The Organic Chemistry of Lipids Chapter 27 The Organic Chemistry of Lipids Organic Chemistry 6th Edition Paula Yurkanis Bruice

Lipids are soluble in nonpolar organic compounds and have a variety of structures and functions:

Common Naturally Occurring Fatty Acids

Fatty acids, a major group of lipids, are carboxylic acids with long hydrocarbon chains. Polyunsaturated fatty acids have more than one double bond. The double bonds in naturally occurring fatty acids have the cis configuration. Double bonds in naturally occurring unsaturated fatty acids are separated by one methylene group.

Unsaturated fatty acids have lower melting points than

Waxes Are High-Molecular-Weight Esters

Fats and Oils The three OH groups of glycerol are esterified with fatty acids. A simple triacylglycerol has three identical fatty acid components. A mixed triacylglycerol has two or three different fatty acid components.

Fats are triacylglycerols existing as solid or semisolid state at room temperature:

Oils are liquid triacylglycerols:

Some or all of the double bonds of polyunsaturated oils can be reduced by catalytic hydrogenation (Section 4.12) Catalytic hydrogenation is used to convert oils to fats (e.g., vegetable oil to margarine). This process also isomerizes double bonds from cis to trans, resulting in unhealthy trans fats.

Oxidation of Polyunsaturated Fats and Oils by O2

Phosphoacylglycerols They are the major components of cell membranes. The terminal OH group of glycerol is esterified with phosphoric acid.

The most common phosphoacylglycerols in membranes are phosphodiesters. The alcohols most commonly used to form the second ester group are ethanolamine, choline, and serine.

Phosphoacylglycerols form membranes by arranging themselves into a lipid bilayer:

Sphingolipids They contain an amino alcohol called sphingosine instead of glycerol: Sphingolipids are the major lipid component in the myelin sheath, the insulator of nerve cells axons

Two of the Most Common Kinds of Sphingolipids

Prostaglandins Prostaglandins are synthesized from arachidonic acid. They are responsible for regulating a variety of physiological responses. They are classified using the formula PGX, where X designates the functional groups of the five-membered ring.

Examples of prostaglandins and physiological processes they regulate: Blood pressure Clotting Fever Pain Labor Sleep–wake cycle

Prostaglandin endoperoxide synthase catalyzes the conversion of arachidonic acid to PGH2. There are two forms of this enzyme: production under normal physiological conditions and in response to inflammation. The enzyme has a cyclooxygenase activity and a hydroperoxidase activity.

Aspirin (acetylsalicyclic acid) inhibits cyclooxygenase activity:

Other anti-inflammatory drugs also compete with arachidonic acid or the peroxy radical for the enzyme’s binding site:

Terpenes Terpenes contain carbon atoms in multiples of five. They are made by joining five-carbon isoprene units. Oxygen-containing terpenes are sometimes called terpenoids. They are a diverse class of lipids.

The Isoprene Rule

Recognizing the Isoprene Subunit Zingiberene, oil of ginger Has three isoprene units

Squalene, a triterpene, is a precursor of steroid molecules:

Lycopene and b-carotene are tetraterpenes called carotenoids:

Vitamin A Is a Terpene

Isopentenyl pyrophosphate is the starting point for biosynthesis of terpenes:

The Conversion of Mevalonic Acid into Mevalonyl Phosphate an SN2 reaction a second SN2 Review Sections 26.2 and 26.3

Isoprene Biosynthesis: Claisen Condensation Starred carbon can be 14C Claisen condensation joins two acetates

Isoprene Biosynthesis: Second Claisen Condensation

Isoprene Biosynthesis: Leaving Group Elaboration

Isoprene Biosynthesis: Elimination and Isomerization Isopentenyl pyrophosphate Dimethylallyl pyrophosphate

Both isopentyl pyrophosphate and dimethylallyl pyrophosphate are needed for the biosynthesis of terpenes

Terpene Biosynthesis Formation of geranyl pyrophosphate:

Geranyl Pyrophosphate Formation with 14C-Labels

Many monoterpenes can be synthesized from geranyl pyrophosphate:

Formation of Farnesyl Pyrophosphate

the Precursor of Cholesterol Formation of Squalene, the Precursor of Cholesterol

Steroids Steroids are nonpolar compounds and are lipids. Hormones are chemical messengers. Many hormones are steroids. Steroids contain a tetracyclic ring system.

Structural Features of Steroids

In steroids, the B, C, and D rings are trans fused: Methyl groups at C-10 and C-13 are called angular methyl groups

The A and B rings are also trans fused in most naturally occurring steroids: Substituents on the same side of the steroid ring system as the angular methyl groups are b-substituents. Those on the opposite side of the plane of the ring system are a-substituents.

Cholesterol Is a Steroid The most abundant member of the steroid family in animals The precursor of all other steroids More rigid than other membrane lipids

Examples of Steroids

Examples of Synthetic Steroids