Organic Chemistry 4 th Edition Paula Yurkanis Bruice Irene Lee Case Western Reserve University Cleveland, OH ©2004, Prentice Hall Chapter 26 Lipids

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

Fatty Acids Are Carboxylic Acid with Hydrocarbon Chains

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 Polyunsaturated fatty acids have more than one double bond

Triacylglycerols The three OH groups of glycerol are esterified with fatty acids glycerol fatty acids a triacylglycerol a fat or an oil

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

Oxidation of Polyunsaturated Fats and Oils by O 2

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

The most common phosphoacylglycerols in membranes have a second phosphate ester linkage Phosphoacylglycerols are classified as phospholipids

Phosphoacylglycerols form membranes by arranging themselves into a lipid bilayer Saturated fatty acids decrease membrane fluidity because their hydrocarbon chains can pack closely together Unsaturated fatty acids have the opposite effect

Sphingolipids They contain sphingosine instead of glycerol They are the major lipid components in the myelin sheaths of nerve fiber They are also found in membranes

Two of the Most Common Kinds of Sphingolipids

Prostaglandins are synthesized from arachidonic acid They are responsible for regulating a variety of physiological responses Prostaglandins postaglandin skeleton Name in the format PGX, with X designating the functional groups of the five-membered ring

PGE 1 PGE 2 PGF 2n Naming Prostaglandins

Aspirin Inhibits the Synthesis of Prostaglandins (prostaglandin endoperoxide synthase)

Terpenes contain carbon atoms in multiples of five They are made by joining together 5-carbon isoprene units Terpenes Oxygen-containing terpenes are sometimes called terpenoids

The Isoprene Rule

Squalene, a triterpene, is a precursor of steroid molecules

Lycopene and  -carotene are tetraterpenes called carotenoids

The five-carbon compound used for biosynthesis of terpenes is isopentyl pyrophosphate

The Conversion of Mevalonic Acid into Mevalonyl Phosphate

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

Formation of geranyl pyrophosphate Terpene Biosynthesis

Formation of Farnesyl Pyrophosphate

Formation of Squalene, the Precursor of Cholesterol

Hormones are chemical messengers Many hormones are steroids All steroids contain a tetracyclic ring system Steroids

Methyl groups at C-10 and C-13 are called angular methyl groups The B, C, and D rings are trans fused

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

Cholesterol Is a Steroid

Examples of Steroids

Biosynthesis of Cholesterol

Examples of Synthetic Steroids