FIGURE 2 The structure of 20-hydroxyecdysone (20HE) and related ecdysteroids. The hydroxyl substituent of 20HE at position 20 confers biological activity to ecdysone. Many insects obtain phytoecdysones from plants and convert them to biologically active forms. These molecules differ from 20HE only in the number of carbons in the alkyl side chain, shown for (A) makisterone A (28 carbons), (B) makisterone C (29 carbons), and (C) ponasterone A, which lacks the hydroxyl group at position 25.

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