisolid/solid at room temperature. • This reaction is used in the food industry to make margarine, peanut butter, and other partially hydrogenated products; however, partial hydrogenation also results in the conversion of cis-double bonds to trans-double bonds (trans-fats).

Chemical reactions of triacylglycerols Reactions of triacylglycerols • Oxidation of the C-C double bonds can also occur. • In this process, both bonds in the double bond are broken and two C=O groups are made: • This kind of reaction occurs when fats become rancid (decompose in air), producing compounds that have bad smells (short-chain aldehydes and carboxylic acids).