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How are carotenoids formed from two 20-carbon isoprenoid compounds?

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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.

Carotenoids are classified as tetraterpenes, which are isoprenoid compounds typically containing 40 carbon atoms. While many smaller isoprenoids are formed through the sequential head-to-tail addition of five-carbon units, larger molecules like tetraterpenes are synthesized differently.12

Rather than incorporating additional five-carbon units, carotenoids are formed through the tail-to-tail coupling of two 20-carbon isoprenoid precursors. This specific mode of attachment occurs at the center of the molecule, resulting in a symmetrical structure. This symmetrical arrangement is a hallmark of tetraterpenes and is distinct from the standard head-to-tail linkage found in smaller terpenes. This structural feature, combined with a long series of alternating single and double carbon-to-carbon bonds known as a conjugated system, allows these pigments to absorb light and exhibit their characteristic yellow, orange, or red colors.21