Responsible for the C-12 hydroxylation of the macrolactone ring of erythromycin. Thus, EryK catalyzes the hydroxylation of erythromycin D (ErD) at the C-12 position to produce erythromycin C (ErC). Erythromycin B (ErB) is not a substrate for this enzyme. erythromycin D + H(+) + NADPH + O2 = erythromycin C + H2O + NADP(+) Binds 1 heme b (iron(II)-protoporphyrin IX) group per subunit. kcat is 108 min(-1). Antibiotic biosynthesis; erythromycin biosynthesis. Monomer. Cells lacking this gene no longer produce erythromycin A but accumulate the B and D forms of the antibiotic. Belongs to the cytochrome P450 family.