A cytochrome P450 monooxygenase involved in corticoid and androgen biosynthesis. Catalyzes 17-alpha hydroxylation of C21 steroids, which is common for both pathways. A second oxidative step, required only for androgen synthesis, involves an acyl-carbon cleavage. The 17-alpha hydroxy intermediates, as part of adrenal glucocorticoids biosynthesis pathway, are precursors of cortisol. Hydroxylates steroid hormones, pregnenolone and progesterone to form 17-alpha hydroxy metabolites, followed by the cleavage of the C17-C20 bond to form C19 steroids, dehydroepiandrosterone (DHEA) and androstenedione. Has 16-alpha hydroxylase activity. Catalyzes 16-alpha hydroxylation of 17-alpha hydroxy pregnenolone, followed by the cleavage of the C17-C20 bond to form 16-alpha-hydroxy DHEA. Also 16-alpha hydroxylates androgens, relevant for estriol synthesis. Mechanistically, uses molecular oxygen inserting one oxygen atom into a substrate, and reducing the second into a water molecule, with two electrons provided by NADPH via cytochrome P450 reductase (CPR; NADPH-ferrihemoprotein reductase). a C21-steroid + O2 + reduced [NADPH--hemoprotein reductase] = a 17alpha-hydroxy-C21-steroid + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] O2 + progesterone + reduced [NADPH--hemoprotein reductase] = 17alpha-hydroxyprogesterone + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] O2 + pregnenolone + reduced [NADPH--hemoprotein reductase] = 17alpha-hydroxypregnenolone + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] 17alpha-hydroxyprogesterone + O2 + reduced [NADPH--hemoprotein reductase] = acetate + androst-4-ene-3,17-dione + 2 H(+) + H2O + oxidized [NADPH--hemoprotein reductase] 17alpha-hydroxyprogesterone + O2 + reduced [NADPH--hemoprotein reductase] = 16alpha,17alpha-dihydroxyprogesterone + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] 16alpha,17alpha-dihydroxyprogesterone + O2 + reduced [NADPH--hemoprotein reductase] = 6beta,16alpha,17alpha-trihydroxyprogesterone + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] 17alpha-hydroxypregnenolone + O2 + reduced [NADPH--hemoprotein reductase] = 3beta-hydroxyandrost-5-en-17-one + acetate + 2 H(+) + H2O + oxidized [NADPH--hemoprotein reductase] 16alpha,17alpha-dihydroxypregnenolone + O2 + reduced [NADPH--hemoprotein reductase] = 3beta,16alpha-dihydroxy-androst-5-en-17-one + acetate + 2 H(+) + H2O + oxidized [NADPH--hemoprotein reductase] 3beta-hydroxyandrost-5-en-17-one + O2 + reduced [NADPH--hemoprotein reductase] = 3beta,16alpha-dihydroxy-androst-5-en-17-one + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] androst-4-ene-3,17-dione + O2 + reduced [NADPH--hemoprotein reductase] = 16alpha-hydroxyandrost-4-ene-3,17-dione + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] Regulated predominantly by intracellular cAMP levels. The 17,20-lyase activity is stimulated by cytochrome b5, which acts as an allosteric effector increasing the Vmax of the lyase activity. Steroid hormone biosynthesis. Steroid biosynthesis; glucocorticoid biosynthesis. Belongs to the cytochrome P450 family.