Specifically catalyzes the 21-hydroxylation of steroids. Required for the adrenal synthesis of mineralocorticoids and glucocorticoids. 17alpha-hydroxyprogesterone + O2 + reduced [NADPH--hemoprotein reductase] = 11-deoxycortisol + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] O2 + progesterone + reduced [NADPH--hemoprotein reductase] = 21-hydroxyprogesterone + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] The leucine-rich hydrophobic amino acid N-terminal region probably helps to anchor the protein to the microsomal membrane. Belongs to the cytochrome P450 family.