CASE FILES: BIOCHEMISTRY
Figure 50-1. Biosynthesis of cortisol. Cholesterol is the starting material for
all steroid synthesis. Cortisol synthesis involves a series of oxidation reactions
catalyzed by cytochrome P450 enzymes (1) The cleavage of cholesterol’s six-
carbon side chain is catalyzed by
—the rate-limiting step in steroid
synthesis. (2) This is followed by dehydrogenation of the hydroxyl group in
pregnenolone, which is catalyzed by 3P-hydroxy steroid dehydrogenase.
(3) This molecule is then oxygenated at carbons 17, 21, and 11 by
respectively, to yield cortisol. All structure modifi-
cations are indicated by dashed lines.