Similar to the active site of KstD1 from R. erythropolis, Y363 is positioned close to the C2 atom of the steroid, and we assume it is in the tyrosyl anion form. In this form, it can act as a catalytic base during substrate activation (see below). Y363 is accompanied by Y118, which, based on the short distance of their tyrosyl O atoms (2.8Å), forms an H-bond with Y363. The Y536 hydroxyl and the G540 NH of the peptide bond form hydrogen bonds with carbonyl oxygen at carbon C3 of ADD (Figure ). Additionally, the steroid core forms π-alkyl or alkylalkyl interactions with the amino acids A52, F338, Y363, and A539.