A cytochrome P450 monooxygenase that catalyzes the side-chain hydroxylation and cleavage of cholesterol to pregnenolone, the precursor of most steroid hormones. Catalyzes three sequential oxidation reactions of cholesterol, namely the hydroxylation at C22 followed with the hydroxylation at C20 to yield 20R,22R-hydroxycholesterol that is further cleaved between C20 and C22 to yield the C21-steroid pregnenolone and 4-methylpentanal. Mechanistically, uses molecular oxygen inserting one oxygen atom into a substrate and reducing the second into a water molecule. Two electrons are provided by NADPH via a two-protein mitochondrial transfer system comprising flavoprotein FDXR (adrenodoxin/ferredoxin reductase) and nonheme iron-sulfur protein FDX1 or FDX2 (adrenodoxin/ferredoxin). cholesterol + 6 H(+) + 3 O2 + 6 reduced [adrenodoxin] = 4-methylpentanal + 4 H2O + 6 oxidized [adrenodoxin] + pregnenolone cholesterol + 2 H(+) + O2 + 2 reduced [adrenodoxin] = (22R)-hydroxycholesterol + H2O + 2 oxidized [adrenodoxin] (22R)-hydroxycholesterol + 2 H(+) + O2 + 2 reduced [adrenodoxin] = (20R,22R)-20,22-dihydroxycholesterol + H2O + 2 oxidized [adrenodoxin] (20R,22R)-20,22-dihydroxycholesterol + 2 H(+) + O2 + 2 reduced [adrenodoxin] = 4-methylpentanal + 2 H2O + 2 oxidized [adrenodoxin] + pregnenolone Lipid metabolism; C21-steroid hormone metabolism. Steroid metabolism; cholesterol metabolism. Interacts with FDX1/adrenodoxin. Localizes to the matrix side of the mitochondrion inner membrane. Belongs to the cytochrome P450 family.