A mitochondrial cytochrome P450 enzyme that catalyzes the side-chain cleavage of C27 cholesterol to C21 pregnenolone in the presence of molecular oxygen and nadph-ferrihemoprotein reductase. This enzyme, encoded by CYP11A1 gene, catalyzes the breakage between C20 and C22 which is the initial and rate-limiting step in the biosynthesis of various gonadal and adrenal steroid hormones.