20-hydroxyecdysone is a steroid hormone that controls the ecdysis or molting of insects. It is formed from the modification of cholesterol by various p450 enzymes. Initially, cholesterol is modified by a cholesterol 7-desaturase, forming 7-dehydrocholesterol. In the endoplasmic reticulum, 7-dehydrocholesterol is modified by cytochrome p450 307a1 to form diketol. Diketol can interact with the cytochrome p450 306a1 enzyme, ecdysteroid 25-hydroxylase, forming 2,22-dideoxy-3-dehydroecdysone. In the mitochondria, 2,22-dideoxy-3-dehydroecdysone can be modified by cytochrome p450 302a1, also known as ecdysteroid 22-hydroxylase, which forms 3-dehydro-2-deoxyecdysone, which in turn can be modified by cytochrome p450 315a1, ecdysteroid 2-hydroxylase, to form 3-dehydroecdysone, one of the final products of this pathway. Diketol can also spontaneously form 3β,5β-ketodiol, which then interacts with the same enzymes as diketol. First, it is modified in the endoplasmic reticulum by cytochrome p450 306a1, ecdysteroid 25-hydroxylase to form 3β,5β-ketotriol. In the mitochondria, 3β,5β-ketotriol is then modified by cytochrome p450 302a1, ecdysteroid 22-hydroxylase, to form 2-deoxyecdysone, and then cytochrome 315a1, ecdysteroid 2-hydroxylase, to form ecdysone. From this point, ecdysone can interact with ecdysone oxidase to form 3-dehydroecdysone, the same product as in the first half of this pathway. In addition to this reaction, ecdysone can also interact with ecdysone 20-monooxygenase in the mitochondria to form 20-hydroxyecdysone (crustecdysone), which is the main molting hormone. Finally, 20-hydroxyecdysone can interact with cytochrome p450 18a1, 26-hydroxylase, in order to form 20,26-dihydroxyecdysone, the final product of this branch of the pathway.

Molting Hormone Biosynthesis References