6.6: Saccharolipids, polyketides, and prenols
- Define and understand the basic structural features of saccharolipids, polyketides, and prenols.
- Define mono-, sesqui-, di-, and tri-terpenes and recognize isoprene units in terpenes, terpenoids, and steroids.
Saccharolipids
Saccharolipids are fatty acids linked to a saccharide (carbohydrate) backbone by linkages other than glycosidic linkages. They are compatible with membrane bilayers. For example, saccharolipid lipid A found in E. Coli is shown in the figure on the right.
Polyketides
Polyketides are a broad class of natural products derived from \(\beta\)-polyketones, i.e., compounds containing \(\ce{[-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-CH2{-}]_{n}}\) repeat units in their backbone or their reduced forms, such as \(\ce{[-\!\!\!\!\!{\overset{\overset{\huge\enspace\enspace{OH}}|}{C}}\!\!\!\!\!-CH2{-}]_{n}}\), and \(\ce{[-CH2-CH2{-}]_{n}}\). Polyketides are important in the pharmaceutical industry. They are used as antimicrobial, antiparasitic, and anticancer agents; some are toxins. About 20% of the top-selling medicines are polyketides. Some examples of medically important polyketide products are shown below.
Prenols
Prenols are synthesized from five-carbon isoprene precursors: isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP), as shown in Figure \(\PageIndex{1}\) (left). The five-carbon skeleton of isoprene units in these products can often be easily distinguished, as shown in by different color sections of some examples in Figure \(\PageIndex{1}\) (right). Since they contain a multiple of five \(\ce{C's}\) in their skeleton, they are also classified as monoterpens (10 \(\ce{C's}\)), sesquiterpenes (15 \(\ce{C's}\)), diterpenes (20 \(\ce{C's}\)), triterpenes ((30 \(\ce{C's}\)), and tetraterpenes (40 \(\ce{C's}\)). Terpenoids are modified terpenes that contain additional functional groups, usually oxygen functional groups as those shown in \(\PageIndex{1}\) (right).
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Steroids -other lipids described later are derived from triterpene squalene, as illustrated in Figure \(\PageIndex{2}\).