How are isoprene units usually linked head-to-tail in isoprenoids?

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Isoprenoid Camphor, an isoprenoid compound classified as a terpenoid ketone, is used in incense and certain medicinal compounds. It is a natural substance obtained from the camphor laurel (*Cinnamomum camphora*), a species of evergreen.

In the biosynthesis of many isoprenoids, isoprene units are typically linked in a "head-to-tail" fashion. In this arrangement, carbon atom 1 of one unit is bonded to carbon atom 4 of the next unit. This pattern of coupling logically follows the expected enzymatic reaction pathways involving pyrophosphate units.12

While head-to-tail linkage is common, some isoprenoids, particularly many triterpenes and tetraterpenes, feature a "tail-to-tail" bond, which connects carbon 4 of one unit to carbon 4 of another. Additionally, some complex isoprenoids may involve other types of linkages that do not follow either the head-to-tail or tail-to-tail patterns.2