Involved in the biosynthesis of mycinamicin, a 16-membered macrolide antibiotic. Catalyzes hydroxylation at the C21 methyl group of mycinamicin VIII, the earliest macrolide form in the postpolyketide synthase tailoring pathway, leading to mycinamicin VII. Uses ferredoxin MycCII in electron transfer for catalysis. kcat is 71.7 min(-1) in the presence of spinach ferredoxin. kcat is 104.1 min(-1) in the presence of MycCII ferredoxin. Antibiotic biosynthesis; mycinamicin biosynthesis. Cells lacking this gene accumulate protomycinolide IV and mycinamicin VIII. Belongs to the cytochrome P450 family. Truncated N-terminus.