text: Steroid 21-hydroxylase is a cytochrome P450 enzyme that is notable for its substrate specificity and relatively high catalytic efficiency.
  Like other cytochrome P450 enzymes, steroid 21-hydroxylase participates in the cytochrome P450 catalytic cycle and engages in one-electron transfer with NADPH-P450 reductase. Steroid 21-hydroxylase is highly specific for hydroxylation of progesterone and 17-hydroxyprogesterone. This is in marked contrast to the evolutionarily and functionally related P450 enzyme 17-hydroxylase, which has a broad range of substrates.
  The chemical reaction in which steroid 21-hydroxylase catalyzes the addition of hydroxyl (-OH) to the C21 position of progesterone, 17α-hydroxyprogesterone and 21-desoxycortisone was first described in 1952.
  Studies of the human enzyme expressed in yeast initially classified 17-hydroxyprogesterone as the preferred substrate for steroid 21-hydroxylase, however, later analysis of the purified human enzyme found a lower K and greater catalytic efficiency for progesterone over 17-hydroxyprogesterone.
  The catalytic efficiency of steroid 21-hydroxylase for conversion of progesterone in humans is approximately 1.3 x 10 Ms at 37 °C. This makes it the most catalytically efficient P450 enzyme of those reported to date, and catalytically more efficient than the closely related bovine steroid 21-hydroxylase enzyme. C-H bond breaking to create a primary carbon radical is thought to be the rate-limiting step in the hydroxylation.