Catalyzes the first oxygenation step of taxol biosynthesis. Can use both taxa-4(5),11(12)-diene and taxa-4(20),11(12)-diene as substrate. O2 + reduced [NADPH--hemoprotein reductase] + taxa-4,11-diene = H(+) + H2O + oxidized [NADPH--hemoprotein reductase] + taxa-4(20),11-dien-5alpha-ol Alkaloid biosynthesis; taxol biosynthesis; taxa-4(20),11-dien-5alpha-ol from geranylgeranyl diphosphate: step 2/2. Belongs to the cytochrome P450 family.