690 likes | 1.3k Views
Chapter 27 The Organic Chemistry of Lipids. Organic Chemistry 6 th 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
E N D
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:
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 saturated fatty acids
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:
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.
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:
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: 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:
Many monoterpenes can be synthesized from geranyl pyrophosphate:
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.
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. • Thoseon 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