The active site of the AcmB binds the flavin part of the FAD and the 3-ketosteroid (here ADD product) and is located in the area of contact of all three domains. Figure shows that most of the amino acids surrounding ADD are non-polar. The axial methyl groups on C10 and C13 carbons face the catalytic domain, and the sterane A ring is almost parallel to isoalloxazine (Figure ). The active site contains six conserved tyrosine residues: Y115, Y118, and Y536 from the FAD-binding domain as well as Y363, Y466, and Y467 from the catalytic domain.